Applicable Systems
BYD Battery Management System
Symptoms
The BYD BMS reports alarm BY008, indicating a relay contactor welded or stuck closed. The main contactor cannot open, preventing the BMS from disconnecting the battery during a fault condition.
Possible Causes
- Contactor has welded closed due to arcing from repeated high-current disconnect cycles
- Contactor coil has failed and cannot generate the electromagnetic force to open the contacts
- Contactor mechanical linkage is jammed due to debris or a broken return spring
Troubleshooting Steps
- Step 1: Check the contactor contact resistance with the contactor commanded open
Contact resistance is infinite with the contactor open, confirming the contacts separate cleanly - Step 2: Test the contactor coil resistance and the coil drive voltage from the BMS
Coil resistance matches the nameplate and the drive voltage is present when the contactor is commanded - Step 3: Inspect the contactor mechanical linkage and return spring for jamming or breakage
Linkage moves freely and the return spring is intact with the correct tension
How to Reset
To reset: 1) Replace the welded or jammed contactor. 2) Replace the contactor coil if failed. 3. Restart BMS. 4. Verify the contactor opens and closes correctly under command and under load.
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Source: BYD Battery Management System Manual, BYD Company Limited
Category: Battery
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