If the battery in your smart key runs low, you may start having trouble locking or unlocking the car, or even starting it. Once the battery is completely drained, the key may stop working altogether, which makes it difficult to access or start your vehicle.
Most Mercedes models will display a “Key Battery Low” message on the instrument cluster when the battery is weak. Don’t ignore this warning—it’s best to replace the battery right away to avoid being stranded.
Mercedes smart keys typically use a CR2025 or CR2032 lithium coin cell battery. Here’s how to replace it:
How to Replace the Battery in a Smart Key
1. Remove any keychain or leather cover.
Take off the leather case or any accessories attached to the key fob.
2. Open the back cover.
On the rear side of the fob, press the small release tab and slide the black cover backward. You’ll now see the battery compartment.
3. Take out the old battery.
Use your fingernail or a small flathead screwdriver to carefully pry the old battery out.
4. Insert the new battery.
Make sure the flat side of the coin cell (the + side) faces upward. If you insert it the wrong way, the key won’t work.
5. Reassemble the fob.
Slide the compartment back in and snap the cover into place.
6. Test the key.
Press the buttons to lock or unlock the car. If it doesn’t respond, check the battery orientation again or gently clean the metal contacts with a cotton swab.