Car Key Fob Battery Dying? Signs, Battery Types and How to Change It

A car key fob battery rarely dies without warning. The range shrinks first. You find yourself pressing the lock button twice, or walking right up to the car before the indicators flash. On a keyless car the dashboard might say "Key battery low" or "Key not detected". Most of the time the fix is a coin cell from the supermarket and two minutes with a small flat screwdriver.

This guide covers how to tell the battery is the real problem, which battery your key fob takes, how to change it without cracking the case, and what it means when a fresh battery makes no difference. It comes from Affordable Car Key, a mobile auto locksmith in Croydon. A fair number of the "my key has stopped working" calls we take turn out to be a flat cell, so it's worth ruling that out before you pay anyone.

Signs your key fob battery is running low

• The range keeps shrinking. The remote used to work from across the car park. Now it only works a few steps away.
• One press isn't enough. You have to press lock or unlock two or three times.
• There's a dashboard warning. Plenty of Ford, Volkswagen, BMW and Toyota models show a key battery message on the instrument cluster.
• Keyless entry has stopped, but the car still starts when you hold the fob against the start button or steering column.
• The little LED on the fob is dim, flickers or doesn't light at all.

One exception: if the remote fails in one particular spot but works everywhere else, the battery may be fine. Strong radio interference near some transmitters and busy car parks can drown out a weak remote signal for a while.

Which battery does my car key take?

Nearly every remote key and smart key runs on a 3-volt lithium coin cell. The type is stamped on the battery itself, so the quickest answer is to open the fob and read it. Your owner's handbook lists it too, usually under keys or maintenance.

The code is easier to read than it looks. On a CR cell, the first two digits are the diameter in millimetres and the last two are the thickness in tenths of a millimetre. So a CR2032 is 20mm across and 3.2mm thick, and a CR2025 is the same width but 2.5mm thick.

• CR2032: by far the most common car key battery, used across many makes.
• CR2025: same width, thinner.
• CR2016: thinner again, often found in older remotes.
• CR1620 and CR1632: smaller 16mm cells for compact fobs.
• CR2450: a wider, thicker cell used in some larger smart keys.

Swapping a CR2032 for a CR2025 because it's what you have in the drawer is a gamble. The thinner cell can lose contact when you press a button, and a thicker one can stop the case closing or crack it. Buy the exact type.

A safety note if you have small children: coin cells are dangerous if swallowed, so keep spares in their packaging and out of reach.

How to change a car key battery

1. Release the emergency blade. Most smart keys have a small slide or button that frees a metal key blade. The slot it leaves is often where the case splits.
2. Find the seam and the notch. Twist a small flat screwdriver or a coin gently in the notch. A layer of tape over the tip stops it marking the plastic.
3. Photograph the old battery before you take it out. You want to be sure which side faces up.
4. Fit the new cell by its edges, positive side the same way round as the old one.
5. Press the halves together until they click all the way round, then test lock, unlock, the boot button and keyless entry if your car has it.

Most keys carry on working straight away. A few older remotes, including some Peugeot and Citroën models, lose their sync and need a short reset afterwards. The handbook procedure usually means switching the ignition on and pressing the lock button shortly after.

How long should a key fob battery last? Most last somewhere between two and four years. Keyless fobs tend to go sooner because they talk to the car whenever they're close to it. A key that lives on a hook by the front door, a few metres from the car on the drive, is in near-constant contact. Moving it further from the car helps the battery and makes relay theft harder too.

The car will still start with a flat fob

This surprises a lot of people. The transponder chip that talks to the immobiliser doesn't use the fob battery at all. The car powers it through an antenna coil, which is how a key with a dead battery can still start the engine.

On a push-button car there's a backup position for exactly this. Depending on the make, you hold the fob against the start button, against a marked spot on the steering column or in a dedicated slot. The handbook shows where. To get in, slide out the emergency blade and use the driver's door lock, which on some cars is hidden behind a small cap on the handle.

It works the other way round as well. If the car's own battery is flat, a brand-new fob battery won't unlock anything, because the car can't receive the signal. If the dashboard is dead, look at the car battery first.

New battery, still not working?

When a fresh cell changes nothing, the fault is somewhere else. The usual suspects:

• Water damage. A trip through the washing machine or a puddle leaves corrosion on the battery contacts, usually a white or green crust.
• A cracked solder joint. After a few drops, the battery holder or a button switch can break away from the circuit board. The giveaway is a key that only works if you squeeze it.
• Worn buttons. The rubber pad wears through and the button stops making contact.
• Lost sync. The remote and the car no longer recognise each other and need reprogramming.
• A failed transponder. The remote unlocks the doors but the engine won't start, or the immobiliser light keeps flashing. That's a chip problem, covered in why transponder keys matter.

A locksmith can check whether the fob is transmitting at all, which tells you quickly whether the problem is the key or the car. If the fob does need replacing, we cut and code a new remote or smart key at your car, and on most makes we can program a replacement car key in the same visit.

If you're stuck in Croydon or South London with a key that won't respond, call 07400 007588. There's no call-out fee, and we agree the price on the phone before anyone sets off.