Our 1S cells are all 1S2P. There are two cells in there connected in parallel. When charging any set of cells from a single connection there is the chance that they will become unbalanced. That connection could easily be for balance charging.
My experience is that it isn't necessary. All the cells I have replaced have been just tired after a lot of use. Cells bought at the same time and used the same amount (I usually buy two and use them by rotation for each race) have become tired at the same time, suggesting to me they are just tired, and not out of balance.
Adon sounds like he knows the real answer though...
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