Applicable Systems
BYD Battery Management System
Symptoms
The battery cell voltage has dropped below 2.8V per cell, indicating over-discharge. The BMS disconnects the battery to prevent cell damage.
Possible Causes
- Continuous load on the battery after the BMS requested a shutdown
- Self-discharge over an extended storage period without maintenance charge
- BMS balancing circuit failed, allowing one cell to discharge below the safe threshold
- Parasitic drain from connected equipment after the main contactor opened
Troubleshooting Steps
- Step 1: Measure each cell voltage with a multimeter and identify the low cells
Cells below 2.5V require careful recovery charging; below 2.0V may be permanently damaged - Step 2: Apply a low-current recovery charge (0.05C) to the low cells individually
Monitor temperature; if any cell exceeds 45C, stop and replace the cell - Step 3: Verify the BMS balancing circuit operates during charging
If the balancing circuit is not functioning, the BMS needs service - Step 4: After recovery, perform a full charge cycle and verify all cells balance above 3.0V
If cells do not balance, replace the weak cells or the entire battery pack
How to Reset
Reset: The BMS reconnects the battery automatically once all cells are above the minimum threshold during charging.
Common Replacement Parts
- Lithium cell: LiFePO4, 3.2V nominal (Match to existing pack specification)
- BMS controller: 16-cell or 24-cell, CAN bus (Match to pack configuration)
Frequently Asked Questions
- Q: What is the minimum safe cell voltage?
For LiFePO4: 2.5V. For NMC: 2.8V. Discharging below these levels causes irreversible chemical damage.
- Q: How do I recover an over-discharged cell?
Apply 0.05C (5% of rated capacity) constant current until voltage reaches 3.0V, then switch to normal CC-CV charging.
- Q: How long can I store a lithium battery?
At 40-50% state of charge in a cool, dry place: 6-12 months. Check voltage every 3 months and top up if below 3.0V per cell.
Source: OEM Technical Manual
Category: Battery