BMS/Cell Balancing

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Cell Balancing is a self-maintenance process by the BMS.

It ensures that cells operate at very well-matched states of charge to ensure even load and better battery performance and overall lifetime.
Mechanism
The BMS has connections to the battery cells via the interconnect wiring and can sense cell voltage.
There are 28 contacts on the side of the back of the board (as viewed from the front of the bike looking at the battery) which connect to each cell in the "brick" series.
The BMS feeds small amounts of current to cells that are abnormally low in order to maintain battery longevity and minimize the possibility of cell damage.
It detects voltage levels on all cells, through interconnects between bricks that harness each cell in the same position in the brick's series, and through this same path able to balance cells by targeting current to the cells with the lowest voltages.
Cell balancing because of the limitations of this circuit can take a long time and is part of why Zero recommends keeping the motorcycle plugged in when unattended by default.
Schedule
Cell balancing often happens at low current levels, mainly during charge tapering when the charger is operating at the voltage limit in "constant voltage" mode.
It also can happen when the bike is long-term plugged in but turned off. The BMS may wake up and control charging specifically to balance the cells.