BMW key battery: how to replace it and what to do if the key is not detected

A weak BMW key battery is one of the most common reasons for a message saying that the key is not detected. Remote locking may also stop working, or work only when you are very close to the car.

On most modern BMW keys, replacing the battery is a quick and simple job you can do yourself. Some older keys and display keys work differently. In this guide we explain how to recognise a weak battery, how to replace it, which keys are rechargeable and how to start the car in an emergency.

Signs of a weak BMW key battery

Typical signs include:

• A message in the instrument cluster saying that the key is not detected or that the key battery should be replaced.
• Shorter range for remote locking and unlocking.
• Comfort Access not working reliably, so the car does not unlock when you touch the door handle.
• The engine not starting with the start button, while the key is inside the car.

Before replacing the battery, also consider other causes. Strong radio signals, a phone or another electronic device next to the key, or a key stored in a metal case can interfere with the signal.

Which BMW key do you have?

BMW has used several types of keys over the years:

• Older keys with a rechargeable battery: many keys from the 1990s and 2000s contain a battery that charges when the key is in the ignition lock. These batteries cannot be replaced by simply inserting a new coin cell, and a key that no longer charges may need professional repair or replacement.
• Keys with a replaceable coin battery: most newer keys, including many Comfort Access keys, use a standard coin battery that you can replace yourself.
• Display keys: the key with a small touchscreen offered on some models has a rechargeable battery that is charged with a cable or a charging tray.
• Digital keys: some newer BMWs can use a smartphone or a key card as a key.

Your owner's manual shows the type of key and the correct battery for your car.

How to replace a BMW key battery

On most modern BMW keys with a coin battery, the process is:

1. Remove the mechanical emergency key by pressing the release button or slide on the key.
2. Open the battery cover, often by using the emergency key or a small plastic tool in the slot that becomes visible.
3. Note the position of the old battery, especially which side faces up.
4. Remove the old battery carefully with a plastic tool to avoid scratching the contacts.
5. Insert the new battery of the correct type, usually a CR2032 or a similar coin cell, with the same orientation.
6. Close the cover firmly and refit the emergency key.
7. Test the key: lock and unlock the car and start the engine.

Use a quality battery from a known brand. Cheap batteries can have a shorter life or slightly different dimensions.

Starting the car when the BMW key battery is flat

If the key battery is completely flat, you can usually still start the car. BMW provides an emergency method:

• Unlock the door with the mechanical emergency key hidden inside the key fob. On some models, the lock cylinder is hidden behind a cover on the driver's door handle.
• Place the key at the marked emergency position, often on the steering column, in the centre console or in a marked area of a storage tray. The car reads the key at very short range, even with a flat battery.
• Press the start button with your foot on the brake or clutch.

The exact position is shown in the owner's manual and often in a message in the instrument cluster. Note that the alarm may sound when you open the door with the mechanical key; it stops when the car recognises the key.

Looking after your keys

• Keep the spare key in a safe place and test it from time to time.
• Replace both batteries at the same time if both keys are of the same age.
• Avoid dropping the key, which can damage the circuit board.
• Keep the key away from water. Dry it immediately if it gets wet and remove the battery if it was submerged.
• Store the key securely at home. Keyless entry signals can be misused by thieves, so keep keys away from doors and windows, or use a signal-blocking pouch where appropriate.

If a key is lost or damaged, a BMW service partner can supply and program a new key. Proof of ownership is required.

When the problem is not the battery

If a new battery does not solve the problem, other causes include:

• A damaged key circuit board, often after a drop or water damage.
• An antenna or control unit fault in the car.
• A weak car battery, which can cause many electronic problems. Our article on BMW battery registration explains why the car battery matters.

A BMW specialist can test the key and the car's keyless system with diagnostic equipment and find the cause.

BMW key battery at a glance

Frequently asked questions

What battery does a BMW key use?

Many modern BMW keys use a CR2032 coin battery, but check your owner's manual because some keys are rechargeable or use another type.

How long does a BMW key battery last?

Often several years, depending on use. Comfort Access keys can use more energy because they communicate with the car more often.

Can I start my BMW if the key battery is dead?

Yes. Unlock with the mechanical emergency key, hold the key at the marked emergency position and press the start button.

Do I need to reprogram the key after changing the battery?

Normally not. The key keeps its programming when the battery is replaced.

Sources

• BMW owner's manuals – vehicle key and emergency start
• BMW Group – digital key information