The battery is composed of cells connected in parallel and in series. Say P parallel strings of S cells connected in series each. You want your cells to be bala

The battery is composed of cells connected in parallel and in series. Say P parallel strings of S cells connected in series each. You want your cells to be balanced: charged the same amount. You can charge every single cell individually, simultaneously, if you design your connections to support it. IIRC our packs were wired to charge the internal cells in pairs. If you think about it, though, with a given max current I = dQ/dt, you need the same amount of time to charge two cells each to a charge Q (mAh), no matter if you charge them simultaneously or one after the other. Because if you charge them simultaneously you can only supply I/2 each.