I have a 2018 S450. The key fob is different from the one I had for the 2015 C300. I have been unable to find anything about starting the car if the key fob battery dies while away from home.
Has anyone run across this info? The old fob could be used as a key by removing the start button, the 2018 version not so much! Maybe there is enough warning from the car to get it replaced before it dies completely...

 

That's the question I would like to know as well. VW Group Brand Cars don't come with removable button and when I asked same question to the VW Dealer the sales agent assured that there was always enough juice to start the car up even with the battery needs replacement .My current car also has a button that's not removable but I have not been into the situation with my current car where I need to remove the button to start the car by cranking up the key .However, in my Mercedes-Benz I had to . The car started up nicely even with dead key fob battery .

 

I just found the answer to my question! A few days ago MB released the on-line PDF owners manual for the S Class 2018 models.I did a search and found what I needed on page 167.
The instructions are to remove the key ring and place the fob on the left hand side of the little compartment directly below the ash tray. There must be an RFID chip or something similar in the fob.

 

Not disputing that it is dead but isn't it a bit surprising that the battery in a fob for a 2018 is dead already? Did something unusual happen to it or do you think it was just defective?

 

No problems with it at all! I noticed the fob had changed and the start button was not removable so I was curious about the new method (if any). I'm looking down the road when I get the low battery warning.
I'm the only one driving the car so the extra fob stays home. I keep the spare fob deactivated to save the battery. The instructions are on page 59 in the 2018 Owners Manual. If you rapidly press the "lock" button twice the fob will switch to low energy mode and preserve the battery.

 

Ah, I see. Thanks for clarifying.

 

I thought the older version where you could pull out the engine start button and use the key fob was a nice idea..