Pack design
Cell balancing
Equalising the state of charge of cells connected in series, either by dissipating excess charge from the highest cells or by moving charge between them.
What it changes in practice
Cells in series see the same current but drift apart because they are not identical. Without balancing, the pack is limited by its weakest group: the BMS must stop charging when the highest cell is full and stop discharging when the lowest is empty, so usable capacity shrinks to the narrowest cell rather than the average. Passive balancing bleeds energy from high cells; active balancing moves it, at the cost of more hardware.
Why it matters when you specify a cell
Balancing cannot fix a badly matched pack; it only stops drift from accumulating. Match cells by capacity, impedance and open-circuit voltage before assembly, then let the BMS hold the string together. Passive balancing burns energy as heat and is slow; check the balance current against the self-discharge spread you expect.
Related terms
Read further
More in Pack design
Other terms in this group
From single cell to assembled pack.
BMS (battery management system)
The electronics that monitor cell voltages, current and temperature and disconnect the pack when any of them leave the manufacturer window.
Read the entry →Series and parallel configuration
How cells are connected to reach the required voltage (series, written S) and capacity or current capability (parallel, written P).
Read the entry →Interconnect welding
Joining cells to busbars by resistance welding or laser welding rather than soldering, which is the accepted method for cylindrical cell assembly.
Read the entry →