Mr. Toad

My car has been parked in my garage for two weeks now and is not connected to a charger. How often must it be driven in order to maintain the battery charge?

northridgeaz

I haven't driven mine in a month and it will still start right up. I just tried it and no problem.

Mr. Toad

Wow! Good to hear; I wasn't expecting that.

laramr

The car battery holds up fine. My key fob batteries not so much. Does everybody else have to replace key fob batteries three times a year?

MikeO3

Never changed any in 21 months of ownership.

Edit: I have a C238 which maybe different fob then yours.

northridgeaz

I replace my key fob batteries once a year as that is about as long as they last.

Mr. Toad

I believe the proximity of the fob to the car has an impact on the fob's battery life. Check your owner's manual; I think you'll find instructions there for temporarily deactivating a fob to keep it from continuously pinging with the car (it's a button sequence).

laramr

Thanks. They sit in the house about 20' from the car so that may be the case.

WIN ER

Frequent battery change (FOB) on E-Class is pretty common. I am changing mine every 3-4 months. Guess I am too lazy to reprogram the key

Mr. Toad

The key I use regularly is kept in my bedroom at the back of the house. My spare key is kept in a kitchen drawer just inside the door coming from the garage. I did the following (from pages 85-86 of the owner's manual) and have only replaced one battery in 2.5 years of ownership.

Deactivating and activating
If you do not intend to use a key for an extended period of time, you can deactivate the KEYLESS-GO function of the SmartKey. The SmartKey will then use very little power, thereby conserving battery power. For the purposes of activation/deactivation, the vehicle must not be nearby.

To deactivate: press the Lock button on the SmartKey twice in rapid succession. The battery check lamp of the SmartKey flashes twice briefly and lights up once, then KEYLESS-GO is deactivated.

To activate: press any button on the SmartKey or insert the SmartKey into the ignition lock. KEYLESS-GO and all of its associated features are available again.

Mr. Toad

Update: After exactly two months of complete dormancy (parked in the garage and untouched), I started my car for the first time this past weekend and took it for a drive. To my surprise and great relief, the engine roared to life on the first touch of the Start button, as if it had just been driven the day before. The total drive was probably about 30-40 miles (at least half of which was on the interstate), and the Eco feature did not shut the engine off at stops until the return leg of the journey. I have learned from experience that a weak battery will prevent the Eco feature from functioning, so I took this as an indication that the battery had been well-charged by the trip.

Harold1898

I change the fob battery each 6 months.

Nyshadow

Yeah i've had the same experience with my E400. I don't dive it often as i have 2 other vehicles, it has sat for a month with out being touched and cranks up fine every time. Sometimes i will put a trickle charger on it to top off the batteries.

WRC-LVR

Yeah , i'll be moving the key fobs further away from the car too