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48 large (150Ah) LiFePO4 prismatic cells were wired in parallel (48P) for days to bring them all into balance; each cell metered 2.800 volts. Perfect!

It would seem possible to initially charge the cells using a bench power supply. When connecting the power leads (+) to one end of the 48 cell array and (-) to the other, charging began at 3.65 volts with a constant current of 16 amps, well within the specifications. The ultimate objective is to get all of the cells up to 3.1v at the same time (top balance) in order to reconfigure them into a 3P16S (48v) configuration. The calculation of charge time shows will take days and days to charge, I get that.

After about 30 minutes of charging I sampled the cells with a multimeter. I noticed the cells at the end of the parallel array, the ones closest to the charge leads, were being charged faster than the cells in the middle. I expected all the cells in the 48P array to charge at the same rate. Not sure if this is normal, I disconnected the power supply and began searching for answers.

All information on parallel charging I found was for smaller cells using a charge board (circuit board), or charging batteries (not cells) in parallel. I couldn't find anything on charging this many cells in parallel at the same time.

THE QUESTION: Is there some sort of theoretical or practical limit to how many cells can be charged in parallel this way? Will the imbalance of end-cells during charging damage the cells in any way? While it is certainly possible to charge cells in parallel, I am concerned there is some sort of limit. Is there an advantage to configure the pack into a 48 volt (3P16S) pack and charge it that way? This would mean the pack is bottom-balanced, but a top balance is what I'm trying to do,

MorningCloud
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  • How big were the cables that you used? At 16 A they could have a large voltage drop on them between each cell. – Rokta Aug 11 '22 at 11:05
  • The wire was 12 gauge which has a rating of 25 amps at 60 degrees Celsius. Applying the 80 percent rule of capacity (NEC 220-2), that drops the ampacity to 20 amps. The 16 amps was well below that. Measuring resistance of the daisy-chain cable from one end to the other (without connected to any cell), the ohms reading was .1 to .2 – MorningCloud Aug 11 '22 at 12:34
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    0.1 Ohms with a current of 16A will give you a voltage drop of 1.6V, which is unworkably large for charging batteries. That would explain the faster charging at supply end. It's also dangerous as you risk overcharging those closer cells. – Finbarr Aug 11 '22 at 14:43
  • That makes a lot sense, @Finbarr. The ultimate configuration of 3P16S will have big heavy buss-bars rated for 400A. I need to re-think how these are being top-balanced. – MorningCloud Aug 11 '22 at 21:26

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