V-Batt vs. BMS Voltage

BMS Voltage is the operating voltage of your battery as determined by the batteries internal battery management system.

V-Batt is the voltage that is recorded across the battery terminals of the inverter.

These values are important when it comes to charging and discharging your batteries. As a safety mechanism if there is difference of 1.5V or larger between V-Batt and BMS Voltage the inverter will stop the flow of current between the inverter and battery to protect against a possible system short or loose connection.

You can use a multimeter to record the voltage across the battery terminals of the battery and then across the battery terminals on the inverter or you can find this information in Sunsynk Connect.

On Sunsynk Connect you can select the parameter in the custom graph to record these values.

These 2 values should always be the same of have a very small difference.

One reason these values may have a difference of 21V or greater is down to the cable length between the inverter and batteries. When designing a system you should account for any voltage losses that may effect system performance when installing long length cables.

Another common reason this voltage difference occurs is because of a fault between the inverter or batteries.

With a standard system designed as follows:

The easiest way to determine where the fault occurs is to check and record voltages at each point of the circuit with a multimeter to determine where the discrepancy occurs.

•             Test the voltage across the slave battery terminals.

•             Test the voltage across the master battery terminals.

•             Test the voltage across the cables on the battery side of the fuse isolator.

•             Test the voltage across the cables on the inverter side of the fuse isolator.

•             Test the voltage across the inverter battery terminals.

When these are recorded you should be able to see where the voltage difference occurs. For example:

Master Battery Voltage – 52.4V

Slave Battery Voltage – 50.4V

This would suggest the battery voltage has become unbalanced due to either a communication issue with the BMS or a connection issue causing voltage losses across the cables.

OR

Voltage on the battery side of the fuse isolator – 50.1V

Voltage on the inverter side of the fuse isolator – 47.3V

This would suggest there is an issue with the fuse isolator, likely a blown fuse or a loose connection.

OR

Voltage on the inverter side of the fuse isolator – 56.4V

Voltage on the inverter battery terminals – 52V

This would suggest an issue or loose connection with one or both power cables passing between the inverter and the fuse isolator.