Where the battery sits and what you need to buy
The battery in a Mercedes keyless entry fob is a small coin-cell battery, usually a CR2032 or CR2025 depending on your fob model and year. You can buy these batteries at any pharmacy, grocery store, or big-box retailer for $3 to $8. The fob itself does not need to go to a dealer — you can open it and swap the battery yourself in about two minutes.
Before you buy a replacement, you need to know which battery your fob uses. The easiest way is to open the fob first and look at the battery itself — the size and model number are printed right on it. If you cannot open it yet, call a Mercedes dealership parts department with your vehicle identification number (VIN), and they will tell you which battery fits your specific fob. Some Mercedes fobs use CR2032, others use CR2025, and a few older models use different sizes, so guessing wrong means a wasted trip.
Key Takeaways
- Mercedes keyless fobs use coin-cell batteries (usually CR2032 or CR2025) that cost $3 to $8 and are sold at pharmacies and grocery stores.
- You can open most Mercedes fobs by sliding a flat tool into the seam on the edge and gently prying the two halves apart.
- The battery sits in a metal holder; note which side faces up before you remove it, then slide the new one in the same way.
- After you replace the battery, press buttons on the fob to test it before you leave the store or drive away.
- If the fob still does not work after a new battery, the problem is likely the fob itself or the receiver in your car, not the battery.
Opening the fob without breaking the plastic
Most Mercedes fobs have a seam running around the edge where the two plastic halves meet. Slide a flat tool — a butter knife, a flathead screwdriver, or a plastic pry tool — into that seam on one side. Push gently and rock the tool back and forth; the two halves will separate. Do not force it or you may crack the plastic.
Some newer Mercedes fobs have a small slot or notch on one edge instead of a full seam. Look for it before you start prying. If you find a slot, insert your flat tool there first — that is the designed entry point and will open more easily than forcing the seam.
Once the fob is open, you will see the circuit board and the battery holder. The battery is usually a small metal cup or clip holding a round coin-cell battery. Do not touch the circuit board itself; just focus on the battery.
Removing the old battery and installing the new one
Before you touch the battery, note which side is facing up. Coin-cell batteries have a positive side (usually marked with a + symbol) and a negative side. The positive side almost always faces up in Mercedes fobs, but looking first takes two seconds and prevents mistakes.
Slide the old battery out of its holder. If it is stuck, use a plastic tool or your fingernail — never a metal tool that could short the circuit. Once the old battery is out, slide the new battery in the same direction, with the same side facing up. You should hear or feel a small click as it seats in the holder.
Do not force the battery. If it does not slide in smoothly, you may have it upside down or the wrong size. Remove it, check the orientation, and try again.
Closing the fob and testing it
Once the new battery is in place, line up the two halves of the fob and press them together firmly until you hear a click. The seam should be flush all the way around. If there is a gap, the halves are not seated correctly — open it again and check that nothing is blocking the fit.
Before you leave, test the fob by pressing the lock and unlock buttons while standing near your car. You should see the lights flash or hear the locks respond. If nothing happens, press the buttons a few more times — sometimes a new battery needs a moment to make full contact. If the fob still does not work after 10 seconds of testing, the problem is not the battery.
What to do if the fob still does not work
A dead fob after a new battery usually means one of two things: the fob itself is broken (water damage, worn buttons, or a failed circuit board), or the receiver in your car is not working. You cannot fix either of these yourself.
If you have a second Mercedes fob, test it on your car. If the second fob works, your first fob is the problem and needs replacement. If neither fob works, the issue is in your car's receiver, and you will need to take the car to a Mercedes dealership for diagnosis. Do not buy another fob until you know which part is actually broken.
If you have only one fob and it does not work after a new battery, you can still drive your car — use the physical key inside the fob to unlock the door and start the engine manually. Then contact a dealership to have them test the receiver and either repair it or program a new fob to your car.
Storing extra batteries and fob care
Keep a spare CR2032 or CR2025 battery (whichever your fob uses) in your glove box or at home. Coin-cell batteries last three to four years in a Mercedes fob under normal use, so you may not need the spare for a while, but having one means you are never stranded with a dead fob.
To extend battery life, avoid leaving your fob in direct sunlight or in a hot car. Heat drains coin-cell batteries faster. Also avoid dropping the fob or exposing it to water — water damage is one of the most common reasons a fob stops working, and no battery replacement will fix it.
Frequently Asked Questions
Can I use a different size battery if I cannot find the exact one?
No. CR2032 and CR2025 are different thicknesses, and using the wrong one may not fit in the holder or may not make proper contact. Stick with the size printed on your old battery or confirmed by the dealership. Most stores carry both sizes, so finding the right one is not difficult.
Do I need to reprogram the fob after changing the battery?
No. Changing the battery does not erase the fob's programming. Once you install the new battery and close the fob, it should work when ready. If it does not, the fob is broken or the car's receiver is not working — reprogramming will not fix either problem.
How long does a Mercedes fob battery usually last?
Most coin-cell batteries in Mercedes fobs last three to four years with normal use. If you use the fob heavily (pressing buttons many times per day), the battery may drain faster. If you rarely use the fob, it may last longer. Heat and water damage shorten battery life significantly.
What if I drop the battery and it rolls away?
Coin-cell batteries are small and straightforward to lose. If this happens, do not panic — just buy another one. They cost a few dollars and are sold everywhere. Keep the new one in a safe place until you are ready to install it, and consider keeping a spare at home so you always have a backup.
Is it safe to handle the battery with my bare hands?
Yes. Coin-cell batteries are safe to touch with bare hands. Wash your hands afterward if you prefer, but there is no danger from handling them briefly. Avoid getting the battery wet or exposing it to extreme heat, and do not try to take it apart or short the terminals intentionally.