When does the key fob battery need replacing?

Modern cars are full of electronics, and a smart key fob is no exception. Inside of it, there is an electronic chip that works as a transmitter; it transmits the signal when you press a button to open the door. A transmitter is powered by a little battery. There is also a security (immobilizer) chip that works without a battery, but only from a very close range. An old-style mechanical key may also be stored inside the fob. This key can open the driver's door in case the fob battery is dead or if the main battery in a car is discharged.
Battery and door key inside the key fob Battery and door key inside the key fob.
In most cars, a key fob battery is not rechargeable, and must be replaced once it loses its charge.

What are the signs of a weak fob battery? As the fob battery becomes weaker, the transmitter loses its range. This means that you have to be closer to the car for the fob to work. In cars with a push button start, a vehicle may not always recognize a smart key fob in your pocket or purse if the fob battery is weak. In this case, you may have to hold the fob closer to the "Start" button or insert it into a special slot to start the engine. In some cars, a warning message "key fob battery low" may also pop up on the driver's display.

How can you check the fob battery? In many cars, a key fob has a little LED light that illuminates when pressing any of the fob buttons. If the LED light doesn't come on at all, the fob battery is dead or there is some other problem with the fob. If you already have the fob battery out, you can check its voltage with a voltmeter. The voltage is marked on a battery.
Checking the key fob battery voltage This battery is bad, as it shows only 2.858 Volts. A good 3-Volt battery should show a little more than 3 Volts.
If it's a 3-Volt battery, and it shows less than 3 Volts, it's weak. For example, this 3-Volt battery in the photo shows only 2.8 Volts; it's bad and must be replaced.

How long does a key fob battery last? Typically a fob battery lasts for up to three years, although it is not uncommon for it to fail sooner.

What kind of a battery does a key fob take? Different cars use different batteries. The most common types are CR2025 and CR2032 3-Volt batteries. You can find the exact type in the owner's manual. Batteries like this are sold in any electronics store or at a dealership.

Changing a fob battery is not very difficult. Check the owner's manual for the instruction and precautions. There are plenty of Youtube videos on how to do it too. You might need a small flat or Phillips screwdriver.
For example, to open the key fob in the top photo, we pried it open with a little flat screwdriver in places indicated in the owner's manual. Be careful; plastic tabs that hold the parts of the fob together break easily. In some cars, there could be some tiny screws or other parts that are easy to lose.

When replacing the battery, watch for the way it's installed. One side of the battery is marked with the "+" sign. It's important to install the new battery the same way, or it won't work.

If you are not mechanically inclined, visit your dealer; they should be able to change the key fob battery for you. Typically it doesn't take more than a few minutes. We checked with several dealers and prices we got were within $10. A key fob doesn't need any programming if only a fob battery is replaced.