Problem Description
- Case #005
- Fault Name: Cell Voltage Imbalance
- The customer reports a significant reduction in the usable capacity of the battery system. The battery reaches 100% SOC unusually fast during charging, and drops to 0% abruptly during discharging, failing to provide the expected backup time.
Symptoms
- The inverter triggers "Battery Over-Voltage" during charging and "Battery Under-Voltage" during discharging prematurely.
- The BMS ALM (Alarm) red indicator lights up constantly or flashes.
- Charge/Discharge current is suddenly cut off by the BMS to protect the battery.
Possible Causes
- Long-term Partial State of Charge (PSOC): Battery cycles in the 20%-80% range and rarely reaches 100%, preventing BMS balancing from activating.
- Loose Busbar Connection: A loose nut creates high contact resistance, causing false high-voltage readings for a specific cell.
- BMS Balancer Failure: The passive or active balancing circuit on the BMS board is burnt or malfunctioning.
- Cell Degradation: Severe internal resistance difference or capacity degradation among aging cells.
How to Verify
Connect the BMS monitoring software (PC or APP) when the battery is charging and SOC is above 90%. Observe the real-time "Individual Cell Voltage" list. Calculate the Max Voltage Difference (ΔV = Highest Cell - Lowest Cell).
Required Tools
- BMS PC Software & RS485/CAN Communication Cable
- Digital Multimeter (High precision)
- Insulated Torque Wrench (10mm/13mm)
- External 3.65V/20A DC Power Supply (If manual balancing is required)
Normal Values
- Standby / Mid-SOC ΔV: ≤ 30mV (0.03V)
- End of Charge ΔV: ≤ 50mV - 80mV
- BMS Balancing Flag: Turns "ON" when a cell > 3.40V and ΔV > 30mV.
- ΔV expands rapidly to > 200mV, or even > 500mV during charging/discharging.
- Cell #1 is at 3.65V (triggering Over-Voltage Protection), while Cell #12 is only at 3.20V.
Diagnostic Procedure
- Step 1: Log in to the BMS software, read real-time data, and locate the highest voltage cell and lowest voltage cell.
- Step 2: Check if the software indicates the "Balance" switch is turning ON for the highest cell.
- Step 3: Power off the battery, open the casing. Use a multimeter directly on the cell terminals of the highest/lowest cells to verify if the physical voltage matches the BMS reading (Ruling out BMS sampling wire faults).
- Step 4: Use a torque wrench to check the tightness of the nuts on the problematic cells' busbars.
- Step 5: Review the inverter's charging history to see if the system has reached a 100% full charge in the last 30 days.
Root Cause
The "Wooden Barrel Effect" (Liebig's law). The total capacity of a series battery pack is determined by the lowest cell. Due to continuous deep cycling without full charges, the BMS passive balancer (which usually only activates above 3.4V) never got the chance to work. The capacity drift accumulated over months, causing the highest cell to hit the 3.65V ceiling early, shutting down charging while the rest of the pack is not full.
Corrective Actions
- If ΔV is between 100mV - 300mV (Software Fix):
- Force a grid charge via the inverter. Set a very small charging current (e.g., 5A or 0.05C) and let it charge slowly. Keep the battery voltage near the fully charged state (e.g., 55V) for 24-48 hours. The BMS will slowly bleed off the high cells and let the low cells catch up.
- If ΔV is > 400mV (Hardware Fix):
- BMS passive balancing is too weak to fix this quickly. Open the battery case (Power OFF). Connect a 3.65V DC Power Supply directly to the positive and negative terminals of the lowest cell, and manually charge it until its voltage matches the others. Reassemble and perform a full charge/discharge cycle test.
⚠ ⚠️ High short-circuit current risk! When opening the case, drop metallic tools into the pack can cause explosions. Always use insulated tools. ⚠️ Never use a charger higher than 3.65V to manually charge a single LFP cell, as it will cause irreversible swelling.
💡 💡 “A battery pack is only as strong as its weakest cell.” For Solar + Storage systems, especially in off-grid setups or during continuous rainy days, cell imbalance is the #1 cause of "capacity drop" complaints. Educate installers and users to configure the inverter to perform a "Maintenance Charge" (Grid charge to 100%) at least once a month. This single habit can eliminate 80% of imbalance related service tickets.