How to Replace the Key Fob Battery on a Mercedes-Benz
⚙Tools Needed
- Small flathead screwdriver or coin
- Plastic pry tool or guitar pick (to avoid scratching)
★Supplies Needed
- CR2025 lithium coin cell battery (Panasonic, Energizer, or Duracell recommended)
Step-by-Step Instructions
- 01
Identify your key fob type
Mercedes-Benz has used several key styles. The most common modern key is the "Chrome Key" (2006-present): a rectangular key with a chrome strip and flip-out mechanical key blade. Older vehicles (pre-2006) use an infrared key that looks like a thick plastic rectangle. The process differs slightly for each type.
- 02
Remove the mechanical key blade (Chrome Key)
On the Chrome Key, locate the small sliding latch on the back of the key fob near the bottom. Slide it sideways and pull out the mechanical emergency key blade. This blade is used to manually unlock the driver door if the fob battery dies. Set the blade aside -- you will need it to access the battery compartment.
- 03
Open the battery cover
With the key blade removed, look into the slot where the blade was stored. You will see a small vertical tab. Use the tip of the emergency key blade or a small flathead screwdriver to push this tab sideways. The battery cover on the back of the fob will slide off or pop open, revealing the CR2025 battery.
- 04
Remove the old battery
Note the orientation of the existing battery before removing it. Typically, the positive (+) side faces up (toward the back of the fob). Gently pry the old battery out using a plastic tool or your fingernail. Avoid using metal tools that could scratch the contacts or short the battery. If the battery is stuck, a small piece of tape pressed against it can help pull it out.
- 05
Install the new battery
Insert the new CR2025 battery in the same orientation as the old one, with the positive (+) marking facing the same direction. Press it firmly into the battery seat until it clicks into place. Ensure the battery sits flat and the contacts are making proper contact on both sides.
- 06
Reassemble the key fob
Slide the battery cover back into place until it snaps shut. Reinsert the mechanical key blade into the slot until it clicks and locks. The fob should now feel fully assembled with no gaps or loose parts.
- 07
Test the key fob
Stand next to your Mercedes and test the lock and unlock buttons. The turn signals should flash when locking or unlocking. Also test the trunk release and panic button. If the fob does not work immediately after battery replacement, hold any button for 5 seconds to reinitialize the communication. In rare cases, the key may need to be resynchronized with the vehicle by inserting it into the ignition or EIS slot.
⚠Warnings & Safety Notes
- 01Use only a CR2025 battery for the Chrome Key. A CR2032 is the same diameter but thicker, and may not fit properly or could damage the contacts.
- 02Handle the new battery by the edges. Oils from your fingers on the flat surfaces can reduce battery life.
- 03Do not dispose of lithium coin batteries in regular trash. Take them to a battery recycling drop-off (available at most auto parts stores and hardware stores).
- 04If both key fobs have dead batteries simultaneously, use the mechanical emergency key to unlock the driver door, then insert the key fob into the EIS slot (between the front seats or in the dashboard) to start the vehicle.
Frequently Asked Questions
How long does a Mercedes key fob battery last?+
A Mercedes key fob battery typically lasts 2-4 years depending on usage. The vehicle will display a "Key Battery Low" message in the instrument cluster when the battery is getting weak, giving you advance warning before it dies completely. Replace it promptly after this warning appears.
My key fob does not work after replacing the battery. What should I do?+
First, verify the battery is installed in the correct orientation (+ side up). If that is correct, try holding the lock button for 10 seconds to resynchronize. If the fob still does not work, insert it into the EIS slot in the vehicle and cycle the ignition. As a last resort, the key may need to be reprogrammed at a dealer or specialized shop.
Can I start my Mercedes if the key fob battery is dead?+
Yes. Use the mechanical emergency key blade to unlock the driver door. Then place the key fob directly into the EIS slot (the slot where you would normally insert a key, located on the dashboard or between the seats). The vehicle can read the transponder through direct contact even with a dead battery. On push-button start models, hold the dead fob directly against the start button and press.
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