In this article we are discussing what happens when the cells in a lithium battery get unbalanced. The battery in question is a Victron Smart Lithium Battery. The screenshots come from the Victron Connect program and are taken from a phone using a Bluetooth connection directly to the battery.

Unbalanced Cells

In the example above the orange icon in the top right (circled in green) is telling me that the Firmware needs to be updated. This would only appear if you had the latest version of the App on your phone and if an update for the battery firmware was available. The last full charge of this battery was 76 days ago and so it is showing cell status as unknown. If you click on the "information" icon it tells us the following: 

Error message

A battery like this is more likely to have BMS shutdowns for Cell over or under voltage. As shown in the drawing below the effective capacity of the battery is reduced. The  BMS will stop charging when the highest of the cells has reached its limit, and it will stop discharging when the lowest of the battery cells reaches its limit. So even though all the cells have the same capacity the effective capacity has been reduced.

Battery balancing graphic
Cell balancing happens during the absorption phase of the charge when the battery voltage is held at a constant voltage of 14.2 volts.  Excess energy from the high cell is burned off as heat and this allows the lower cells to catch up.  As long as this happens on a regular basis the cells will not get out of balance.  If balancing does not happen on a regular basis and the cells get more out of balance it will be necessary to modify the charge profile temporarily to get them back in line.  The Absorption time would be extended and the charge rate would be reduced.  This allows longer time for the balancing to take effect.  By reducing the charging rate the cells will not so easily hit their upper limit.  For a more detailed discussion of this please refer to the cell balancing section of the battery instruction manual.

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