I had my battery box sandblasted and then I painted it with etching primer and coated it with some reasonably durable spray paint. All this time I've been trying to figure out how to mount the batteries without bolting through the bottom of the battery box. There isn't much clearance under the box and seeing the deep scratches on the bottom I felt that if I high-centered on something I could do some damage to the box and possibly the batteries as I sheared off a bolt head or something. After talking to several people I finally settled on the idea of mounting the batteries to a 3/4" sheet of exterior plywood. I made sure to get at least 7 ply so that it would be reasonably stiff. The aluminum box bottom was bowed down about a 1/2" or more and I didn't want a flexible bottom for the batteries to sit on. I put 3/4" of rigid insulation board under the plywood.
Next I needed to figure out some way to bolt the batteries to the board. I didn't want anything conductive at the top of the batteries. Someone suggested wide nylon straps over the center of each string of batteries but I couldn't come up with a way to attach the straps and then a way of tightening them. I tried building a frame around each set with holes to bolt through to the plywood but then I didn't have enough room to fit all the batteries in. I finally ended up with a 3/4" strip of aluminum between each row of batteries and a 3/4" angle aluminum on the ends. the holes you see in the plywood are for some 1/4" x 20 cap head bolts. I installed some Tee nuts in the plywood for the bolts to go into. You can also see the 3/4" angle aluminum I used on each side of the board to hold it down. I put three bolts through each side of the box and through the angle aluminum to hold things in place.
I did several test fits of the batteries to make sure everything was going to fit. Below you can see one of the middle bars. This one is 1/8" thick because I couldn't get 1/4" x 3/4" aluminum bar stock. I finally ordered some from McMaster-Carr because I didn't like how flimsy the 1/8" bar was.
When I test fit the batteries I still had about a 1/4" at each end of the box. I needed a way to make sure that the 3/4" angle aluminum end pieces didn't slip off the narrow ledge of the batteries so I made some 1" x 2" x 3/4" shims and attached them to the angle aluminum at each bolt location. As it turned out I had to use 1/8" on the other side of the box. In the picture below you can also see one piece of the 3/4" insulation installed under the hold down strip.
After thinking about the fact that aluminum conducts heat quite well I decided to look for something else for the shims. I found that McMaster-Carr had some 1/4" ABS plastic so I ordered a 2' piece and replaced the shims as shown below.
Because of voltage limitations I could not fill the whole box with 40 cells. I decided to start with 18 buddy pairs which left a hole. Below is what I did to keep the ends of the hold down straps from slipping off the battery slot edge. I used some aluminum pipe for spacers.
Here is a picture between the batteries. They are tightly packed together and don't seem to move at all when I try to move them out of place. I'll be checking them regularly to make sure the hold down method is working. If it doesn't work, I can use some threaded rod and get some non-conductive rigid material to hold the batteries from the top edge.
Here are the batteries in the battery box before I installed 3/4" insulation around the edges.
Battery straps installed. Notice the orange 1 gauge cable to the front set of 8 cells. I turned around the front set so that the positive end of the pack wasn't against the front edge of the box. The front set of cells is not easily accessible through the battery access hole in the tub of the Gizmo. Since I had to use a cable any way it was an easy thing to do. If you look closely at some of the bolts you will see a little hole in them. This is a tapped hole which the BMS modules will attach with.
The Black Sheep Technology BMS modules are installed and I'm giving the pack the first charge as a pack. Earlier I charged all the cells up to 4.00V with a bench top power supply and then put a load on them until the first BMS module gave a low voltage trip. I then disconnected the batteries and measured their voltages to 3 decimal places and ranked them based on voltage. I paired the highest voltage with the lowest, then the next highest with the next lowest and so on until I had my 18 buddy pairs. I'll check them again in a year or so. While this isn't the best way to measure capacity it is what I had. I used several 500W shop lights and a bank of ceramic base light bases as my load. 100Ah is a lot of energy to dissipate! It took quite a while.
If you look carefully at the photo below and compare it to the one above you will see that I moved a couple of BMS modules in the second row from the back. This is the second row from the bottom of the picture above and second row from the top of the picture below. The reason is that I wanted to get the whole pack to the same SOC and my Zivan NG1 wasn't working too well for that since it wanted to see a 48V pack of lead acid batteries. I used my bench top power supplies to charge two parts of the pack and I didn't know if there would be an issue with both hooked together in series. They are supposed to be able to be put in series or parallel but I accidentally hooked up my battery pack backwards and burned out the series parallel circuitry in my dual power supply unit :(. It will be going in for repair soon. By moving a couple of BMS modules I can now split my pack at the 8 pair mark without having to remove any BMS modules. Two bolts out, remove the connecting strap and I'm done. In the photo below you can see the BMS interconnecting wires. I still have to hook up the +12V, fuse, and HVT and LVT test buttons at one end of the string and the HVT, OK, LVT circuitry to the other end. The small black wire coming in from the top center of the photo goes to the "outdoor" probe of a digital thermometer to measure pack temperature. The blue-green foam piece in the center of the pack is holding the probe against the second row of batteries. The black and red wires which join and trail off the bottom of the picture are to my volt meter. I've been playing around with the voltage trim pot on my Zivan NG1. So far I have it so that it is charging less than 0.5A when the pack hits 71V. I want to stop sooner, however since this is 3.944vpc. If I turn the voltage down sooner then my pack doesn't get fully charged. I may send in the charger to get it reprogrammed.
BTW, the picture above is of the battery pack installed in the Gizmo. That is why you cannot see the front row of batteries.Coming up...
- Performance comparison
- weight change
- efficiency differences
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