Charging an Accumulator Stack
Consider an accumulator stack that consists of two cells connected in series. Both have the same rated capacity, for example 5Ah, but are slightly different charged due to different thermal stress during operation for example. Cell A is charged to 80% of its rated capacity, ie 4Ah, while cell B is charged to 70% of its rated capacity, which is 3.5 Ah. During charge no problem occurs, as long as cell A is not fully charged and less than 20% charge is added.
After some time cell A is fully charged and now has its nominal capacity of 5Ah. At this time cell B is charged to 4,5 Ah. The difference of 0.5 Ah in capacity from the beginnig of charge remaines unchanged! Overall the pack has now at 90% of its maximum capacity, as the weakest cell determines the overall system capacity. The question is: Continue chargeing or not? This question can't be answered in general:
- NiCd and NiMH batteries require an ongoing charge with significantly reduced charge current to fully charge cell B as well. An overcharging of cell A is tolerable as long as the temperature rise of cell A stays below a certain level. Cell A starts to convert the added electrical energy to heat due to the fact that it is fully charged and not loger able to convert it in chemical energy.
- Lithium accumulators allow no further charging. Otherwise the permitted end-of-charging voltage exceeds the limit and takes damage to the cell if a protection circuit allocated in the stack doesn't prevent this anyway. An automatic leveling during charge isn't possible without other methods! Therefore, in high quality lithium stacks, the protective circuit bypasses a part of the charging current to the cells which the highest cell-voltage. Cells with lower voltage are charged at full current.
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