BQ76940: Recommendation on criteria for choosing cells to balance

Hi  Aakash,

Correct, BQ76940 does not have a built-in algorithm for balancing cells.  The MCU must implement an algorithm for balancing. 

A first question is whether you want to balance.  Some designers know they use good matched cells and do not want to balance.  However loading can be unequal and many times balance is desired.

Often a battery is balanced during charge since that is when capacity is added and the dissipative balance loses energy from the pack.  However the I*R drop of the cell is added to the voltage of the cell, so voltage based balancing can make a cell with high resistance look like it has a higher voltage during high current charge.  So you might want to balance during low current charge or while the pack is idle but at high voltage after charging.   Generally you don't want to balance during discharge since you are dissipating energy in the balance circuit and generally you want to keep that energy for your load.  You also don't want to balance at low state of charge since cells may differ more dramatically near the discharged state.  You must choose something suitable for your system.   BQ76940 does not have high voltage accuracy, so setting a tight balance threshold may not be helpful.

As you note the part does not allow balancing adjacent cells, you could balance every other cell in the group.  For example if you decide cell 1 is low and all other cells should be balanced down you could balance even cells in each group for some time, then odd cells in the group for a similar time. Note that the cell balancing runs on the scheduler timeline of 250 ms, so allowing balancing to run several seconds will be more effective than switching quickly.