Unbalanced Batteries

In multiple battery systems, sometimes the batteries may become unbalanced. This means that the SOC (State of Charge) of each individual unit has a difference of >10%. This can cause a number of issues with the functionality of your system. 

The batteries can become discharged below their set Shut Down % which can negatively affect the battery SOH (State of Health) and warranty conditions. The batteries may also request unexpected force charges from the GRID causing unnecessary costs to the end user. 

For example, in a scenario involving a three-battery system with SOC values of 30%, 30%, and 5%, the recorded SOC on the inverter by the combined BMS (Battery Management System) would hover around 20-25%, calculated as an average of all individual battery SOC values. Despite this aggregated SOC reading, the battery with the lowest individual SOC may trigger a force charge upon reaching its critical low voltage state. Consequently, a grid charge is initiated until the battery exits its low voltage range. However, because of the system unbalance, the batteries fail to reach the restart percentage before reinitiating discharge.

This causes a loop of grid charge spikes in a sawtooth pattern similar to those pictured below:

The L5.1/L5.3/G5.3 batteries allow us to access individual battery information on the inverter screen. To do this:

Settings > Li BMS > Details Data (you can then use the up and down arrows to scroll between all of the connected batteries)

For other Sunsynk batteries you can see the individual SOCs by looking at the SOC indication lights on the front of each battery unit. These should all match if the batteries are balanced.

Causes of Battery Unbalance

Batteries can become unbalanced by a number of causes. The main cause being a voltage difference between each battery. If a stack of batteries are connected in a ‘daisy chain’ the voltage in the Master Battery will be the highest with each subsequent battery in the chain being lower than the battery previous. 

Ideally in systems with 2 or more batteries connected a DC busbar should be used to further maintain a voltage balance. You can use any DC rated busbar with your batteries as long as they are rated for the maximum amount of current coming to/from the batteries. 

If this is not possible and issues are occurring please double the positives and negatives at either end of the stack as follows:

Another cause of battery unbalance is insufficient charging. If the batteries do not reach 100% ideally once every 7-10 days the BMS does not recalibrate. This means that the batteries do not have the chance to reduce any voltage differences and maintain a consistent SOC. During darker months if there is insufficient solar production it may be necessary to charge the batteries via the grid to make sure they are reaching 100% at least once during the recommended time frame.

Batteries can also be unbalanced when adding a new battery to the system. Please reference the following guide when adding an additional battery to a system:

https://sunsynk.freshdesk.com/a/solutions/articles/103000272400

Rectifying a Battery Unbalance

If a battery unbalance has been confirmed please follow the below instructions to rectify the issue:

1. Disconnect each battery.
2. Reconnect each battery individually and charge up so SOC matches on all units.
3. Reconnect the batteries as detailed above.
4. Grid charge the batteries to 100% (this allows a calibration of the BMS)