Have you actually measured each module's true capacity (discharge test)? It
sounds like their capacity varies quite a bit. The one with high voltage
probably has the least capacity, so it was fuller than the others. It might
also have more internal resistance.
Batteries that looked
to be in sync based on resting voltage being out of sync after a small
discharge and on recharge.
Resting voltage doesn't tell you much about capacity. A sulfated battery
might read low, but there are many other depreciation mechanisms. Such a
battery might really be fully charged, but can deliver only 10 or so amp hours
instead of 26. Thus the only time the pack is ever balanced is at full charge
(if then).
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