What is BMS failure?

A BMS failure is the malfunction of a battery management system that can lead to unsafe charging or discharging, unreliable state-of-charge readings, and compromised protection features.

What a Battery Management System does

A battery management system (BMS) monitors and protects a battery pack. It tracks cell voltages, temperatures, and state-of-charge, manages balancing, and enforces safety protections such as overvoltage, undervoltage, overcurrent, and thermal cutoffs. It also facilitates communication with chargers, vehicle controllers, or hosts to coordinate safe operation.

Why BMS failures happen

Faults in a BMS arise from a mix of hardware wear, environmental stress, and software shortcomings. Below are the most common categories of causes observed in contemporary battery packs across electric vehicles, energy storage systems, and consumer devices.

Common causes of BMS failure include:

• Sensor or measurement faults: voltage and temperature sensors drift or fail, leading to incorrect state-of-charge or thermal readings.


• Communication failures: degraded CAN/SMBus/I2C links cause data loss or misinterpretation of cell conditions.


• Firmware/software bugs or miscalibration: faulty balancing logic, protection thresholds, or watchdog behavior.


• Thermal and environmental stress: overheating, moisture ingress, corrosion, or mechanical impact degrade electronics and connections.


• Aging and degraded cells: imbalanced or damaged cells stress the BMS’s protection and balancing circuits.


• Hardware failures: faulty protection FETs, regulators, power rails, or PCB traces can prevent proper disconnection or monitoring.


• Manufacturing defects or improper assembly: poor solder joints or connector issues lead to intermittent faults.


• Improper charging or abusive use: consistently high C-rates, deep discharges, or unstable charging conditions strain the BMS and batteries.


• Power-supply or harness issues: voltage spikes or ground faults in the BMS supply and wiring.


• Electrical transients or ESD: static events or surge conditions can damage sensitive electronics.

Understanding these causes helps technicians diagnose why a pack is not operating as intended and guides design improvements to reduce recurrence.

Common signs that the BMS has failed or is failing

Owners and technicians look for a pattern of symptoms that point to BMS problems, rather than isolated battery faults alone.

Typical symptoms include:

• Inconsistent or incorrect state-of-charge readings that wander unusually or lag behind actual usage.


• Unexplained protective events such as abrupt, frequent overvoltage, undervoltage, short-circuit, or thermal cutoffs.


• Cells that fail to balance, or persistent cell imbalances despite balancing attempts.


• Communication errors with charging equipment or vehicle controllers (diagnostic codes, warning lights, or failed data exchanges).


• Unusual temperature readings or trapped heat in areas of the pack, even under normal use.


• Pack or system goes into a degraded or limited power mode, reducing available output.


• Failure to power up or intermittent operation of the BMS itself (no fault code, but no monitoring).

Detecting these symptoms early can help prevent safety incidents and extend the life of the battery system.

What to do if you suspect a BMS failure

Immediate steps focus on safety and preventing further damage. Work with qualified personnel when dealing with high-energy packs.

Recommended actions:

• Cease charging and discharging for the pack and disconnect from loads if safe to do so.


• Do not bypass protections or force the pack to operate outside its design limits.


• Inspect the system for obvious signs of damage, moisture, or loose wiring; check fuses and connectors.


• Consult the manufacturer’s diagnostics or service manual to read fault codes and logs from the BMS.


• Update or reflash BMS firmware only if the manufacturer approves and provides a verified release.


• Schedule a professional inspection or module replacement if faults persist, especially for high-energy packs (EVs, stationary storage).

Timely, professional diagnosis helps ensure safety and prevents cascading damage to cells and ancillary systems.

Prevention and maintenance

Proactive care can reduce the likelihood of BMS failures and improve reliability across battery systems.

Preventive measures include:

• Maintain proper thermal management to keep cell temperatures within design limits and prevent thermal runaway risks.


• Perform regular diagnostic checks, including sensor calibration and module health assessments.


• Keep firmware and software up to date with manufacturer-recommended releases and patches.


• Use certified chargers and follow recommended charging procedures to avoid unusual current or voltage spikes.


• Avoid deep discharges; operate within the pack’s specified state-of-charge range.


• Ensure connectors, harnesses, and seals are intact and protected from moisture, corrosion, and vibration.


• Design consideration: build in robust protection logic, redundant pathways where appropriate, and clear fault reporting for technicians.

Adopting these practices helps extend BMS life, maintain safety margins, and reduce unexpected outages.

Summary

BMS failure is the malfunction of the battery management system, which can compromise safety, protection, and performance of a battery pack. Failures arise from a mix of hardware, software, and environmental factors, and they manifest as erratic readings, protection trips, communication issues, or degraded performance. Recognizing symptoms, following safety-first troubleshooting steps, and investing in preventive maintenance and quality components are essential to minimizing risk and keeping energy storage systems reliable.

What does BMS mean on a Toyota?

Battery Management System
BMS stands for “Battery Management System.” It's the assembly that houses and keeps the batteries operational. The BMS monitors battery levels, optimizes battery performance, and controls battery temperature.

What happens when a BMS fails?

If a Battery Management System (BMS) fails, the battery can become unusable, unsafe, or both, as it loses its ability to protect against overcharging, over-discharging, and overheating. A BMS failure can cause sudden power loss, reduced efficiency, or even cause the battery to shut down or overheat and potentially catch fire.
 
Common consequences of a BMS failure

• Overcharging or over-discharging: A failed BMS can lead to cells being charged beyond their safe voltage limits or discharged too deeply, which can shorten the battery's lifespan or cause catastrophic failure. 
• Overheating and thermal runaway: Without proper temperature monitoring, a battery can overheat. For some chemistries like ternary lithium (NCM/NCA), this can lead to thermal runaway, where the battery becomes extremely hot and can explode or catch fire. 
• Cell imbalance: A failing BMS can stop properly balancing the voltage between cells in a pack. This results in some cells being overcharged while others are undercharged, leading to poor performance and reduced lifespan. 
• Sudden power loss: In high-voltage systems like electric vehicles, a BMS failure can lead to a sudden and unexpected loss of power, which is a significant safety hazard. 
• Reduced efficiency or inability to charge: A failed BMS may prevent the battery from charging or providing power, rendering it unusable. It can also lead to a general reduction in overall efficiency. 
• Device malfunction and potential fire: Ultimately, a failed BMS can lead to the failure of the entire device and, in the worst-case scenarios, result in a fire or explosion. 

What to do if your BMS fails

• Do not bypass the BMS: For large batteries, it is extremely dangerous to bypass a failed BMS because you will lose all the safety protections it provides. 
• Do not use the battery: A failed BMS means the battery is no longer safe to use and could cause serious damage or injury. 
• Troubleshoot and replace: A failed BMS needs to be repaired or replaced. You should check for any simple issues like loose wiring or a short circuit first, but if those are not the problem, the BMS itself is likely faulty and needs replacement. 
• Look for a secondary safety device: Some battery systems include a secondary safety measure, such as a thermal fuse, that can shut the battery down if the BMS fails to. 

How to fix BMS communication fault?

Connection To BMS Failed

1. Ensure the BMS is powered when connecting to the BMS..
2. Verify the correct baud rate is selected.
3. Ensure proper wiring of the CAN bus, including proper termination of the network.
4. Check the status of the LEDs on the CANdapter.

How do I reset my BMS battery?

A Battery Management System (BMS) reset is a process that recalibrates the system, often necessary after replacing a battery or if the system is malfunctioning. Common methods include a vehicle-specific manual reset, such as flashing the high beams and pressing the brake pedal, or a more general method of letting the battery discharge and then fully charging it. Always check your owner's manual for the exact procedure for your vehicle.
 
This video demonstrates a manual BMS reset procedure for Ford vehicles: 59sSmackey's GarageYouTube · Jan 27, 2024
Manual reset (vehicle-specific)

• For many Ford vehicles:
  1. Turn the key to the "on" position without starting the engine, or just put the car in accessory mode.
  2. Make sure the headlights are off.
  3. Flash the high beams five times.
  4. Press the brake pedal three times.
  5. Watch for the battery light on the dashboard to start flashing, which indicates the reset is complete.
  6. Turn the vehicle off and then start it normally. 

Discharge and recharge method (for EVs and hybrid vehicles) 

• For electric and hybrid vehicles:
  1. Drive the vehicle until the battery charge level is 10% or lower.
  2. Let the battery sit and stabilize for at least an hour.
  3. Charge the battery to 100% and continue to charge until the vehicle no longer accepts any more energy.
  4. The system is now recalibrated. 

Other methods

• Disconnecting the battery:
  1. Fully charge the battery and disconnect the power source.
  2. Disconnect the negative terminal, then the positive.
  3. Press the brake pedal for about 30 seconds to drain any residual power.
  4. Wait 30-60 minutes before reconnecting, positive first, then negative.
  5. Some systems may have a specific reset button on the battery terminal itself. 

You can also watch this video to learn about a general approach to BMS reset by discharging and recharging the battery: 38sEFFICIENTSELFYouTube · Nov 5, 2022