Sunday, October 2, 2011
Gizmo 48V Battery Meter values
While I don't use one any more a friend of mine is restoring a couple of Gizmos and I thought it might be nice to know what voltage the different marks are on the Gizmo 48V Battery Meter. Assuming each 48V meter is the same you probably can use these values. According to the Gizmo User Manual early Gizmos had a 12V meter hooked to the first battery so these values might not match. In the picture the tip of the V is where the value is to be read. The voltage where the needle first starts to move is at about 27V and where it hits the upper end of the range is about 66V. When I had lead acid batteries in my Gizmo I knew when the resting voltage was near or at the bottom of the green it was definitely time to charge! I usually charged at every opportunity I had which meant every time I got home.
Battery Balance Meter 9V Battery Replacement
As it turns out I got tired of replacing the 9V battery which powers the Battery Balance Monitor volt meter. I was going through them faster than my smoke detectors were "producing" them and the place I had them installed required a zip-tie each time I replaced the battery.
I decided to see if I could find an isolated DC-DC. I have some Tyco 1/2 brick DC-DC converters which would run from pack voltage but it seemed a bit overkill to use a 150W unit to power something which draws in the mA range of current. Not being an EE I didn't have much to go by as to where to look or what kinds of parts are available. I found that Jameco Electronics had a small isolated DC-DC converter which took 9-18V input and had 9V output. I ordered this, two 3.5mm terminal blocks, a small ABS case, a PCB which fit the case, and some stand-offs to mount it.
The DC-DC said it had filtering built in but I decided to install an inductor and capacitor on both the input side and the output side to smooth out the current. I didn't find out if I need an inductor in both the positive and negative leads. I only put the inductors in the positive leads. I used two small inductors out of a dead PC power supply. The fuse holder, 1/2A fuse and 22uF capacitors came from a dead digital mulit-meter. Here is a picture of the finished board.
I didn't realize that the 3.5mm terminal blocks were not spaced correctly for mounting in the holes of the board. As it turned out they mounted diagonally just fine which actually made it easier for the wires to make the bend to go out the hole in the bottom of the case. The block on the left is for input power and the right is for output. They are connected to match the order/placement of the markings on top of the DC-DC. On the back side I used leads cut off of LEDs used in other projects. They are plenty large for the minimal currents involved and they don't flop around with road vibrations. I only had to use one insulated wire which I decided to put on the top side of the board.
I mounted the box portion of the case to the underside of the dash in the Gizmo. There is a little fiberglass tab which was unused in my Gizmo. Looking at another Gizmo I see that this tab is where the fuse block used to be mounted. I put some soft wide weatherstripping on the underside of the box, drilled holes to match the holes in the fiberglass tab and then ran a large zip-tie through the holes to mount the box. I figured that mounting in this way would lessen the sharp jarring and vibration that Gizmos get on rough roads. I then mounted the PCB to the lid of the box and drilled a small hole in the lid for the wiring. This allows me to get to the unit by merely removing the lid to the box making installation/removal much easier.
So far, this has worked flawlessly.
My parts list:
I decided to see if I could find an isolated DC-DC. I have some Tyco 1/2 brick DC-DC converters which would run from pack voltage but it seemed a bit overkill to use a 150W unit to power something which draws in the mA range of current. Not being an EE I didn't have much to go by as to where to look or what kinds of parts are available. I found that Jameco Electronics had a small isolated DC-DC converter which took 9-18V input and had 9V output. I ordered this, two 3.5mm terminal blocks, a small ABS case, a PCB which fit the case, and some stand-offs to mount it.
The DC-DC said it had filtering built in but I decided to install an inductor and capacitor on both the input side and the output side to smooth out the current. I didn't find out if I need an inductor in both the positive and negative leads. I only put the inductors in the positive leads. I used two small inductors out of a dead PC power supply. The fuse holder, 1/2A fuse and 22uF capacitors came from a dead digital mulit-meter. Here is a picture of the finished board.
I mounted the box portion of the case to the underside of the dash in the Gizmo. There is a little fiberglass tab which was unused in my Gizmo. Looking at another Gizmo I see that this tab is where the fuse block used to be mounted. I put some soft wide weatherstripping on the underside of the box, drilled holes to match the holes in the fiberglass tab and then ran a large zip-tie through the holes to mount the box. I figured that mounting in this way would lessen the sharp jarring and vibration that Gizmos get on rough roads. I then mounted the PCB to the lid of the box and drilled a small hole in the lid for the wiring. This allows me to get to the unit by merely removing the lid to the box making installation/removal much easier.
So far, this has worked flawlessly.
My parts list:
- CONVERTER,DC-DC,5W,9V@0.556A 9-18Vin,REGULATED,ENCAP,FCC/CE Jameco PN:2107477
- CASE,ABS SPEEDY,3.125 x2 x.875 Jameco PN: 18922
- PROTOTYPE BUILDER,1.6 x2.7 Jameco PN: 105100
- MOUNTING HARDWARE KIT,CIRCUIT BOARD Jameco PN: 106551
- HEADER,3.5mm,TERM BLOCK,2 POS, TOP SCREW Jameco PN: 2094506
- Two 50V 22uF capacitors
- 250V Fast Acting 0.5A fuse and holders
- Two small inductors
Thursday, September 8, 2011
CycleAnalyst Control Board
It has been a while since I installed
the CycleAnalyst and I've received a couple of questions about how I
built the control board so I figured it was time I documented it. I
have gone through a couple of iterations so the one here is the final
version. There really isn't much difference between the two versions
except that the first one had a reed relay which would turn off the
CA's back light when the headlights were off where as this version
doesn't switch the back light on and off but instead has a 1Kohm 15
turn potentiometer used to adjust the back light brightness. The
other change is that I added a toggle switch to the side of the box
the CA is in so I could turn on the CA when the key was off.
[edit: I wanted to have the CA operate without a 12V system under normal charging circumstances which is why I went the route I did. The only time I need the 12V system to work is when driving and when using the manual over ride switch. If you don't need/want to operate the CA without the 12V system a simpler solution might be to hook the CA to the switched side of the main contactor and use a SPDT relay to switch on the CA when charging or with a manual over ride.]
The CA would always seem to record
either +0.1A or -0.1A when sitting idle which would increment or
decrement the amp-hours faster than reality so I decided to figure
out a way to have the CA only come on when the key was turned on or
when charging. Since the CA is a high resistance device it doesn't
draw much current. I also noticed that the relays in the Zivan
charger are rated at 1A at 30VDC and 0.3A at 110VDC which meant that
I could likely get by with a 10A 28VDC relay to switch the CA to a
pack voltage of 70VDC without any problem. I also figured that if the
relay welded shut there wasn't much that could go wrong other than
the CA not turning off. In any case, if you use a relay outside of
its ratings you do so at your own risk. I'm also blessed with a DC-DC
converter from Sure Power Industries which has a switched and
unswitched outputs. As far as I can tell it is not an isolated DC-DC
converter so both input and output grounds are shared with pack
negative. I hooked the coil of the relay to the switched 12V so that
when the key is turned on the CA is connected to the pack. So far
this has been working just as I want.
Since I don't charge with the key on I
needed a way to turn on the CA during charging so I could see the
pack SOC without having to fully charge to get to a known SOC. This
has been handy when I have had to go farther than the round trip
range of my rig. I could just charge until I knew I had enough to
make it back home. Fortunately the Zivan NG line of chargers has two
auxiliary relays, each with a NC and NO contacts. AUX1 closes the NO
contact when the equipment is switched on. AUX2 opens the NC contact
at the end of charge. All I did was run a wire from B+ to the NO
contact on AUX1, a jumper wire from the C contact on AUX1 to the NC
contact on AUX2 and then from the C contact on AUX2 back to the CA.
This will turn the CA on while charging and off when it is done.
 |
| Zivan NG1 relay connection |
I recently had my NG3 reprogrammed with
the LiFePO4 profile with the same 69.3V saturation voltage. I also
installed another 50A Anderson connector to hook this charger to for
those times I need a quick charge. The problem is that this charger
is usually mounted on my shop wall so doesn't have the wiring to
automatically turn on the CA so I installed the toggle switch to turn
on the CA at any time. Since I didn't want another high voltage
switch in my dash I merely wired in +12V from the always on side of
my DC-DC converter and ran this to the +12V input of my CA control
board. Since I didn't want anything back feeding to the rest of the
12V system I installed a diode between the key switched input and the
toggle switch. If you look by pins 1 and 2 in the picture below you
can see the diode. BTW, since the Zivan chargers are isolated I can charge with both at once for a 55A charge rate!
 |
| CA Control board |
Note that there are 16 wire locations.
I didn't really have to have so many but I wanted to be able to
disconnect the CA without having to desolder things or disconnect
things back at the source so I took the liberty to also make the CA
control unit a convenient disconnect location. Here are the pin
connections:
Vertical set
1 – +12V switched with key
2 – +12V manual over ride switch
3 – Speedometer input
4 – Ground from pack negative
5 – Shunt -
6 – Shunt +
7 – Out to Zivan NG1 relays from B+
8 – B+ Pack positive
Horizontal set
9 – CA Back-light
10 – CA Back-light
11 – Speedometer to CA
12 – Ground to CA
13 – Shunt - to CA
14 – Shunt + to CA
15 – In from Zivan relays
16 – B+ out to CA
Pin 1 is connected to pin 2 through the
diode. Pins 7 & 8 are connected to each other as are pins 15 &
16. Pins 3, 4, 5, and 6 are connected directly to pins 11, 12, 13,
and 14, respectively. The wiper and one end of the 1K pot are
connected to pins 9 & 10. Apparently the CA has a constant
current LED driver for the back light so it is perfectly safe to
completely short out the LEDs or connect a resistor in parallel with
them so it doesn't matter which side hooks to pin 9 and pin 10. I
found through experimenting that about 600 ohms of resistance across
the LEDs would dim them to the brightness I liked for night driving.
When they were full on it affected my night vision. [Grin Tech
(ebikes.ca) now make a CA which has optional red back lighting which
I would definitely use if I had the choice now.] If you look
carefully you can see where I drilled a small hole in the side of the
box so I can adjust the back-light brightness without opening up the
box.
 |
| Wired up |
This is what the box looks like wired
up. Note the large zip tie on the bundle of wires. This is to provide
strain relief on the connections in the box. It seems to work well.
Also, if you look at the lower left corner of the picture you can
just see a zip tie going through the corner of the box. There isn't
really any where to mount more things under my dash so I suspended
the control box from opposite corners using zip ties. The box doesn't
bounce much at all and doesn't pull on the wires.
The speedometer input comes directly
off the wire feeding my main digital speedometer. I did have to put a
22nF capacitor at the C6 position on the CA board. Note that I have
CA rev 2.2. When I would get to 48-50mph the speed would start
jumping around erratically and eventually just go to 0. Changing the
capacitance of the RC filter would filter out the controller noise.
When I was going over 50mph I could let off the throttle and the
speed would be rock solid. As soon as the controller was asked for
power the speed would go weird again.
Monday, July 25, 2011
Battery Pack Balance Monitor
It has been 19 months since I installed my LiFePO4 battery pack made up of 40 (originally 36) Thunder Sky TS-LFP100AHA cells in a 2p20s arrangement for a nominal pack voltage of 64V and an energy capacity of 12.8kWh. From January 2010 until the end of July 2010 I ran with 18 cell pairs and top balanced the pack at 4.00vpc. At this point I received my reprogrammed charger back from Elcon set for a saturation voltage of 69.3V which, until recently, I had bumped up to 69.7V. At this point I also installed the remaining 4 cells for a total of 20 cell pairs. I also did my final top balance at this point in time since I had noted that there was very little if any balancing going on and that it wasn't the same cells which hit top voltage first. I left the Black Sheep Technology BMS boards in place but they did not do any shunt balancing since my charge was ending at 3.485vpc and the boards don't shunt until 4.00V.
With the BMS boards, however, I had no way of knowing if any cell was going high or low relative to the others. When the BMS alarm would sound all I knew was that at least one cell was low. I wanted some way to narrow down if I had a weak cell or a bad connection. Lee Hart has posted on the EVDL Library a Batt-Bridge Battery Balance Alarm. This device used resistors selected for a particular pack voltage and a series of LEDs. I emailed Lee about some alternatives to his circuit. I wanted to have some sort of meter which would deflect to show me what was going on rather than wait for a large enough voltage swing to occur to light one of the LEDs. You can read about my dialog with Lee and some other discussion on the DIYElectriccar site. What I ended up doing is ordering a 9V battery powered volt meter from eBay seller clinia. He will setup the meter to what ever specs you want. Here is a picture from his listing of the meter:
With a battery balance circuit such as this the voltage reading will be exactly half of what the difference is between the two halves. Because of this I had the meter setup to display a reading double what the input voltage was. Now I can glance at the meter and see the actual voltage difference between the most negative half of the pack and the most positive half of the pack.
The circuit is constructed by connecting a resistor between the most negative post of the battery pack, to one end of a potentiometer, and the other end of the potentiometer to another identical resistor and then to the most positive end of the pack. A volt meter is then connected between the middle post of the battery pack and the wiper on the potentiometer. The potentiometer is used to calibrate the device so that when each half of the battery pack is exactly the same voltage the meter will read zero. One of the challenges with hooking wiring up to high voltage DC in a vehicle is the potential for a short which could cause all kinds of damage, including fire. The challenge is then to find small, low amperage, high voltage fuses. Radio Shack fuses won't cut it. I then searched for name brand fuses and holders but didn't know that I wanted to have to mount them on PC boards and such. Next is to determine where and how to mount them. I then inquired Lee about using a resistor as a safety device. Since I wanted this device to draw as little power as possible I used resistors with a very high resistance. This meant that any short would cause only a very low current and wouldn't burn something. With this in mind I went about coming up with a way to mount the fuses right at the terminals so that no matter where any wire might short the current would be very low and not cause any problems. Naturally I wanted to secure things where there would be no shorts but I wanted to be safe.
In playing around with Ohm's Law, V=IR, and trying a couple of fuses I had laying around and my DMM, I determined that it really doesn't matter what resistance I use as long as it is high enough. I ended up getting 100K Ohm, 1/2 Watt, 5% resistors. Since the max my pack voltage would ever go is 80V, if I were to top balance at 4.00vpc, this means that the maximum current would be I=V/R=80V/100KOhm=0.0008A=0.8mA. This is only 0.064W so well under the resistor's rating. This is if one wire shorted between the extreme most posts which is highly unlikely to happen! For the potentiometer I picked up a 1KOhm 15 turn pot. This means that between the negative post and the most positive post would be a total of 201KOhm of resistance and with a fully charged pack at 70V (I'm really having charging end at closer to 69.3V now) the current draw of the device is under 0.35mA. This is quite acceptable and means that even if I left for a year and didn't charge that the pack would only be drained about 3Ah. Furthermore, this is evenly across the full pack so there will be no imbalance caused by the device. Perfect!
These resistors take care of protecting two wires but what about the center tap? What if it shorts to something? I played around with different resistors to see what difference they made to voltage readings. They do make some difference so I decided to go with a 10KOhm resistor on the center tap wire. This only changed the reading by about 0.01V which is acceptable to me. The most voltage the center tap could ever see was 40V so the current would be at most 4mA and 0.16W, again well under the 0.5W rating of the resistor.
For mounting I bought some double clad PC board material and cut out 25mm squares. At the moment I'm not using the BMS boards so I can use the screw mounts for them. If you have read earlier posts on the BMS boards you know I am using brass bolts with a tapped hole in the center. I drilled an off center hole for the screw to go through and then used a dremel tool to grind away a gap in the copper on both sides of the board. I used a #57 Wire Gauge drill bit to drill three holes, two for the resistor and one for the wire to attach. Below are the three boards I built. The two at the top are the most positive/negative boards with one showing the back side. The one marked with an M is for the center tap.
The small unfilled hole is where I attached the wire to the board. Note that I soldered both sides of the board. I did this to provide as many path ways for conduction as I could to hopefully minimize the chance for a bad connection. I haven't done it but I probably should conformal coat the boards except for the solder pad for the screw. It was very easy to make the pad and draw the M. The copper is textured somewhat and is very ready for soldering.
The next thing to do was to decide how to connect everything. While I like the good connection achieved with soldering I didn't want such a permanent hookup since I would have to do the soldering on the vehicle or in the battery compartment. These Gizmo's don't have much extra room for things, especially after all the things I have added over the years. I decided to get a 4.5cm square PC Board from Radio Shack and mount the pot and some PCB terminals to it. The set of four terminals is for hooking up the battery connections (only 3 are used) and the double terminal is for hooking up the volt meter. This makes a simple and hopefully reliable connection point. Time will tell how well this will hold up.
Except for the insulated jumper wire I just used the leads I cut off of the resistors to make the connections. For the wire connections from the terminals to this board I used 24 gauge aircraft wiring I got from Aircraft Spruce. The wire is tinned and rated to 600V. Most importantly the insulation is very tough. I accidentally set some on a hot soldering iron and it barely made a mark on it! The wire is rated to 150°C which explains the minor damage from touching the soldering iron.
Next came the need to mount the volt meter. Since a Gizmo dash is curved it is very difficult to mount flat meters. I also don't want big holes in the dash, especially if I change something in the future. I decided to mount this meter the same way I mounted the CycleAnalyst, by carving the back of an ABS project box to match the contour of the dash and then silicon the unit to the dash. I first needed a pattern to go by so I used some air dry clay as I did before. I used a 4"x2"x1" Radio Shack project box wrapped by a strip from a cereal box and pressed the clay onto the dash and then let it dry.
Unfortunately the dry, sunny, warm days we were supposed to get turned into cool wet days so it took a while to dry. After a couple of days I finally carved out the center and then used a hair dryer to finish the drying so I could remove it from the dash. Here is the result:
I carved the edges so I could place this inside the project box. After cutting the back off the project box I put this inside and held the sides firmly while I used my Dremel tool to cut the project box to match the contour.
While the clay was drying I took the screws which hold the lid to the project box in place and screwed them into a piece of cardboard. I sprayed them with metal etching primer and then sprayed them with a couple coats of gloss black paint. This seems to work well and the paint doesn't chip when inserting the screws. I did have to cut the screws shorter since they would have hit the dash otherwise. On the thinnest corner of the box I reinforced the screw hole by gluing in a piece of the ABS box I cut out between the screw hole cylinder and the wall of the box. Regular ABS glue for black sewer pipe works great for this. Without the reinforcement the screw mount would tend to twist and deform the edge of the box.
To mount the display I placed the box cover face down on a piece of #2 plastic scrap, scored the opening I wanted with a small tipped Xacto knife and then went to work repeatedly running the knife down the groves until I cut through the front. Working from the back makes sure that any slips don't show through on the front. The hole was then filed with a standard 8" mill file until it was smooth and the correct size to hold the bezel. This part took me about an hour to do. Here is the finished result:
The ammeter really doesn't look like it is crooked. When sitting in the seat it all does look just fine. I have the meter hooked up so that a positive display represents how low the most positive part of the pack is from the most negative part and a negative reading is the opposite. I'm still trying to decide how I want to label the meter so that my wife and any one else who drives my Gizmo will know how to interpret the reading. As you can see the front (most positive) half of the pack is 0.02V lower than the back (most negative) half. When I did my last pack balance I had to do each half separately. I believe this is why there is a difference. However, under a >1.5CA load the difference is +0.13V and when regen current is over 50A I get a -0.00 reading so I may have a poor connection or weak cell pair in the front half of the pack.
Here is a parts list:
Now if I have a cell going bad I'll see the voltage difference show up right away unless, of course, I have a cell in each half of the pack doing the same thing at the same time.
In my next blog post I plan on relaying the results of not doing any pack balancing for the past 11 months.
Edit: I remembered to take some pictures the other day when I was checking pack balance. Below are different views of the batt-bridge installation. The pack really looks dirty in these pics!
With the BMS boards, however, I had no way of knowing if any cell was going high or low relative to the others. When the BMS alarm would sound all I knew was that at least one cell was low. I wanted some way to narrow down if I had a weak cell or a bad connection. Lee Hart has posted on the EVDL Library a Batt-Bridge Battery Balance Alarm. This device used resistors selected for a particular pack voltage and a series of LEDs. I emailed Lee about some alternatives to his circuit. I wanted to have some sort of meter which would deflect to show me what was going on rather than wait for a large enough voltage swing to occur to light one of the LEDs. You can read about my dialog with Lee and some other discussion on the DIYElectriccar site. What I ended up doing is ordering a 9V battery powered volt meter from eBay seller clinia. He will setup the meter to what ever specs you want. Here is a picture from his listing of the meter:
The circuit is constructed by connecting a resistor between the most negative post of the battery pack, to one end of a potentiometer, and the other end of the potentiometer to another identical resistor and then to the most positive end of the pack. A volt meter is then connected between the middle post of the battery pack and the wiper on the potentiometer. The potentiometer is used to calibrate the device so that when each half of the battery pack is exactly the same voltage the meter will read zero. One of the challenges with hooking wiring up to high voltage DC in a vehicle is the potential for a short which could cause all kinds of damage, including fire. The challenge is then to find small, low amperage, high voltage fuses. Radio Shack fuses won't cut it. I then searched for name brand fuses and holders but didn't know that I wanted to have to mount them on PC boards and such. Next is to determine where and how to mount them. I then inquired Lee about using a resistor as a safety device. Since I wanted this device to draw as little power as possible I used resistors with a very high resistance. This meant that any short would cause only a very low current and wouldn't burn something. With this in mind I went about coming up with a way to mount the fuses right at the terminals so that no matter where any wire might short the current would be very low and not cause any problems. Naturally I wanted to secure things where there would be no shorts but I wanted to be safe.
In playing around with Ohm's Law, V=IR, and trying a couple of fuses I had laying around and my DMM, I determined that it really doesn't matter what resistance I use as long as it is high enough. I ended up getting 100K Ohm, 1/2 Watt, 5% resistors. Since the max my pack voltage would ever go is 80V, if I were to top balance at 4.00vpc, this means that the maximum current would be I=V/R=80V/100KOhm=0.0008A=0.8mA. This is only 0.064W so well under the resistor's rating. This is if one wire shorted between the extreme most posts which is highly unlikely to happen! For the potentiometer I picked up a 1KOhm 15 turn pot. This means that between the negative post and the most positive post would be a total of 201KOhm of resistance and with a fully charged pack at 70V (I'm really having charging end at closer to 69.3V now) the current draw of the device is under 0.35mA. This is quite acceptable and means that even if I left for a year and didn't charge that the pack would only be drained about 3Ah. Furthermore, this is evenly across the full pack so there will be no imbalance caused by the device. Perfect!
These resistors take care of protecting two wires but what about the center tap? What if it shorts to something? I played around with different resistors to see what difference they made to voltage readings. They do make some difference so I decided to go with a 10KOhm resistor on the center tap wire. This only changed the reading by about 0.01V which is acceptable to me. The most voltage the center tap could ever see was 40V so the current would be at most 4mA and 0.16W, again well under the 0.5W rating of the resistor.
For mounting I bought some double clad PC board material and cut out 25mm squares. At the moment I'm not using the BMS boards so I can use the screw mounts for them. If you have read earlier posts on the BMS boards you know I am using brass bolts with a tapped hole in the center. I drilled an off center hole for the screw to go through and then used a dremel tool to grind away a gap in the copper on both sides of the board. I used a #57 Wire Gauge drill bit to drill three holes, two for the resistor and one for the wire to attach. Below are the three boards I built. The two at the top are the most positive/negative boards with one showing the back side. The one marked with an M is for the center tap.
 |
| Terminal connections |
The small unfilled hole is where I attached the wire to the board. Note that I soldered both sides of the board. I did this to provide as many path ways for conduction as I could to hopefully minimize the chance for a bad connection. I haven't done it but I probably should conformal coat the boards except for the solder pad for the screw. It was very easy to make the pad and draw the M. The copper is textured somewhat and is very ready for soldering.
The next thing to do was to decide how to connect everything. While I like the good connection achieved with soldering I didn't want such a permanent hookup since I would have to do the soldering on the vehicle or in the battery compartment. These Gizmo's don't have much extra room for things, especially after all the things I have added over the years. I decided to get a 4.5cm square PC Board from Radio Shack and mount the pot and some PCB terminals to it. The set of four terminals is for hooking up the battery connections (only 3 are used) and the double terminal is for hooking up the volt meter. This makes a simple and hopefully reliable connection point. Time will tell how well this will hold up.
 |
| Front view |
 | |
| Rear view |
Except for the insulated jumper wire I just used the leads I cut off of the resistors to make the connections. For the wire connections from the terminals to this board I used 24 gauge aircraft wiring I got from Aircraft Spruce. The wire is tinned and rated to 600V. Most importantly the insulation is very tough. I accidentally set some on a hot soldering iron and it barely made a mark on it! The wire is rated to 150°C which explains the minor damage from touching the soldering iron.
Next came the need to mount the volt meter. Since a Gizmo dash is curved it is very difficult to mount flat meters. I also don't want big holes in the dash, especially if I change something in the future. I decided to mount this meter the same way I mounted the CycleAnalyst, by carving the back of an ABS project box to match the contour of the dash and then silicon the unit to the dash. I first needed a pattern to go by so I used some air dry clay as I did before. I used a 4"x2"x1" Radio Shack project box wrapped by a strip from a cereal box and pressed the clay onto the dash and then let it dry.
While the clay was drying I took the screws which hold the lid to the project box in place and screwed them into a piece of cardboard. I sprayed them with metal etching primer and then sprayed them with a couple coats of gloss black paint. This seems to work well and the paint doesn't chip when inserting the screws. I did have to cut the screws shorter since they would have hit the dash otherwise. On the thinnest corner of the box I reinforced the screw hole by gluing in a piece of the ABS box I cut out between the screw hole cylinder and the wall of the box. Regular ABS glue for black sewer pipe works great for this. Without the reinforcement the screw mount would tend to twist and deform the edge of the box.
To mount the display I placed the box cover face down on a piece of #2 plastic scrap, scored the opening I wanted with a small tipped Xacto knife and then went to work repeatedly running the knife down the groves until I cut through the front. Working from the back makes sure that any slips don't show through on the front. The hole was then filed with a standard 8" mill file until it was smooth and the correct size to hold the bezel. This part took me about an hour to do. Here is the finished result:
Here is a parts list:
- Low Cost LCD Volt meter ($16.21)
- 4"x2"x1" Project Box ($3.19)
- 2-sided PC Board material ($4.19)
- 45mm square PC board ($2.19)
- 2 packages of 2-pin 5mm PCB Terminals ($2.39 each)
- 5 pack of 10KOhm resistors ($1.19)
- 5 pack of 100KOhm resistors ($1.19)
- 1KOhm 15-turn potentiometer ($3.19)
- #57 Wire Gauge Drill bit ($2.89)
- 24 gauge wire ~ 25 feet total (~$6.00)
- Air Dry clay ($???)
- 9-volt battery (from my smoke detector) [Note: Since I don't turn off the meter with the key the batteries were going dead too fast so I installed a 12V-9V isolated DC-DC. See the post at http://2003gizmo.blogspot.com/2011/10/battery-balance-9v-battery-replacement.html]
- miscellaneous zip ties (had on hand)
- a donation to Lee Hart for all his help. Have you benefited from his help? Send him a Thank you donation for the Sunrise EV2 project via paypal to leeahart_at_earthlink.net
Now if I have a cell going bad I'll see the voltage difference show up right away unless, of course, I have a cell in each half of the pack doing the same thing at the same time.
In my next blog post I plan on relaying the results of not doing any pack balancing for the past 11 months.
Edit: I remembered to take some pictures the other day when I was checking pack balance. Below are different views of the batt-bridge installation. The pack really looks dirty in these pics!
 |
| Main Board |
 |
| Main Board and Center tap |
 |
| Negative Terminal connection |
 |
| Positive Terminal connection |
Monday, January 17, 2011
A year with LiFePO4 batteries: What Have I Learned?
One year ago today I took my first drive in my Gizmo with 36 TS-LFP100AHA installed in a 2p18s configuration. See the blog posts in January and February 2010 for my setup. In that time I have learned quite a bit about LiFePO4 batteries. Playing with them, using them, measuring them, keeping an open mind about how they operate and avoiding the "conventional wisdom" about batteries are all good things to do. Being a physics type is also good but not necessary. It just means there is a chance that you learned that you record everything even if you don't think it is useful and especially if it doesn't support your current notions about things or something you are trying to prove.
In the past year the odometer went from 8640 miles and now sits at 14139 miles: one mile short of 5500 miles.
I also saw this nice pattern of numbers on the odometer! The aluminum over the speedometer was an attempt to shield sun from hitting the display. Sometimes I can't see how fast I'm going and don't want to get a ticket. I've since removed it since the CycleAnalyst has a speed function I can use for the infrequent times I need it.
I spent $46.80 on electricity to recharge and used 993.43kWh from the wall (representing 12,365.37Ah) to travel that distance. This comes out to a conservative average of 137Wh/mile from the battery pack. I'll post about current efficiency values in another post. I see a trend and think it might be tied to pack temperature so I'll post in the summer about it.
This is a CycleAnalyst from ebikes.ca showing the total cycle count, total Ah and total Miles driven. The miles is higher than my speedometer gives. I'm assuming that the CA is more accurate because I calculated it from 10 tire rotations but I wanted to keep a consistent count from before I installed the CA and after so I only record the speedometer values. Also, the difference in the CA total of 12,544Ah and my total of 12,365Ah is due to the fact that the CA records total Ah that the batteries deliver whereas my figure is from adding up all the individual trip Ah which include regenerative braking. From this you can calculate that regenerative braking gained me 1.4% range over this time period. [edit: I just realized that 7597Ah came from back calculating for the time I did not have the CA and are based on the kWh used to charge from the wall. A more accurate representative result would be a 3.7% gain from regen.] Not a whole lot, however I have gone over three times farther on this set of brake pads than I did before I had variable regenerative braking so it is definitely a benefit. Not to mention the ability to come down steep long hills and not have brake fade and the much quicker panic stops than without.
The Cycle Count of 290 represents the total number of times I have charged the battery pack until it was full and I reset the CA. The Ah value is not always exactly at zero after a charge. It ranges from a fraction of an Ah above or below to as much as 2.5Ah above or below depending on how long the drive was and how much the battery temperature changed. I think it is also dependent some what on how closely calibrated it is to the shunt. With my 500A shunt the CA only claims accuracy to 0.1A so at the end of charge when the Zivan ramps down to near zero amps it may or may not register on the CA. The 290 cycles represent an average (mean) discharge per cycle of ~21% with a median of 13.6%. The highest discharge amount was 89.9% (179.6Ah) and the smallest was 4.34% (8.68Ah). At this rate I hope the pack will last 10 years. Even then I will probably have spent more than if I had stuck with 6V flooded Golf Cart batteries. However, the utility of the vehicle is so much better that it is hard to put a price on it. Not to mention I am getting valuable information on a $5000 pack rather than on a $20,000 pack in a full size car. (I need lots of range for what I do with my car so I would need a much larger pack even though a $6500 pack would work well in a small car.)
One thing is for sure, when the range is more than 20 miles it is much easier to drive more. There were several times where the 20 mile range of lead acid were not enough for what I needed. I remember heading out to one of my rental houses to show it to someone and getting most of the way there and realizing I had forgot the key. I had to turn around and go home to get it. Unfortunately I didn't have enough range to make the trip a second time so I had to take an ICE vehicle. With LiFePO4 and an easy 60-70 mile range meant those situations are no longer an issue. I've driven from Kelso to Battle Ground, WA to work on a rental house knowing I would be there long enough to charge enough to make the return trip.
If you read my February 2010 blog and the two comments you will see there was talk about my original ending charge of 71V or about 3.94vpc. Jack Rickard correctly pointed out that charging this high would shorten the life of my cells. On July 28, 2010 at 11,365 miles on the odometer I installed my newly reprogrammed Zivan NG1 with the #612 charging algorithm for 19 LiFePO4 cells. I did adjust the voltage trim pot up so now the ending charge is 69.6-69.7V. At the same time I installed the remaining two pairs of cells. I top balanced all the cells using my BMS boards which shunt at 4.00V. This means my pack voltage was at 80V when I was finished. My DC-DC won't come on at this voltage and neither will my Sevcon PP745 controller. I used a bank of light bulbs I built as a load to discharge batteries and brought the voltage down to below 70V. This took very little time since there is very little energy above 3.45V with this particular chemistry.
One might be wondering about the top balancing if my BMS boards don't shunt until 4.00V. After all, 69.7V/20 cells = 3.485vpc. Well, since there is so little energy above 3.45vpc (with my 18 "cell" 200Ah pack it was only enough to get me up to about 15mph from a stop) there is really nothing lost by stopping earlier in the charging curve. The Zivan is incredibly consistent in stopping at its programmed cutoff voltage. It even will shut down if for some reason it can't control the voltage. I found out quite by accident on the first pack charge I did. Being naive about the characteristics of these batteries I went to the store thinking that since the voltage hadn't risen much it would still be a while before they were full. When I got back the charger was beeping an error code which turned out to be "unable to control voltage" or some thing like that. Zivan chargers may not be too great to end user adjustment but I sure am thankful that they just plain do what they are supposed to. They are truly plug-n-play if you don't make any changes to your battery pack. Another point about the small amount of energy at the top of the voltage curve. I've watched the voltage at the end of charge. From the time the charger first starts to ramp back on the current it is only 10-15 minutes before it is putting less than 500mA. This last phase (yellow light) of the charging curve lasts for 1 hour. Even if it continuously put in 500mA (it doesn't) that would only be 0.5Ah into a 200Ah pack. It comes on periodically and draws only 40W from the wall and that drops down rather quickly to under 20W and then shuts off. I'm sure if I didn't have any parasitic loads on the battery pack that it would shut off and not come on again during the remainder of the final charging curve.
Back to top balancing: I was wondering too what would happen. If these batteries drift I should start seeing it after a few charge cycles. I started recording the voltage at the end of charge when the charger was not charging. This was during the yellow light phase of charging. I could see the wall power on my kill-a-watt meter so I knew when the charger was not putting anything into the pack. When not running the charger draws about 4W. I can also hear when the charger shuts off and turns back on because I hear a high pitched squeak which is probably the PWM of the charger coming on or turning off. Other than the first time I took cell readings the charger was off. Below is a screen capture of the spreadsheet I'm using to record the information.
The first column lists the cell number and location in the pack. Cell pairs 19 and 20 were installed after the charger was reprogrammed. They only had about 3 cycles on them before being merged into the pack. I did have a mishap with pair #19 a day or two after installing them. I missed tightening one of the bolts which held the straps between this pair and pair #20. It had worked its way up about a centimeter or so. Amazingly, there was no melted metal but one cell of the pair took several Ah to bring it up to the rest. What I think was happening was that the high currents were draining the cell but the charge currents were not getting into it very well. It is possible that since I am using brass bolts rather than the stainless steel ones supplied by TS I avoided problems. I tightened this bolt down and charged this one pair with my bench top power supply. You will see that it is one of the higher voltage pairs, though not the highest, of the set. The next columns are a date followed by High/Low. the High/Low column shows an H for the highest cell and an L for the lowest cell for that day's reading. At the bottom of this column is the difference between these two values and represents the maximum voltage range of the pack. The cell immediately below the individual voltages is the sum of the voltages, below that is the total pack voltage as given my my ExTech Instruments EX830 multimeter. This is an awesome mid-range meter, BTW. Note that the ending voltage for the first reading is 70V. I lowered the ending voltage of my charger after this to keep my DC-DC happier. The next cell in the spreadsheet is the Watt reading on the Kill-a-Watt meter, below that is the pack temperature, in degrees Celsius, by an inexpensive indoor-outdoor digital thermometer. I placed the outdoor probe between the middle rows of batteries against the case. It is held in place by a folded up piece of closed cell foam to try to get a more accurate case temperature. The temperature was generally stable by the time the charging was finished so this value likely is very close to the internal temperature of the cells. The whole box is surrounded by 3/4" hard pink insulation foam as sold by home hardware stores. The box is by no means air tight but generally there is not much wind in my carport. Finally, the last cell is the average Volts per cell value.
As you can see, the voltages don't move around very much. The high and low cells don't change much. (Note that the dates are farther apart later in the table.) As Jack Rickard and others told me I'd get bored taking these measurements and eventually quit. Well, I like data and I'm out to test out a hypothesis that has been forming in my mind for a while. I'm no longer convinced I need a cell level BMS. I think a half pack voltage comparison is enough to spot a dying cell and take care of it. Before you tune me out read on.
After watching Jay Whitacare's talk to a group of Carnegie Mellon students and professors about their EV experiments I became less convinced that the risks of a cell level BMS were lower than a string level monitoring system. See the video linked to on the ChargeCar website or on the EVTV.ME blog. When I read "Shuttle reaction" in the comments to Jack Rickard's January 9, 2011 blog it finally hit me why these batteries are so different from other chemistries. I have since read about it in other places. It was hinted at in one of the responses to an EVDL post I made asking why my efficiency from the wall numbers were so much better with LiFePO4 batteries than with lead acid. A poster mentioned a secondary reaction in lead acid that didn't participate in the storage of energy but used energy in the reaction. I believe this was the shuttle reaction which provides the self discharge of so many other batteries. Lithium ion batteries in general and LiFePO4 in particular merely move a Lithium ion from one plate to the other during charge and back on discharge. There really isn't a chemical reaction, per say, taking place to make this happen. Barring a mechanical defect in the cell there is no way for self discharge to happen! When people put BMS boards on each cell, this then becomes the discharge mechanism for the cell, not the cell it self. If you have any LiFePO4 cells in storage make sure there is nothing on the terminals. If you want just check the voltage every 6 months or so. Unless there is something on the cell allowing charge to transfer between the terminals you will be pleasantly surprised at or bored with the fact that the voltage doesn't change much if at all, even out to the thousandth of a volt.
Another source for this information is on Gold Peak Industries (Taiwan), Ltd. website. In section 3.8 of the linked pdf it states,
Another sentence in the above quote caught my eye, "Initially the cell suffers from irreversible capacity loss." This may explain why my efficiency numbers were so high right after I installed my pack. Last winter wasn't as cold as this winter but it may be that as the capacity dropped in my cells it took a little more energy to put back what I used. Another, more likely, possibility is that each time I drove I didn't put back as much energy as I took out. I didn't have an amp hour counter at the time so I have no way to support either possibility. This may cause you to wonder how I came up with my total Ah numbers for my total. What I did was average the number of Ah/kWh out of the wall for the first dozen or so charge cycles after receiving the CA and then calculated the Ah used for all previous drives. The total I arrived at, number of cycles and miles driven were then entered into the CA to give reasonably accurate lifetime pack values.
So, what about my existing BMS boards? As you can tell I haven't been using the top balancing function of them. I think Black Sheep Technology did a very good job of designing a robust board so I don't expect any failures as some people have experienced with other products. These batteries do sag significantly when cold, so that a 1.5C discharge rate was enough to get the low voltage trip, LVT, to sound my low voltage alarm even though the pack was fully charged and had less than 1Ah removed. The total pack voltage was in the range of 58V so the 2.93 LVT value leads me to believe that maybe all boards were sending the alarm and not just a few. This isn't a big issue except that I'm about to install a switch to turn off the siren I installed so I'm not bothered by it when the pack is cold. If I forget it is off I may reverse a cell without realizing it. Or, I might get complacent and merely ignore the warning as a false positive and still kill a cell. I need to either remove the boards or at least the siren and install another type of monitoring system or leave the boards and install another type of monitoring system.
I'm planning on keeping the boards and installing another monitoring system. Why would I want to do this? Well, I could be asking for trouble but I don't expect my particular BMS boards to short out and drain a cell. They could quit working and not alert me to an over voltage or under voltage condition but I don't expect that either. The most likely failure would be the three wire connecting jumper wires running from board to board. If this happens they would fail to send the appropriate signal to alert me. I'm planning on buying or building and installing a Lee Heart type Batt-Bridge circuit. The difference is that I will either install a meter with zero at the center and have the needle swing to one side or the other if the two haves of the pack don't have the same voltage or I'll have a bar of LEDs where the middle one is lit and the ones to one side or the other will light up to indicate pack imbalance. The benefit of this type of circuit is that I can see what is going on under different load conditions. It is possible that under little or no load that the pack is balanced but that under load a weak cell will drop more than the others. I can then look for this cell knowing it is in one half or the other of the pack. It would also be possible to hook up a circuit to this which would turn off the charger on a pack imbalance condition during charging in case a bad cell wasn't caught before hand.
By having both systems in place, the cell level BMS and the Batt-Bridge type circuit, I can see what system gives me the information I need first. If the Batt-Bridge doesn't warn me before the BMS then it may not be the best method. If, the BMS doesn't warn me before the Batt-Bridge then it may not be the best method. If they warn me at the same time then the Batt-Bridge is the winner because it has fewer connections to the pack, only three wires, and it cost significantly less than the BMS system. Furthermore, it has fewer points of failure. It won't drain a single cell in the case of a short and it won't heat up from shunting current and potentially lead to a fire. While I hope I don't need either one this will hopefully provide one more data point to either support or deny the "conventional wisdom."
I hope this helps someone who is trying to decide how to manage their pack of LiFePO4 batteries in their EV. Moving to Lithium is a huge step up from using lead acid. It has very different characteristics than lead acid and what you learned about lead acid or other chemistries in the past needs to be either forgotten in its entirety and relearned for LiFePO4 or you need to re-check all previous notions against data for this exciting battery technology. If someone tells you that you have to do such and such or use this or that just ask, "how do you know that? What data do you have to support that?" You will be glad you did.
[EDIT: 01-19-2011 Be sure to read the comments. Also, if you didn't go read Jack Rickard's January 9, 2011 blog be sure you do and read comment #85 posted January 19, 2011]
In the past year the odometer went from 8640 miles and now sits at 14139 miles: one mile short of 5500 miles.
I also saw this nice pattern of numbers on the odometer! The aluminum over the speedometer was an attempt to shield sun from hitting the display. Sometimes I can't see how fast I'm going and don't want to get a ticket. I've since removed it since the CycleAnalyst has a speed function I can use for the infrequent times I need it.I spent $46.80 on electricity to recharge and used 993.43kWh from the wall (representing 12,365.37Ah) to travel that distance. This comes out to a conservative average of 137Wh/mile from the battery pack. I'll post about current efficiency values in another post. I see a trend and think it might be tied to pack temperature so I'll post in the summer about it.
This is a CycleAnalyst from ebikes.ca showing the total cycle count, total Ah and total Miles driven. The miles is higher than my speedometer gives. I'm assuming that the CA is more accurate because I calculated it from 10 tire rotations but I wanted to keep a consistent count from before I installed the CA and after so I only record the speedometer values. Also, the difference in the CA total of 12,544Ah and my total of 12,365Ah is due to the fact that the CA records total Ah that the batteries deliver whereas my figure is from adding up all the individual trip Ah which include regenerative braking. From this you can calculate that regenerative braking gained me 1.4% range over this time period. [edit: I just realized that 7597Ah came from back calculating for the time I did not have the CA and are based on the kWh used to charge from the wall. A more accurate representative result would be a 3.7% gain from regen.] Not a whole lot, however I have gone over three times farther on this set of brake pads than I did before I had variable regenerative braking so it is definitely a benefit. Not to mention the ability to come down steep long hills and not have brake fade and the much quicker panic stops than without.The Cycle Count of 290 represents the total number of times I have charged the battery pack until it was full and I reset the CA. The Ah value is not always exactly at zero after a charge. It ranges from a fraction of an Ah above or below to as much as 2.5Ah above or below depending on how long the drive was and how much the battery temperature changed. I think it is also dependent some what on how closely calibrated it is to the shunt. With my 500A shunt the CA only claims accuracy to 0.1A so at the end of charge when the Zivan ramps down to near zero amps it may or may not register on the CA. The 290 cycles represent an average (mean) discharge per cycle of ~21% with a median of 13.6%. The highest discharge amount was 89.9% (179.6Ah) and the smallest was 4.34% (8.68Ah). At this rate I hope the pack will last 10 years. Even then I will probably have spent more than if I had stuck with 6V flooded Golf Cart batteries. However, the utility of the vehicle is so much better that it is hard to put a price on it. Not to mention I am getting valuable information on a $5000 pack rather than on a $20,000 pack in a full size car. (I need lots of range for what I do with my car so I would need a much larger pack even though a $6500 pack would work well in a small car.)
One thing is for sure, when the range is more than 20 miles it is much easier to drive more. There were several times where the 20 mile range of lead acid were not enough for what I needed. I remember heading out to one of my rental houses to show it to someone and getting most of the way there and realizing I had forgot the key. I had to turn around and go home to get it. Unfortunately I didn't have enough range to make the trip a second time so I had to take an ICE vehicle. With LiFePO4 and an easy 60-70 mile range meant those situations are no longer an issue. I've driven from Kelso to Battle Ground, WA to work on a rental house knowing I would be there long enough to charge enough to make the return trip.
If you read my February 2010 blog and the two comments you will see there was talk about my original ending charge of 71V or about 3.94vpc. Jack Rickard correctly pointed out that charging this high would shorten the life of my cells. On July 28, 2010 at 11,365 miles on the odometer I installed my newly reprogrammed Zivan NG1 with the #612 charging algorithm for 19 LiFePO4 cells. I did adjust the voltage trim pot up so now the ending charge is 69.6-69.7V. At the same time I installed the remaining two pairs of cells. I top balanced all the cells using my BMS boards which shunt at 4.00V. This means my pack voltage was at 80V when I was finished. My DC-DC won't come on at this voltage and neither will my Sevcon PP745 controller. I used a bank of light bulbs I built as a load to discharge batteries and brought the voltage down to below 70V. This took very little time since there is very little energy above 3.45V with this particular chemistry.
One might be wondering about the top balancing if my BMS boards don't shunt until 4.00V. After all, 69.7V/20 cells = 3.485vpc. Well, since there is so little energy above 3.45vpc (with my 18 "cell" 200Ah pack it was only enough to get me up to about 15mph from a stop) there is really nothing lost by stopping earlier in the charging curve. The Zivan is incredibly consistent in stopping at its programmed cutoff voltage. It even will shut down if for some reason it can't control the voltage. I found out quite by accident on the first pack charge I did. Being naive about the characteristics of these batteries I went to the store thinking that since the voltage hadn't risen much it would still be a while before they were full. When I got back the charger was beeping an error code which turned out to be "unable to control voltage" or some thing like that. Zivan chargers may not be too great to end user adjustment but I sure am thankful that they just plain do what they are supposed to. They are truly plug-n-play if you don't make any changes to your battery pack. Another point about the small amount of energy at the top of the voltage curve. I've watched the voltage at the end of charge. From the time the charger first starts to ramp back on the current it is only 10-15 minutes before it is putting less than 500mA. This last phase (yellow light) of the charging curve lasts for 1 hour. Even if it continuously put in 500mA (it doesn't) that would only be 0.5Ah into a 200Ah pack. It comes on periodically and draws only 40W from the wall and that drops down rather quickly to under 20W and then shuts off. I'm sure if I didn't have any parasitic loads on the battery pack that it would shut off and not come on again during the remainder of the final charging curve.
Back to top balancing: I was wondering too what would happen. If these batteries drift I should start seeing it after a few charge cycles. I started recording the voltage at the end of charge when the charger was not charging. This was during the yellow light phase of charging. I could see the wall power on my kill-a-watt meter so I knew when the charger was not putting anything into the pack. When not running the charger draws about 4W. I can also hear when the charger shuts off and turns back on because I hear a high pitched squeak which is probably the PWM of the charger coming on or turning off. Other than the first time I took cell readings the charger was off. Below is a screen capture of the spreadsheet I'm using to record the information.
The first column lists the cell number and location in the pack. Cell pairs 19 and 20 were installed after the charger was reprogrammed. They only had about 3 cycles on them before being merged into the pack. I did have a mishap with pair #19 a day or two after installing them. I missed tightening one of the bolts which held the straps between this pair and pair #20. It had worked its way up about a centimeter or so. Amazingly, there was no melted metal but one cell of the pair took several Ah to bring it up to the rest. What I think was happening was that the high currents were draining the cell but the charge currents were not getting into it very well. It is possible that since I am using brass bolts rather than the stainless steel ones supplied by TS I avoided problems. I tightened this bolt down and charged this one pair with my bench top power supply. You will see that it is one of the higher voltage pairs, though not the highest, of the set. The next columns are a date followed by High/Low. the High/Low column shows an H for the highest cell and an L for the lowest cell for that day's reading. At the bottom of this column is the difference between these two values and represents the maximum voltage range of the pack. The cell immediately below the individual voltages is the sum of the voltages, below that is the total pack voltage as given my my ExTech Instruments EX830 multimeter. This is an awesome mid-range meter, BTW. Note that the ending voltage for the first reading is 70V. I lowered the ending voltage of my charger after this to keep my DC-DC happier. The next cell in the spreadsheet is the Watt reading on the Kill-a-Watt meter, below that is the pack temperature, in degrees Celsius, by an inexpensive indoor-outdoor digital thermometer. I placed the outdoor probe between the middle rows of batteries against the case. It is held in place by a folded up piece of closed cell foam to try to get a more accurate case temperature. The temperature was generally stable by the time the charging was finished so this value likely is very close to the internal temperature of the cells. The whole box is surrounded by 3/4" hard pink insulation foam as sold by home hardware stores. The box is by no means air tight but generally there is not much wind in my carport. Finally, the last cell is the average Volts per cell value.As you can see, the voltages don't move around very much. The high and low cells don't change much. (Note that the dates are farther apart later in the table.) As Jack Rickard and others told me I'd get bored taking these measurements and eventually quit. Well, I like data and I'm out to test out a hypothesis that has been forming in my mind for a while. I'm no longer convinced I need a cell level BMS. I think a half pack voltage comparison is enough to spot a dying cell and take care of it. Before you tune me out read on.
After watching Jay Whitacare's talk to a group of Carnegie Mellon students and professors about their EV experiments I became less convinced that the risks of a cell level BMS were lower than a string level monitoring system. See the video linked to on the ChargeCar website or on the EVTV.ME blog. When I read "Shuttle reaction" in the comments to Jack Rickard's January 9, 2011 blog it finally hit me why these batteries are so different from other chemistries. I have since read about it in other places. It was hinted at in one of the responses to an EVDL post I made asking why my efficiency from the wall numbers were so much better with LiFePO4 batteries than with lead acid. A poster mentioned a secondary reaction in lead acid that didn't participate in the storage of energy but used energy in the reaction. I believe this was the shuttle reaction which provides the self discharge of so many other batteries. Lithium ion batteries in general and LiFePO4 in particular merely move a Lithium ion from one plate to the other during charge and back on discharge. There really isn't a chemical reaction, per say, taking place to make this happen. Barring a mechanical defect in the cell there is no way for self discharge to happen! When people put BMS boards on each cell, this then becomes the discharge mechanism for the cell, not the cell it self. If you have any LiFePO4 cells in storage make sure there is nothing on the terminals. If you want just check the voltage every 6 months or so. Unless there is something on the cell allowing charge to transfer between the terminals you will be pleasantly surprised at or bored with the fact that the voltage doesn't change much if at all, even out to the thousandth of a volt.
Another source for this information is on Gold Peak Industries (Taiwan), Ltd. website. In section 3.8 of the linked pdf it states,
“There is no shuttle-based self-discharge reaction in the Lithium Ion cell like that found in the NiMH and NiCd. As the cell ages, the self-discharge eventually becomes zero. Initially the cell suffers from irreversible capacity loss. This is a reaction of the electrolyte with the the active components if the cell. It occurs more rapidly with increasing temperature and cell voltage. For this reason, cells should not be stored fully charged at temperatures approaching 60°C. Optimally they should be stored at 25°C or less and between 30-50% state of charge. The lower limit is chosen because they are often stored in packs witch circuitry that demands a small drain on the battery. When one considers the circuitry needed for li-Ion, it becomes the most important source of self-discharge.”Note that it states that the lower limit, 30%SOC, is chosen because so many put BMS circuits on them. Leave them off means almost zero self discharge. Furthermore, as the cell ages, what ever self-discharge there may be drops to zero any way. It sounds like the cells will become less and less prone to drift apart in any particular given pack.
Another sentence in the above quote caught my eye, "Initially the cell suffers from irreversible capacity loss." This may explain why my efficiency numbers were so high right after I installed my pack. Last winter wasn't as cold as this winter but it may be that as the capacity dropped in my cells it took a little more energy to put back what I used. Another, more likely, possibility is that each time I drove I didn't put back as much energy as I took out. I didn't have an amp hour counter at the time so I have no way to support either possibility. This may cause you to wonder how I came up with my total Ah numbers for my total. What I did was average the number of Ah/kWh out of the wall for the first dozen or so charge cycles after receiving the CA and then calculated the Ah used for all previous drives. The total I arrived at, number of cycles and miles driven were then entered into the CA to give reasonably accurate lifetime pack values.
So, what about my existing BMS boards? As you can tell I haven't been using the top balancing function of them. I think Black Sheep Technology did a very good job of designing a robust board so I don't expect any failures as some people have experienced with other products. These batteries do sag significantly when cold, so that a 1.5C discharge rate was enough to get the low voltage trip, LVT, to sound my low voltage alarm even though the pack was fully charged and had less than 1Ah removed. The total pack voltage was in the range of 58V so the 2.93 LVT value leads me to believe that maybe all boards were sending the alarm and not just a few. This isn't a big issue except that I'm about to install a switch to turn off the siren I installed so I'm not bothered by it when the pack is cold. If I forget it is off I may reverse a cell without realizing it. Or, I might get complacent and merely ignore the warning as a false positive and still kill a cell. I need to either remove the boards or at least the siren and install another type of monitoring system or leave the boards and install another type of monitoring system.
I'm planning on keeping the boards and installing another monitoring system. Why would I want to do this? Well, I could be asking for trouble but I don't expect my particular BMS boards to short out and drain a cell. They could quit working and not alert me to an over voltage or under voltage condition but I don't expect that either. The most likely failure would be the three wire connecting jumper wires running from board to board. If this happens they would fail to send the appropriate signal to alert me. I'm planning on buying or building and installing a Lee Heart type Batt-Bridge circuit. The difference is that I will either install a meter with zero at the center and have the needle swing to one side or the other if the two haves of the pack don't have the same voltage or I'll have a bar of LEDs where the middle one is lit and the ones to one side or the other will light up to indicate pack imbalance. The benefit of this type of circuit is that I can see what is going on under different load conditions. It is possible that under little or no load that the pack is balanced but that under load a weak cell will drop more than the others. I can then look for this cell knowing it is in one half or the other of the pack. It would also be possible to hook up a circuit to this which would turn off the charger on a pack imbalance condition during charging in case a bad cell wasn't caught before hand.
By having both systems in place, the cell level BMS and the Batt-Bridge type circuit, I can see what system gives me the information I need first. If the Batt-Bridge doesn't warn me before the BMS then it may not be the best method. If, the BMS doesn't warn me before the Batt-Bridge then it may not be the best method. If they warn me at the same time then the Batt-Bridge is the winner because it has fewer connections to the pack, only three wires, and it cost significantly less than the BMS system. Furthermore, it has fewer points of failure. It won't drain a single cell in the case of a short and it won't heat up from shunting current and potentially lead to a fire. While I hope I don't need either one this will hopefully provide one more data point to either support or deny the "conventional wisdom."
I hope this helps someone who is trying to decide how to manage their pack of LiFePO4 batteries in their EV. Moving to Lithium is a huge step up from using lead acid. It has very different characteristics than lead acid and what you learned about lead acid or other chemistries in the past needs to be either forgotten in its entirety and relearned for LiFePO4 or you need to re-check all previous notions against data for this exciting battery technology. If someone tells you that you have to do such and such or use this or that just ask, "how do you know that? What data do you have to support that?" You will be glad you did.
[EDIT: 01-19-2011 Be sure to read the comments. Also, if you didn't go read Jack Rickard's January 9, 2011 blog be sure you do and read comment #85 posted January 19, 2011]
Sunday, January 16, 2011
Rear Wheel Bearing Modification
I have two Gizmos and both have the same issue with rear wheel bearings. I assume that any Gizmo with a belt drive would have the same issue. I believe that due to the tension required in the belt, the weight on the rear wheel, loads due to acceleration and regen, road shock, and the narrow R12 bearing outer surface the aluminum cannot handle the pressure. I believe that it actually cold flows slightly allowing the bearing to start to move and continually rounds out the hub. On both Gizmo hubs the inner seal was warn down to the metal support due to the bearing slopping around as the wheel turned. It also caused a lot of noise.
As a proof of concept I took a pop can and cut a strip of aluminum just long enough and wide enough to slip into the groove made by the bearing. This worked just fine for about 50 miles and showed me that the noise was caused by the bearing movement. (Ok, for you physics types like me it was actually hub movement on the bearing but relative movement none the less.) Since this worked I decided to use a harder material so I bought some brass sheet and repeated the process. On the second try I finally got the piece made to fit and everything was fine for a while. Then one day I noticed that the outer seal was working out of the hub. Later it was even further out. Finally I took the wheel off and the seal just pulled out easily. What I found was that the brass sheet was working its way out from around the bearing and pushing out the seal. Time for a different approach.
Since the inner, belt side, wheel bearing also wears out quite fast I thought I would like to install a tapered roller bearing on each side and gain an adjustable bearing, more bearing surface, a wider outer race surface, and longer bearing life at the expense of a little more friction. The problem I ran into was that I couldn't find a bearing with the dimensions I needed. The aluminum hub would have to be machined out and the aluminum may not have enough strength to keep the outer race compressed. I confirmed this with a couple of knowledgeable bearing people. I then went to Waite Specialty Machine Inc. in Longview, WA and talked to Keith Warring. Keith is an awesome guy and knows his stuff. I took in one of the hubs to show him what the issue was. I explained what I had tried and what I had found about bearing options. He suggested that a steel sleeve be inserted and that two bearings be installed on the belt side. They regularly sleeve motors and other applications where the bearing seat has rounded out too large to properly hold the bearing. For these types of applications they machine the hole just slightly smaller than the sleeve then they heat the outer piece and freeze the sleeve and insert them. When the two are at the same temperature they create an interference fit where the sleeve won't move. They also cut and faced the spacer that goes between the bearings and installed the bearings so the hub was ready to go.
Note the extra piece between the bearing outer race and the hub. The picture below shows a different angle where you can see the sleeve.
The outer bearing where the castle nut goes did not show any signs of wear. In fact, the inner wheel bearing on all three wheels seems to be the first one to go. I believe this could be due to the fact that the shaft flexes a fair amount so those bearings don't get the same road shock.
The cost of all of this was a little over $500 for two hubs. As with much machining the setup cost is quite high so doing two hubs at once was much cheaper than doing one at a time.
The other issue I've had with Gizmo bearings is that the inner seal seat, the collar on the shaft, had rusted and then wears out the seal rapidly. I was able to sand down the collar, the metal is quite soft, and install a speedy sleeve which is a stainless steel sleeve that goes over the seal race to gives a good surface for the seal to ride on. These are not cheap! One cost me over $40 but I'm sure it was less than replacing the axle or the entire rear swing arm. I'm actually suspicious that the collar that the seal rides on were damaged first by a bearing failure which would lead to premature seal failure and then dirt and water damage. Inspect those bearings and seals regularly.
As a proof of concept I took a pop can and cut a strip of aluminum just long enough and wide enough to slip into the groove made by the bearing. This worked just fine for about 50 miles and showed me that the noise was caused by the bearing movement. (Ok, for you physics types like me it was actually hub movement on the bearing but relative movement none the less.) Since this worked I decided to use a harder material so I bought some brass sheet and repeated the process. On the second try I finally got the piece made to fit and everything was fine for a while. Then one day I noticed that the outer seal was working out of the hub. Later it was even further out. Finally I took the wheel off and the seal just pulled out easily. What I found was that the brass sheet was working its way out from around the bearing and pushing out the seal. Time for a different approach.
Since the inner, belt side, wheel bearing also wears out quite fast I thought I would like to install a tapered roller bearing on each side and gain an adjustable bearing, more bearing surface, a wider outer race surface, and longer bearing life at the expense of a little more friction. The problem I ran into was that I couldn't find a bearing with the dimensions I needed. The aluminum hub would have to be machined out and the aluminum may not have enough strength to keep the outer race compressed. I confirmed this with a couple of knowledgeable bearing people. I then went to Waite Specialty Machine Inc. in Longview, WA and talked to Keith Warring. Keith is an awesome guy and knows his stuff. I took in one of the hubs to show him what the issue was. I explained what I had tried and what I had found about bearing options. He suggested that a steel sleeve be inserted and that two bearings be installed on the belt side. They regularly sleeve motors and other applications where the bearing seat has rounded out too large to properly hold the bearing. For these types of applications they machine the hole just slightly smaller than the sleeve then they heat the outer piece and freeze the sleeve and insert them. When the two are at the same temperature they create an interference fit where the sleeve won't move. They also cut and faced the spacer that goes between the bearings and installed the bearings so the hub was ready to go.
Note the extra piece between the bearing outer race and the hub. The picture below shows a different angle where you can see the sleeve.
The outer bearing where the castle nut goes did not show any signs of wear. In fact, the inner wheel bearing on all three wheels seems to be the first one to go. I believe this could be due to the fact that the shaft flexes a fair amount so those bearings don't get the same road shock.
The cost of all of this was a little over $500 for two hubs. As with much machining the setup cost is quite high so doing two hubs at once was much cheaper than doing one at a time.
The other issue I've had with Gizmo bearings is that the inner seal seat, the collar on the shaft, had rusted and then wears out the seal rapidly. I was able to sand down the collar, the metal is quite soft, and install a speedy sleeve which is a stainless steel sleeve that goes over the seal race to gives a good surface for the seal to ride on. These are not cheap! One cost me over $40 but I'm sure it was less than replacing the axle or the entire rear swing arm. I'm actually suspicious that the collar that the seal rides on were damaged first by a bearing failure which would lead to premature seal failure and then dirt and water damage. Inspect those bearings and seals regularly.
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