There can be many reasons why the indicated SoC is incorrect or may drift. The BMS uses various parameters to calculate the SoC, including the battery current, cell voltages and battery capacity. If you are interested in how a BMS determines the SoC, please check our dedicated article.

Checklist for troubleshooting incorrect SoC indication #

Use the following checklist to ensure everything is configured and working properly.

1. Check the battery capacity #

In the 123\SmartBMS app at Settings, you have the Battery capacity. Ensure this capacity is correctly entered. The capacity can be calculated with the following formula:
Battery capacity = Battery Ah x amount of cells x nominal voltage
I.e. if you have 8 LiFePO4 cells in series where each cell is 300Ah, the battery capacity is 300Ah x 8 x 3.3V = 7920Wh, which is 7.9kWh.

2. Check the current sensor(s) mounting #

If you have 1 current sensor, make sure that all loads and chargers are connected on the same cable which goes through this current sensor. All charging and load currents should flow through the cable with the current sensor, even computers like the Victron GX. Only then, the BMS is able to measure all currents to and from the battery and determine the SoC correctly.

If you have 2 current sensors, make sure that currents are not measured twice. You should have two cables from the + of the battery. Current sensor 1 should be on one of the cables, current sensor 2 on the other cable. The current from/to the battery should either flow through sensor 1 or through sensor 2, but never be able to flow through sensor 1 and then through sensor 2

Besides having the current sensor mounted on the high power cable, you have to check if the current flow direction is correct. Open the 123\SmartBMS app and go to the dashboard. Next to the battery icon is info about the battery, including the current (A). When you charge the battery, this current value should be positive and when you discharge the battery, this value should be negative (for example -3.1A). If this is not the case, then you need to either rotate cable through the current sensor, or make sure that you have the latest 123\SmartBMS firmware and invert the sensor direction via Settings in the app.

3. Check the current sensor configuration in the app #

The 123\SmartBMS gen3 current sensors are all dual range 500A/20A. If you have not any loops through the current sensor for higher accuracy, then leave the setting to the default value which is “dual range 500A – 20A”

4. Calibrate the current sensor(s) #

The first time you use the BMS, you need to calibrate the current sensor(s). This calibration removes any offsets, so that no current is really seen as 0A. See the 123\SmartBMS manual for the calibration procedure.

5. Verify the current measured by the current sensor #

If you have a charger or inverter which indicates the charge or load current, or if you have a current clamp, use this to verify that the indicated current in the app is about the same. Note that the value do not completely have to match, when a charger indicates 4.5A and you have a computer connected, it can be that the BMS will indicate 4.3A. At least, the values should be near each other.

6. Check if the charger’s absorption voltage is configured correctly #

The charger’s absorption voltage must be configured correctly to ensure that the cells all reach Vbalance at the end of the charging session and are able to balance. When the charge voltage is set too low, the cells will never all be able to balance and the SoC will never be synchronized at 100%. A voltage that is too high again causes Vmax to be reached, causing the charger to stop temporarily.

Use the following formula to calculate the correct absorption voltage:
Absorption voltage = (Vbalance + 0.03V) x amount of cells in series
For example a Vbalance set to 3.5V and 4 cells in series gives 3.53V x 4 cells = 14.12V.

If you want, you can check to see if all cells start balancing when the battery is almost full. All cells in the app should get orange. When all cells are balancing (orange voltage) and the charging current is reduced enough, which should happen automatically, the BMS will sync the SoC to 100%.

Note: if you have a 123\SmartBMS with USB cable to a Victron GX and DVCC enabled, the BMS will automatically pass the correct charge voltage to the Victron.

7. Bad cell or one cell with less capacity #

Sometimes a battery bank contains a bad cell. The SoC should indicate 100% when all cells are full and 0% when all cells are (almost) empty. However if a cell is in bad health, it has less capacity than the others. The BMS may still indicate that the SoC is not 0%, but one cell may already be near/at Vmin. In general you can see that a cell is bad when the battery was recently balanced, one cell is near/at Vmin and the other cells have a voltage in rest which corresponds to a battery still partly full.