Battery discharging....My 2009 MB C300 4matic sport seems to be prone to discharging the battery if left unused for a week or so. Originally we replaced the old battery, which was unable to be fully charged, with a brand new one about a month ago. This is my wife's car and she doesn't like to use it too often. I finally recharged the new battery and we drove it today without any trouble . We are seeing the garage on Thursday but they have already tried to diagnose the issue without success or finding any undetected short or draining. Anyone have similar experience and maybe some ideas/solutions to or about what is happening, please?

 

moved to W204 forum...

 

My 2008 C300 has had a slow battery discharge for years. Old battery or new one. I connect a battery maintainer if the car is not going to be used for 3-4 days.

 

A Mercedes Benz should be able to be left at the airport for two weeks and start reliably on your return.

It sounds like your Quiescent Current Relay might not be functioning. It removes most of the load from the battery after the car is parked and the key removed for 6 hours. It will also open if the battery voltage drops below 11.8 volts. It closes and wakes up the systems in the car if a door is opened or the smart key is operated/detected to prepare the car for driving. It’s located on the prefuse bloc under the hood.

You don’t leave the key fob in the car overnight, do you?

The battery sensor and the rear SAM control the quiescent current relay, so the problem could also lie there.

Since the prefuse bloc is a $600 part and Mercedes doesn’t sell individual components in it, JimMBC300’s suggestion to use a battery maintainer or trickle charger seems like the simplest and most practical remedy.

It can be repaired if you:
1-have the patience to test it to be sure that’s the problem;
2-have a bit more than basic DIY auto repair skills;
3-can locate the part in electronics catalogs from the manufacturer’s number on it;
4-have basic electronics repair skills and the tools.