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How to Reset the Battery Management System

The high-voltage battery pack in an electric car is often rated for hundreds of miles of range. The average range for an EV in 2021 was 217 miles with the longest-range model rated for 520 miles. These vehicles are powered by large, heavy Lithium-Ion battery packs, and if the maximum battery range is less than you expect when it’s fully charged, you might be led to believe that the battery is degrading.

That’s not always the case, and sometimes, it just takes calibrating the battery management system, or BMS. You might be new to electric cars or unaware that the BMS needs to be reset from time to time. When it’s completed, you’ll have a more accurate picture of your battery health and the true driving range.

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What is a Battery Management System (BMS)

What is a BMS? Like any battery, from chemistries including lead-acid and lithium-ion, degradation is a factor that needs to be watched. Over time, batteries will lose the ability to recharge to their original capacity, and it’s the Battery Management System, or BMS, that keeps watch over the battery pack. The BMS is responsible for monitoring all of the factors that contribute to its State of Health, or SoH including the current, voltage, and temperature. It also balances the battery pack to ensure each cell in each module has the same voltage.

With the data it monitors and collects, the BMS controls how much charge the battery will accept and projects the range until the battery reaches its discharge limits. 

What causes BMS failure

Like the electronic modules in a car with an internal combustion engine, calibrations can sometimes go off-track. The BMS for battery electric vehicles is typically thrown off when it can’t monitor the battery over a broad State of Charge (SoC) range.

What does that mean? The BMS needs to measure the battery’s health when it’s fully charged as well as at various other charge levels. The BMS does this checkup on the battery’s status when the battery pack is stable, which is usually after it’s been sitting for several hours. That’s when the kinetic energy inside has settled and become consistent across all the cells.

If it’s usually plugged into an EV charger immediately after it’s parked, the BMS will only check the battery’s health when it’s fully charged or close to it, and the calibration slowly slips out of its normal values. It also happens if the SoC is kept in a mid range – say 70% – and it only goes on short trips.

Ideally, the BMS would experience the extreme charge ranges occasionally to measure the SoH across a spectrum of charge levels.

Symptoms of a failing BMS

As an EV battery’s ‘brain’, the BMS is crucial to maintaining safe and long-lasting performance. Since it’s responsible for keeping tabs on the safety limits, variations in its calibration will usually narrow the window of how the battery can be used. 

Symptoms of improper BMS automotive calibrations can include: 

  • Reduced electric vehicle driving range.
  • The display shows less than 100% when it’s fully charged.
  • Longer charging times than normal.
  • Shorter lifespan of the EV battery.

How to reset a BMS

Fortunately, the BMS calibration can usually be reset. It’s a good idea to perform a BMS reset if your EV doesn’t get the opportunity to get stable readings across a wide range of charge levels. Dealerships may have a procedure that can reset or calibrate the BMS on the spot, but these steps can help bring the BMS data back into normal ranges and restore your range.

1. Bring the battery to a low state of charge

Drive the car until the charge level drops to 10% or below. Single-digit SoC is ideal. When the battery is discharged to this level, the BMS is able to monitor the battery and more accurately determine the lower limits of the discharge rate.

2. Let the battery charge stabilize and monitor

To ensure the BMS can monitor the EV battery pack, let the battery stabilize for at least an hour while keeping the systems ‘awake’. All this takes is checking your car in its app or locking or unlocking the door.

3. Charge the car fully

After an hour or so, charge the car completely. If you’ve set your battery charge limit below 100%, change it back to 100% so the BMS can calibrate most accurately.

After the battery has been fully charged, check the range on your display. It should read its full capacity again. If it doesn’t, verify that you’ve dropped the SoC below 10% in the first step. While one cycle is usually enough for a BMS reset, it might take two cycles for some vehicles. Or if you decide that it isn’t something you want to tackle on your own, let AutoZone help you find qualified professional mechanics through our Shop Referral Program.

FAQ/People Also Ask

How do I reset my car battery?

Resetting your EV’s battery management system essentially involves retraining it to detect the upper and lower limits of its state of charge.

How do I reset my BMS on my car?

The process may vary according to make and model, but the BMS reset procedure usually entails discharging the battery to single digits and recharging it completely.

What causes BMS failure?

In most cases, BMS failure is caused by improper charging procedures or keeping the State of Charge in a small window.

How long does it take for a battery to reset on a car?

Typically, it doesn’t take any longer to perform the BMS calibration procedure than it normally takes to recharge the EV battery.

Advice, how-to guides, and car care information featured on AutoZone.com and AutoZone Advice & How-To’s are presented as helpful resources for general maintenance and automotive repairs from a general perspective only and should be used at your own risk. Information is accurate and true to the best of AutoZone’s knowledge, however, there may be omissions, errors or mistakes.

Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

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