Cell Balancing
Cell balancing is the process a battery management system uses to reduce differences in state of charge among series-connected cells or cell groups.
Why balancing is needed
Small differences in capacity, self-discharge, temperature, and aging cause cell voltages to drift apart. Charging or discharging must stop when the first cell group reaches a safe limit, so an imbalance can leave part of the pack's energy unavailable even when the other groups could continue.
Passive balancing removes a small amount of energy from higher-voltage groups, usually as heat. Active balancing transfers energy between groups or through another storage path. The chosen method affects speed, efficiency, cost, and circuit complexity; it does not change the battery's fundamental chemistry or rated capacity.
Balancing is not a repair for a damaged or severely degraded cell. It can align the usable state of charge, but it cannot restore lost capacity, eliminate excessive self-discharge, or make a weak cell match healthy cells under load.
For the detailed methods, see Cell Balancing.