Balance chargers for small packs, like you'd have for a radio controlled aircraft, have separate wires for the positive and negative plus in between each cell.

Balance chargers for small packs, like you'd have for a radio controlled aircraft, have separate wires for the positive and negative plus in between each cell. So, a 6-series configuration would have 7 wires and the charger uses them to charge each cell separately to the correct voltage. The problem with that is that each wire has to be able to handle the full charge current, but at a low voltage. So, if you're dealing with, say, an electric car with dozens or hundreds of cells and thick copper cables the size of garden hoses it's no longer practical. Instead, you charge it the simple way (by applying a large voltage to the whole series string through a positive and negative lead) and use a battery management system that measures voltages between cells and drains any high-voltage cells gradually through a resistor. It's kind of wasteful, but it's fairly simple and if your batteries are well balanced the BMS shouldn't have to do much at all. Using the high cells to charge the low cells would be a nice feature; I'm not aware of any EV BMS that does that, but I'm not an expert and I'm really only aware of what's going on with DIY conversions. I don't know what the state of the art is for BMSs in commercially manufactured vehicles.