Applicable Systems
BYD Battery Management System
Symptoms
The BYD BMS reports alarm BY007, indicating a cell balancing module fault. One or more balancing circuits are not operating, causing cell voltage divergence and reduced pack capacity.
Possible Causes
- Balancing MOSFET has failed open or short, preventing the balancing current from flowing
- Balancing resistor has opened or changed value, reducing or preventing balancing current
- BMS balancing control signal is not reaching the balancing module due to a wiring fault
Troubleshooting Steps
- Step 1: Check the balancing MOSFET gate signal and drain-source resistance on each module
Gate signal toggles correctly when balancing is commanded and the MOSFET switches cleanly - Step 2: Measure the balancing resistor value with an ohmmeter on each module
Resistor value is within 5 percent of the rated value, confirming no open or value shift - Step 3: Verify the BMS balancing control wiring and connector continuity to each module
All wiring is continuous and connectors are seated properly with no bent or missing pins
How to Reset
To reset: 1) Replace the failed balancing MOSFET or resistor. 2) Repair the balancing control wiring. 3) Restart BMS. 4) Verify balancing current flows on all modules during a balance cycle.
Source: BYD Battery Management System Manual, BYD Company Limited
Category: Battery