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When do you need to replace your car key battery?

Replacing a key fob is more costly than simply replacing the battery, so it’s worth checking if the problem can be resolved with a new battery.

If your remote key fob has stopped remotely opening the doors, it’s possible that the battery needs replacing. Similarly, if it only opens the doors every now and then, the battery may be getting weak, in which case a replacement is also a good idea.

It’s worth remembering that if the battery in your fob is dead, you can still enter and start the car – it’s the remote locking and unlocking functionality that won’t work.

As the battery only supplies power to the remote opening system, there won’t be any effect on the immobiliser, either. The immobiliser communicates with a transponder chip in the key, which is passive and doesn’t require a battery.

car key

Ensure the battery is the problem

There are several quick checks you can carry out to help ensure that it’s the battery that’s the problem.

Check if the manual locks on the car still work. If they don’t, then it’s a possibility that the issue lies with the car locking mechanism as opposed to the car key or battery.

It’s also possible that security conditions could have disabled the remote entry feature, in which case it’ll need reprogramming.

Of course, if your car came with several fobs, try another one. If the other key successfully unlocks your car, you can assume that battery failure is the issue.

How to replace a car key battery

Thankfully, in most cases replacing a car key battery doesn’t require any specialist skill and can be done quickly and cheaply at home.

Car key batteries are round and flat, and shaped like a coin. Before buying a new battery, it’s worth checking that you’re getting the right one.

This will require opening the fob. Lots of the time, you can do this just using your fingernail, but you may need a flathead screwdriver. Gently pry the fob open – to ensure you don’t damage it, apply gentle pressure from a few locations around the fob.


You may need to lift out the circuit board to reveal the battery. At this point, check what the battery code is. It’ll start with a two-letter prefix, such as CR (see the image below), and your key will only work with a battery with that code on.


Don’t use a battery if you’re unsure of its ID, as if it’s too strong it can result in leakage and damage the fob. It’s not the case that using a more powerful battery will improve the battery life of your car key!

Once you’ve lifted the original battery out, replace it with the new one and clip the two halves of the fob back together.

Top Tips

To save money, buy a bulk pack of batteries for just a few pounds and keep them at home for quick replacements.If the battery is left out of the fob for too long, in some cases this can make the fob stop communicating with the car. At this point, it’ll need reprogramming, which may require specialist help.

If your car key seems to be quickly using up batteries, get the advice of a specialist as you may need a new fob. Remember, batteries should last for a few years.

Also, if you can’t open your fob for whatever reason and looking online doesn’t give you any answers, seek specialist help from an auto locksmith.

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