Yesterday I had an interesting experience with my MY 2017 E300. Parked by my local Walmart a couple miles away to take my daily 3 miles walk. Parked the car and within less than a minute all doors locked, rear sunshade went up and Panoramic sunshade closed as well

Hit the start button and received display message in the instrument cluster to place key in slot and check user manual. I figured the battery for the key was dead. I have a leather protective case on the key and needed the small screwdriver to remove the key. Unfortunately this along with the replacement battery was at home. I had removed everything from the car sometime ago when it went in for scheduled routine service

I hit the info button on the overhead console and spoke with a representative who provided me a toll free number for MBUSA Customer service which would be opened at 9:00 AM Eastern Time. Called a couple minutes after 9:00 AM and received recording. Called back a couple minutes later and received recording again so I left a message.

Used the Mercedes Me app to lock the car. I then noticed that the mirrors did not fold in as it usually does . Went for my walk and was back in about an hour. Tried starting the car again and still no lock. Called again an no answer so I decided to walk home for the small screwdriver and replacement battery

Locked the car again with Mercedes Me app and noticed the mirrors folded in this time. I immediately unlocked the car with the app and tried starting the car. I was in luck, the car finally started and I immediately drove home and changed the key battery. I then placed the small screwdriver and another batter in my armrest.

Hopefully this will help others to remember to place your key in the charging slot periodically to keep it charged up or keep a replacement battery in your car. Has anyone else ever experienced anything like this. During the week I will call that MBUSA Customer service to see if there is a way to start the car with a dead key

I may have been incorrect on charging the key in the slot. It’s the emergency operation mode

 

I don't think the slot is a charging one. It simply reads your key when the battery is dead or can't communicate for other reasons. When my car's main battery got very low I could not start as usual. Once I placed the key in that slot it started right up.

 

I believe you are correct: it is a slot to read the key FOB - not charge the battery in the FOB.

I recently changed the battery in my FOB: Mercedes wanted $11 for the battery, bought two on EBay for $1.99 - not from China: In any event when I used the FOB with the new battery, the car would not start and the notice on the dash was to insert FOB into slot. I did that and the car started. Thereafter I was able to lock, unlock and start the car with the FOB in my pocket.

Just my $.02.

 

I may have been incorrect on charging the key in the slot. It’s the emergency operation mode. I just added the information from the user manual