Problem Description
- Case #007
- Fault Name: One Cell Voltage Too High
- During the charging phase, the battery system abruptly stops accepting charge even though the total SOC displays a value well below 100% (e.g., stuck at 85%). The inverter indicates that the battery has cut off the charging current, preventing the system from utilizing the available solar or grid energy fully.
Symptoms
- The BMS turns OFF the "Charge MOS" prematurely.
- The ALM (Alarm) indicator flashes or stays solid red during charging.
- The BMS software reports "Cell Over-Voltage Protection (COVP)", pointing to one specific cell (e.g., Cell #3) hitting the upper limit (e.g., 3.65V).
- The moment the charging stops, the high cell's voltage might drop back down quickly.
Possible Causes
- Severe Cell Imbalance: Accumulated capacity drift over months of operation without full charges; the leading cell fills up much faster than the rest.
- High Internal Resistance (IR): The specific cell has aged or degraded, causing its voltage to artificially "spike" under high charging currents (Ohm's Law: V = I * R).
- BMS Sampling Error: The BMS voltage sensing wire has poor contact, or the BMS ADC chip is faulty, causing a false high reading.
- Weak Passive Balancing: The BMS passive balancer (usually bleeding only 30mA-100mA) is simply too weak to hold back the leading cell during a heavy 50A charge.
How to Verify
Monitor the BMS PC software during an active charging cycle. Watch the "runner" cell. Compare its voltage to the pack average. Once it triggers protection and charging drops to 0A, closely watch how much the cell's voltage drops instantly. Next, use a multimeter to measure the physical cell terminals to rule out a false BMS reading.
Required Tools
- BMS PC Software & Communication Cable
- High-Precision Digital Multimeter
- Power Resistor (e.g., 5W 10-Ohm Ceramic Resistor) or a 12V Halogen Car Bulb (Used as a manual "bleeder")
- External Active Balancer module (Optional)
Normal Values
- Max cell voltage at 100% SOC: ~3.45V to 3.55V (for LiFePO4)
- Voltage Drop when charge current stops: < 50mV
- Max voltage difference (ΔV) at the end of charge: < 80mV
- Cell #3 reaches 3.65V (triggers alarm) while Cell #8 is barely at 3.30V.
- When charging stops (0A), Cell #3 instantly drops from 3.65V to 3.40V (indicating extremely high internal resistance).
Diagnostic Procedure
- Step 1: Verify reality vs. software. Measure the physical voltage of the highest cell. If the multimeter says 3.35V but BMS says 3.65V, fix the BMS sampling wire harness.
- Step 2: If the physical voltage is high, observe the voltage drop upon cutoff. A massive instant drop (>150mV) means the cell is degraded (high resistance) and needs replacement. A slow stabilization means it's just a capacity imbalance.
- Step 3: If it's a simple imbalance, check the inverter settings. Was the charge current set too high, giving the BMS no time to balance?
Root Cause
Due to slight manufacturing variances, not all cells have the exact same capacity. A cell with slightly less capacity will always "fill up" first. If the user charges the battery rapidly (e.g., 100A), this smaller cell hits the 3.65V safety ceiling rapidly. The BMS protects this cell by turning off the overall charge, effectively leaving the other larger, healthier cells undercharged.
Corrective Actions
- Software/Trickle Charge: Lower the inverter's Max Charge Current to a very small value (e.g., 5A). This slows down the charging process, giving the BMS's built-in passive balancer time to burn off energy from the high cell while the lower cells catch up.
- Manual Bleeding (Hardware): Power OFF the battery. Open the casing. Connect a power resistor or 12V bulb directly across the positive and negative terminals of the highest cell only. Discharge it manually until its voltage matches the pack average.
- Active Balancer Injection: Temporarily plug in a high-current (e.g., 5A) active equalizer board across the cell pack for 24 hours to rapidly transfer energy from the high cell to the low cells.
⚠ ⚠️ NEVER increase the BMS over-voltage protection limits to "force" the battery to 100% SOC. Pushing a LiFePO4 cell beyond 3.65V continuously will cause the electrolyte to gas, expanding the aluminum casing (swelling) and permanently destroying the cell.
💡 💡 “The runner finishes the race first, but ruins it for the team.” Installers often falsely believe a cell hitting High Voltage means it is a "strong" cell. Paradoxically, the cell that jumps to 3.65V the fastest is usually the weakest (lowest capacity) cell in the pack! If manual bleeding and slow charging cannot fix the issue, that single cell has degraded and must be replaced to restore the pack's full usable capacity.