Applicable Systems
BYD Battery Management System
Symptoms
The BYD BMS reports alarm BY004, indicating SOC calculation anomaly. The state of charge estimate has jumped, drifted, or become inconsistent with the actual pack capacity, causing unreliable range or capacity reporting.
Possible Causes
- Coulomb counter current integration has accumulated error due to sensor drift or an offset bias
- Cell balancing has not been performed, causing the pack SOC to diverge from the weakest cell SOC
- BMS firmware SOC algorithm parameters have not been updated after a cell module replacement
Troubleshooting Steps
- Step 1: Compare the BMS SOC against the open-circuit voltage SOC table after 2 hours of rest
BMS SOC matches the OCV-based SOC within 5 percent, confirming the calculation is accurate - Step 2: Verify the current sensor zero-offset by checking the reading with no load connected
Zero-offset is less than 0.5A, confirming no significant bias in the current measurement - Step 3: Check the cell balance status and verify that balancing has been performed on all modules
All cell modules show balanced voltages within 20mV, confirming active balancing is effective
How to Reset
To reset: 1) Perform a full charge and discharge cycle to recalibrate the SOC. 2) Replace the current sensor if drifted. 3) Run active cell balancing. 4) Restart BMS and verify SOC accuracy over a full cycle.
Source: BYD Battery Management System Manual, BYD Company Limited
Category: Battery