I have 48 prismatic LiFePO4 cells that are perfectly balanced. They have been sitting in parallel for days, and each cell is within .001 volt of each other. Each cell nicely measured 2.800 volts.
While the cells were in parallel, I went to charge them using a bench power supply. The positive and negative leads were attached to opposite ends of the very large string of 48P. The charge voltage was 3.6, the CC started charging at about 16.4 amps. All seemed okay. I'm guessing these large 150a prismatics will take days to fully charge.
HOWEVER, after about 20 or 30 minutes I measured the voltage of the cells. The two cells at the ends had a higher voltage than the cells in the middle. Remember, the end cells is where the power leads were attached, (+) on one end, (-) on the other. The difference was more than .100 volt. I freaked out a bit and stopped charging. Right now the cells are all in parallel, getting in balance.
THE QUESTION: During a parallel charge, shouldn't all cells be at the same voltage? Is a 48 cell bank simply too large to charge this way? Should I reconfigure the cells to charge in series? Everything I've seen on parallel charging uses a charge board. Obviously there is no charge board for a large array of large capacity cells.
Am I being too cautious / too paranoid?
While the cells were in parallel, I went to charge them using a bench power supply. The positive and negative leads were attached to opposite ends of the very large string of 48P. The charge voltage was 3.6, the CC started charging at about 16.4 amps. All seemed okay. I'm guessing these large 150a prismatics will take days to fully charge.
HOWEVER, after about 20 or 30 minutes I measured the voltage of the cells. The two cells at the ends had a higher voltage than the cells in the middle. Remember, the end cells is where the power leads were attached, (+) on one end, (-) on the other. The difference was more than .100 volt. I freaked out a bit and stopped charging. Right now the cells are all in parallel, getting in balance.
THE QUESTION: During a parallel charge, shouldn't all cells be at the same voltage? Is a 48 cell bank simply too large to charge this way? Should I reconfigure the cells to charge in series? Everything I've seen on parallel charging uses a charge board. Obviously there is no charge board for a large array of large capacity cells.
Am I being too cautious / too paranoid?