Safety hazards of lithium battery modified BMS
15,Aug 2025
Lithium batteries have become the core energy source for electric vehicles, energy storage systems, and portable devices due to their high energy density and long cycle life. However, the battery management system (BMS)—as the "brain" of lithium batteries—is often overlooked in illegal modification practices. According to data from the U.S. Consumer Product Safety Commission (CPSC), 63% of lithium battery fires in 2023 were related to illegal BMS modifications, with an average of 60 fire incidents per day involving lithium batteries in waste treatment facilities alone. This article will systematically analyze the safety hazards of modified BMS, supported by real cases, technical principles, and authoritative warnings, to reveal the risks behind "pursuing higher performance at all costs."
In October 2022, an 18-year-old teenager purchased a second-hand electric bike that had been resold six times and illegally modified. Only 12 days after purchase, the bike exploded during charging, causing 90% of his body to be burned. He died two days later due to multiple organ failure.
Investigation Results:
The original lead-acid battery was replaced with a lithium battery, and the BMS was modified to remove speed limits, increasing the top speed to over 100 km/h.
The fire originated from the battery compartment under the seat, and the fire department determined the cause was "battery failure."
The court ruled that 7 defendants (modifiers and previous owners) jointly bore 40% liability, paying compensation equivalent to $73,000.
In April 2025, a fire broke out in an electric bike repair shop in an urban area, killing 8 people, including a 10-year-old child. The fire started from a backyard stacked with used batteries and repair tools, and the flames quickly spread due to the lack of fire separation facilities.
Key Hidden Dangers:
The shop used second-hand dismantled batteries for modification, with BMS circuits manually altered to bypass overcharge protection.
Over 200 used batteries were stored outdoors without temperature monitoring, and the charging area lacked smoke detectors.
The original BMS strictly limits the charging voltage (typically 3.65-4.2V per cell). Illegal modifications often increase the voltage threshold or bypass the protection circuit to "improve charging speed."
Technical Principle:
When the battery is overcharged to >4.3V, lithium dendrites grow inside, piercing the separator and causing internal short circuits. Tests show that modifying the BMS to allow overcharging to 4.5V increases the thermal runaway probability by 20 times.
Original BMS maintains cell voltage consistency through active/passive balancing (voltage difference <20mV). Modified BMS often omits balancing circuits to reduce costs.
Consequences:
After 100 cycles, the cell voltage difference can reach 200mV, causing some cells to overcharge while others are over-discharged.
Battery cycle life is shortened by 50%, and the risk of spontaneous combustion increases by 8 times.
| Performance Indicator | Original BMS | Modified BMS | Risk Difference |
|---|---|---|---|
| Overcharge Protection | Voltage ≤4.2V, multi-level protection | Voltage limit removed or ≥4.5V | Overcharge risk increased by 15x |
| Over-discharge Protection | Cutoff voltage ≥2.8V | No protection or <2.5V | Permanent capacity loss ≥30% |
| Short-Circuit Response Time | <10ms | >40ms, some without protection | Fire probability increased by 8x |
| Temperature Monitoring | NTC sensors per cell string | 1-2 sensors for entire pack | Thermal runaway warning accuracy ↓70% |
| Balancing Function | Active balancing (±5mV precision) | No balancing or simplified passive | Cell inconsistency leads to ↓50% life |
| Safety Certification | UL 9540, ISO 26262 compliant | No certification | Non-compliant with safety regulations |
In April 2024, the U.S. Consumer Product Safety Commission (CPSC) issued an urgent warning to recall Unit Pack Power (UPP) e-bike batteries (models U004/U004-1), citing 13 overheating reports, including 7 fires. The agency emphasized: "Modified BMS cannot reliably monitor temperature and voltage, posing fire and burn hazards."
The IEEE Battery Management System Safety Guidelines that modified BMS lacks:
Real-time monitoring of cell health (SOH).
Communication with vehicle controllers (CAN bus data disconnection).
Redundancy protection design (single-point failure leads to total system collapse).
Refuse "Performance Upgrades": Never modify battery capacity, voltage, or BMS parameters.
Choose Certified Products: Purchase batteries with UL/IEC certification, and check the BMS model (e.g., original Tesla BMS).
Regular Inspections: Check for bulging, leakage, or abnormal heat during charging; test BMS functionality annually via professional tools.
Proper Charging Habits: Use original chargers, avoid over 8-hour charging, and never charge in bedrooms or stairwells.
The battery management system (BMS) is the "safety lock" of lithium batteries. Illegal modifications may temporarily improve performance but permanently sacrifice safety. As the NFPA 855 Standard emphasizes, any unauthorized alteration of battery parameters violates international safety regulations and may face fines or criminal liability.
FAQ
A: Yes. According to NFPA 855 Standard and IEC 62133 regulations, unauthorized modification of BMS parameters violates product safety laws in over 180 countries. In the U.S., CPSC enforces fines up to $50,000 for selling modified battery systems, while the EU's CE certification explicitly prohibits BMS tampering.
A: Look for these warning signs:
Missing UL/CE certification labels on the battery casing
Voltage fluctuations exceeding ±0.3V during charging (detectable with basic multimeters)
Unusual charging speed (e.g., 0-100% in <1 hour without fast-charging certification)
Inability to connect to manufacturer's official diagnostic apps
A: Rarely possible. Most illegal modifications involve replacing BMS hardware or overwriting firmware. According to IEEE technical bulletin, only 12% of modified BMS can be fully restored, with 43% developing new faults within 3 months of attempted restoration. It's safer to replace with a certified OEM BMS module.
A: Key standards include:
UL 9540: Covers energy storage system safety
IEC 62133: Defines rechargeable battery safety requirements
NFPA 855: Specifies installation standards for stationary energy storage
UN 38.3: Regulates lithium battery transportation safety