Replace Key Battery Message
Thread Starter
Senior Member
Joined: Jul 2008
Posts: 296
Likes: 65
2015 E400
Replace Key Battery Message
My 2015 E400 Cab has two sets of keys, the battery voltage in both were over 3v, but on fob would trigger the 'Replace Key Battery Message'. I used Energizer Lithium C2025s in both fobs, but only one would trigger the message. I tried swapping the batteries, and but the fob I use daily would not clear that message. Then I read somewhere that these fobs are very finicky and they seem to work best with Panasonic batteries. I ordered the Panasonic batteries and as soon as I put the Panasonic in, the red light on the fob worked and no more message. Crazy...
Junior Member
Joined: Nov 2013
Posts: 25
Likes: 0
From: South Carolina
E350; GLK 350 4MATIC
Voltage is voltage, dumkopf....
My 2015 E400 Cab has two sets of keys, the battery voltage in both were over 3v, but on fob would trigger the 'Replace Key Battery Message'. I used Energizer Lithium C2025s in both fobs, but only one would trigger the message. I tried swapping the batteries, and but the fob I use daily would not clear that message. Then I read somewhere that these fobs are very finicky and they seem to work best with Panasonic batteries. I ordered the Panasonic batteries and as soon as I put the Panasonic in, the red light on the fob worked and no more message. Crazy...
Super Member
Joined: Aug 2004
Posts: 954
Likes: 245
E550 Coupe 2wd (2016)
I wrote a whole spiel about having the same issue, and short batt life. My fix was to charge the batt to a higher voltage, like 3.45, and problem solved. I also modded the key to take a bigger 2032 batt.
If the batt dies early, and you want to try my way, search for my post.
If the batt dies early, and you want to try my way, search for my post.
Thread Starter
Senior Member
Joined: Jul 2008
Posts: 296
Likes: 65
2015 E400
It's not necessarily brand, it's more min voltage and drain. The Panasonics are known to be over 3.2v and don't drain as fast. My other batteries, were over 3v but not quite 3.2. Really quirky spec, not sure if it's really documented anywhere but buried in a forum or YT. The main issue is if the fob no longer shows a red light when you press a button, you know the voltage has dropped below 3.2 and you need to replace. I can't explain why the Energizers worked for my spare key, i.e., red light worked and car recognized.
Last edited by pl626; Mar 28, 2026 at 06:44 AM.
