Got this message today while driving the car. It reads "change key battery" and I had just started the car with no problems. No matter what keystroke I used, I could not clear the message, thinking it was just a temporary or false alert. Pressing "return" on the steering wheel only cleared the alert temporarily; checking the status messages it was still saved.

When I arrrived home, I checked the range of the key fob for unusual activity. The range was still remarkable - e.g. I can still set the alarm / set the panic mode (see the lights blink) over 500 feet away! Also, the battery check LED on the keyfob is still working fine! BTW the car was still able to start - just the message continued.

So just for kicks I take out my second (never used) keyfob and try to start my car -- the message is gone! I then tried to start my car using the first key and the "replace key battery" message returned. Apparently the car knows one key from the other. So now it's time for surgery.

I removed the suspect battery and found the voltage to be 3.02V, which seems fine for a 3V CR2025 battery. I replaced with a "new" one, voltage measured at 3.24V and tried to start my MB - all is normal!

Morale to the story:
Appears that MB set the low voltage limit of the RFID keyless go system VERY SAFE - to the point that most batteries would still be rated as good!

SO --- if you have yet to replace the key fob battery on your W212 with keyless go; my advice, keep a few new batteries around; you may need them sooner than you think.

 

Or maybe the battery just needed to be reseated?

You went through alot of hassle for nothing.

 

Re-seating the battery was the first thing I tried. All along, the red led on the key fob was working fine.

 

Yeah, plus I drove around for a few months with that message popping up. I wouldn't worry to the point of carrying extra batteries.... and I also have keyless go. The warning on the IC (as the OP found out) comes on way too early. The low battery warning light on the key FOB is a lot more accurate.

If it gets too low to open the door via keyless go, use the mechanical key in the FOB. Just unlock the driver's door, hop in the car and stick it in the ignition and head down to the drugstore and buy some batteries

 

I had a similar problem with older model of Mercedes. I was really frustrated since car was just 2 years old and certainly battery shouldn't go down considering using the car on weekends only, However problem was corrected itself, I started the car with other key, keeping mine one away. So the warning message gone after and mine key started working as usual. Moral, the system can be triggered for some reason thinking that key battery gone and to make it back of the glitch you have to use other key making sure that the key issues the glitch is away.

 

Seems Keyless Go doesn't come without it's PITA aspects to contrast what it's meant for [ease].

 

pretty cool feature I think. nice to know the car's looking out for you!!

 

Your key uses small flat batteries. Some use a single large battery (a CR2032 I believe) others use two small ones. Pop the panel off the back of the key and you will see two small philips screws. Unscrew them and the back cover should come off. Replace the batteries and then put the cover back on. If you do it quickly, your key should work just fine immediately. If not, then the key will need to be reinitialized....

 

I've have batteries out for hrs and didn't lose any settings at all.

 

Yeah, I've swapped out batteries from the spare smart key and left it without batteries for days on end. Works fine once you put the batteries back in.