Battery Design Debacle
After reviewing the battery layout in the car I realized that I made a major mistake in the number of batteries. The DMOC/Siemens combination only works at 400V or less. The DMOC will permit charging to over 400V but will not run the motor until the voltage drops to that level. I had thought I could put cells together in a parallel connection to increase that capacity of the pack while keeping the pack voltage down. The problem is that all batteries would have to be connected together in parallel pairs to have a system that works. Having a subset of cells in parallel effectively makes larger capacity cells than the other cells in the system. So when all the cells discharge though running the car, the different capacity cells will discharge at a different rate. They will also charge a different rate. Having all cells in parallel pairs would drop the system voltage by a factor of two down to 246V which is too low for the DMOC and Siemens motor to operate. There is no room to increase the number of batteries by 40% to get the pack voltage up. My new plan is to use 118 or 120 batteries, all wired in series. That will give a charge voltage of 420V for the 120 cell system. When the charging voltage is removed, the pack voltage drops considerably. Also, when the DC-DC converter is connected there will be an additional drop in the pack voltage. I will have to experiment to see what the optimum battery number will be. The lower number has a couple of advantages. One, it reduces the weight of the battery pack by nearly 100 pounds and two, reduces the design constraint for the number of cells in the engine compartment. Unfortunately the reduced number of cells will reduce the driving range. The original driving range for the 141 cell pack was expected to be 100 miles. The new driving range will be 80 to 85 miles.