Found this information to be very helpful.
How a BMS Balances the Pack
If your charger shows a green light (indicating that its full), our theoretical 13S pack will read 54.6V when its hooked up, but if…as soon as you unplug it from the charger, the battery pack plug reads 53.0V on a voltage meter? This means that one or more of your sub-packs is not taking and holding the full charge.
The way that a BMS works is that it allows a simple bulk charge to pass through it to the battery until it reaches the programmed “full” charge. Then it stops the charge and takes a moment to sense what each of the sub-packs’ voltage is at. Most sub-packs will be at 4.20V, but one cell might be at 4.15V
Most BMS’s then drain the other sub-packs to the voltage of the lowest pack (in this example, 4.15V). and then the BMS allows the bulk charger to send another full charge to all the cells. This drain and charge happens several times until the BMS senses that all the sub-packs are all close enough in voltage to be considered “balanced”.
Due to the varied cells’ resistances, when the LVC cuts power at the end of a ride cycle, each of the sub-packs will be at a slightly different voltage (which is natural), but…as long as they are not too far away from each other, the BMS can then manage getting them to a balanced charge state without it taking so many drain/charge cycles that…it seems like it takes forever for the pack to finish charging.
Out of all the different ways that a piece of electronics could fail, the sub-pack “drain” function (as part of the drain/charge cycles at the top of the charge for balancing) can fail, and completely drain that sub pack down to zero. If you have a 13S / 48V pack, and the packs highest voltage is 4.2V less (50.4V instead of 54.6V), you have a dead sub-pack that will no longer take any charge. If a BMS uses the “drain the high cells” method to get the pack balanced, that is called a “resistor bleed”.