ElECTRIC Karmann Ghia Project
Because of the decision to integrate a true BMS we had to utilize the cell taps and thermistors integrated into the Tesla battery modules. I looked, but Digikey, Mouser, etc didn’t have the connectors in stock to work with the stock connectors on the modules. This meant we had to change all the connectors which meant the arduous task of removing all the batteries we had already installed in the back of the car. We took this as an opportunity to add 2 additional thermistors to the middle of each module to supplement the 2 thermistors on each pole. This will fill out all of the thermistor connections that the BMS accommodates. While crimping on new connectors and testing the wire harnesses we were making we found that one of the cell taps on one of the modules had popped its weld. I tried soldering is back on but it would not take solder at all. There is no way I was going to try spot welding it myself so close to so many cell fuses. I found some silver impregnated conductive epoxy and gave that a go. So far that repair has held up quite nicely.
We got the charger back and reinstalled and used the ABS as a convenient mounting point for the charge controller that will integrate the charger with the new BMS.
October - November
We didn't get a whole lot done in this time frame. Most of the effort was spent on figuring out how to mount the batteries in the front of the car. We were originally going to recess one module into the space previously occupied by the gas tank and stack the other in a 2x2 block above it, but after additional analysis it would have been a huge pain to wire all the modules in that configuration. Instead we would stack the fifth module on top of the other 4 and we have just enough room to do this without getting in the way of the hood closing. The design for this battery mount is a sheet of steel mounted to the gas tank bolt holes with angle iron and square tubing welded to it to mount the first layer of batteries, then more welded angle iron structures bolted through the rails to lock in the first batteries and provide support for the second set, then 2 bands of metal bolted to the base plate and the center support to mount the top battery to.
The first attempt at this failed because I went too fast and too hot with the welder and turned the base plate into a big banana. The second attempt I was much slower and more careful and it turned out fine. The whole contraption doesn't look too shabby either if I do say so myself.