The CR2025 battery on the key fob of my 2012 C350 got weak and I was getting the “Replace Key Battery” warning on startup of my car.

I have 6 brand new (Chinese Knock-Off) Harbor Freight CR2025 batteries each of which register greater than 3.0 volts (a few registering 3.24v) Hovever, each of these batteries once installed still gives the “Replace Key Battery” warning. The car will start and the keyless go lock/unlock feature still work. It is interesting to note the red LED does NOT illuminate when you depress a button on the key.

I also have 2 brand new Ray-O-Vac CR2032 which also register above 3.0 volts. The CR2032 is the same diameter but a little thicker than the CR2025. The CR2032 battery works as well as the CR2025, because it is nearly twice the thickness as the CR2025 I suspect it will last longer. One thing about the CR2032 is The key fob battery cover does not close. The reason is due to a rubber gasket/seal that keeps the battery in place when the key fob cover is closed.

Over the years I have amassed several key fobs for the W210 and W204 Mercedes-Benz. So I took one of the battery covers from one of my extra keys and removed the outer rubber gasket from that battery cover. In doing so I removed a lot of the thickness of the seal once. Once removed the battery cover closed properly. I keep the original (unaltered) battery cover in the glovebox so I can switchback to the CR2025 battery whenever I want.

See photos below. First photo you will see the CR2032 and CR2025 side by side with the unaltered battery cover in the foreground. The second photo you will see a close up of the unaltered key fob, and the third photo is the altered key fob battery cover with only the center rubber.

Side View of the Batteries






Close up of unaltered cover






Altered key fob cover with removed rubber