Saturday, February 6, 2010

Performance is WAY UP!

I haven't finished installing everything in the Gizmo yet. I still have a little wiring to do on the BMS warning system, I don't have an emeter type device yet, I don't have a latching relay to kill the AC to the charger if something goes wrong, and I don't have the charger installed yet since I'm still fiddling with the finish voltage trim pot. It is finishing at about 71V right now so it isn't going too high for the BMS modules on a balanced pack. [(7-25-2010) see comments below about this ending voltage.] I have 18 "cells" and the BMS modules have a HVT (High Voltage Trip) of 4.00V. When I get things finished, or nearly so, I'll go weigh it at the Airport. A friend of mine is an A&P mechanic. He has some scales he uses to calculate the weight & balance on airplanes. I will post the data along with my CG calculations for the 6V pack and the new pack. Suffice it to say, the Gizmo is much lighter than it was with the lead acid pack.

Each pack of 10 cells with connecting straps & bolts weighs 80lbs I have a total of 36 cells. With the mounting hardware I used I figure this pack weighs about 300lbs. The Interstate Batteries U2200UTL are 62lbs each so 8 of them were 496lbs and this is without the connecting cables. It looks like I was able to reduce the weight buy about 200lbs. This is lighter than with the original Trojan T-875 batteries. They are 63 lbs each so a total of 378lbs.

The 18 "cells" I went to raised the voltage I see to about 61V nominal. After a short run to drain off the top 1% of charge or so the pack sits about 61V. After a several mile run it sits at about 60V. I find that while cruising along at about 125A or so the voltage sags to about 56-57V. A 200A load (this is 1C since I'm using buddy pairs) lowers this to about 55V. This is with the batteries at 45-50°F. Maybe when things warm up they won't sag as much. Even with this, it is much better than with the lead acid batteries.

My top speed on level roads with no wind is about 42 mph. Just after I installed the pack it appeared to be about 45-48mph so maybe the batteries were still warm from being in my shop. With the higher voltage I'm seeing current readings a little lower than before. This is to be expected. When climbing my hill I now only slow to 33mph where before I slowed to 24mph on a fully charged pack. I attribute the climbing performance to a higher voltage and being 200lbs lighter.

After installing the pack I didn't get a chance to back off on the spring tension on my coil-over shocks. I definitely sat higher and it seemed that I could feel every pebble on the road. I think I only had a 1/4" travel before the rear shock was at its maximum extension. I have since reduced the tension a couple of notches but I still ride a little higher and it is still a little stiff. I'm going to reroute the wires going to the motor so that I have more travel before the motor bottoms out against the tub of the Gizmo and the lower the tension in the spring to see how that feels.

Acceleration is much better than before. I have to watch my speed-o-meter to make sure I don't get a speeding ticket now. I can easily out accelerate the other cars on the road now. I'm not drag racing them but just comparing to what the typical driver does when a light turns green. When accelerating with the lead acid pack I almost never saw 400A from the batteries. Now I can pull 400A on every acceleration if I want. I tried bumping up the maximum amperage to the armature to 500A and noticed significant increase in acceleration. I didn't leave it there, however, since I want to stay below 2C on these batteries. I need them to last several years longer than the lead acid batteries did to recoup my investment.

In short. I love having a Lithium Ion battery pack!


Jack Rickard said...

The best thing you can do to prolong the life of your cells is lower your charge voltage. You're likely overcharging your cells. We use 3.65v per cell on TS cells in series which would mean you would charge to 66v instead of 71.

The cells don't mind bursts of 3C and it won't hurt them. Overcharging them will. As they have different capacities, 3.65 works well.

Thundersky's own charger is hard set at 87v for a 24cell pack. Same numbers.

Jack Rickard

Gizmo said...

I've been thinking about that very thing for quite a while. If I lower the voltage on my Zivan NG1 it quits too early because it times out. That is the problem with using the algorithm for a Trojan T-875 lead acid pack.

When I first contacted Elcon at I couldn't get any info from them about their charging algorithms. I finally called them a couple of weeks ago and they said I would need to give them the size of my pack and type of battery. I said that the size of my pack was going to depend on the finish voltage. I emailed them this info and they sent back two sets of charging profiles for me to choose from. I chose the one which ended at 3.65vpc and told them I had 19 cells so gives me an ending voltage of 69.35V, just under the cut-out voltage of my DC-DC and max turn-on voltage of my controller. I'm planning on putting in my remaining cells for a 20 cell pack which will give me an ending voltage of 3.47vpc. If I have some cells go bad I can always go back to 18 cells and still not be quite as high a finish voltage as I'm doing now, besides, there should be some room to adjust the calibration pot.

When I did a hand test of the amp hours between 3.45 and 3.65vpc I came up with just a hair over 1Ah on a 100Ah cell. This is not much to leave on the table and is well under my BMS shunt turn on voltage of 4.00V. I'll top balance the pack and then check it in a year and see how close things are. This will give another data point to support or refute that the cells tend to stay balanced with no sort of BMS system. Assuming of course that the BMS boards don't have a problem.

The single cell charger I got from EV Components charged to 4.17V. I wish I could figure out how to lower this voltage. I couldn't find any adjusting pots inside so I only use it if I'm right there watching cell voltage and can manually shut it off.