When combining cells, the degree of mismatched ESR and SoC affects reliability. For serial, voltage sensing on charger per cell is needed, which is why they have 3 pin on multi cell batt's. Serial, doubles ESR and ESR rises rapidly when any cell approaches 10% SoC so reverse protection is often used or better, cell equalization FETs.
For parallel same thing. If not matched by factory, parallel cells can age faster by current hogging and if not matched Voc, results in self discharge until equal, which can result in excessive current as ESR is almost a short circuit. In this case if cells are of unknown age and capacity, then series equalization Rs is added to match ESR of all batteries, which may be in range of 100 mOhm and now adds more loss but more reliable.
In series the weakest cell drains fastest. In parallel, strongest battery drains faster until equal, but mismatch weakens initially faster and Rs if added is more lossy. So factories may make arrays in Series parallel like LEDs 2S2P for more capacity and resilient voltage to match load.
Rule of thumb is cell ESR must be 10% of motor coil ESR for fast start and low dip.
But in cars it is often 33% so starter current in coldest winter allows 4V drop from 12V when battery ESR is highest.(cold) thus the infamous CCA lead acid test at 0deg C and7.2V
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Typically factory matched cells have ESR and capacity of 1% worst case, only when new. Aging increases the degree of mismatch as ESR rises with aging and not all at the same rate.
Something like 10% mismatch if in series will fail quickly, but in parallel will not, but then Ah capacity is reduced a lot.