BYD BY015: BYD BYD Battery Management System Error Code B...

Symptoms

The main contactor has welded closed. The BMS software shows the contactor as open, but battery voltage is present at the output terminals.

Contactor contacts welded from interrupting a fault current above its rating
Contactor coil driver transistor short-circuited, keeping the coil energized
Mechanical binding in the contactor armature
Contactor selected with insufficient make/break current rating for the application

Step 1: Disconnect the battery from the inverter and verify zero voltage at the output
If voltage is present with the BMS showing open, the contactor is welded
Step 2: Measure the coil voltage - it should be zero when the BMS commands open
If voltage is present, the coil driver on the BMS is faulty
Step 3: Replace the welded contactor with an OEM-specified unit
Verify the new contactor's make/break rating matches the application
Step 4: Inspect the BMS coil driver circuit for shorted transistors or ICs
Replace the BMS board if the driver circuit is damaged

Reset: After replacing the contactor and/or BMS board, the system resumes normal operation. No manual reset is required beyond a power cycle.

Main contactor: 24V DC coil, 200A continuous (Verify make/break current rating)
BMS board: With coil driver circuit (Match to battery pack configuration)

Q: What causes contactor welding?
Interrupting current above the contactor's breaking capacity. Pre-charge circuits protect the main contactor by limiting inrush into capacitor loads.
Q: What is a pre-charge circuit?
A resistor and small contactor that slowly charge the inverter's input capacitors before the main contactor closes. This prevents high inrush current from welding the main contactor.
Q: How do I select a battery contactor?
Continuous current: rated battery current. Make current: inrush to capacitors (use pre-charge to limit). Break current: maximum fault current the BMS can interrupt.