A couple of months ago my E550 2011 began draining the battery overnight. This happened after one day that I moved my car and it displayed a message like “take car to a shop without changing gears”; I turned the engine off and the motor fan stock in high unresponsive for some time. When I turned the car back on I was not able to shift gears, however after several minutes it began shifting like normal. The next morning the battery was dead. I took the car to an auto parts mega store to test the battery and they said that although it was still good it would require replacement shortly; so I decided to put a new battery (AGM type). After replacement, my car was starting OK every morning until the weekend arrived. When sitting for more than 24 hours the battery drained completely (4.9 V). The same happened every week. I took it to an independent mechanic (specialized in MB and BMW), that a friend of mine recommended, but he could not figure out what the problem was, even with the specialized scan tools he has including the software by MB.



I followed a procedure that requires testing every fuse for voltage drop. I finally found that fuse #27 (ME-SFI control unit or engine computer) had the highest voltage drop (I think it was about 16 mV). Then I followed another procedure to measure the amps. It resulted in a current draw of 1.8 A! Bingo, when I removed the fuse (#27) the current dropped to almost zero. I left the fuse out overnight and the battery stopped draining.



Now I am wondering what is causing the ME-SFI control unit to stay awake. Is it the computer itself that is malfunctioning or another system that is trying to communicate with it? Could it be a communication Issue? Any help or experience is welcome.



Some other things that I have observed during the past couple of months include:

1. One day a red battery displayed on the dashboard and the car began to disable systems until it completely stopped. I had to call road assistance to tow the car. I put the battery to charge overnight and next day it was working OK. The mechanic that I took the car to tested the alternator and it passed all the tests.
2. I have monitored the battery voltage (using the hidden menu) on the dashboard. Every morning the battery voltage is between 11.1 and 11.3 and it still starts. For the first 10 to 15 minutes of driving the voltage fluctuates between 13.5 and 14.5V and then drops to 12.6 for the rest of the trip. I normally drive about 45 minutes to work and when I get to the parking and turn the engine off the battery voltage is about 12. 4V. When I leave work the voltage is about 11.6 V.
3. When the battery drains and I disconnect the battery for charging or when I start jump the car it takes some time and several attempts for the shifter (located by the steering wheel) to shift gears. Also “Auxiliary Battery Malfunction” illuminates on the dashboard when I turn the engine on.
4. In other occasions, when the main battery drains, the keyless entry does not work after connecting the charged battery. If I disconnect and reconnect the battery the keyless begins to work.

 

Do you have ANY aftermarket electrical items in the car? Radio, remote start, lights, radar detector, Anything? Those are often the problem area. Are the ANY other electrical devices that do not work correctly?

Your car depends on the various modules to go to sleep, when the car is parked and they usually take 30 minutes, maybe 1-hour max to go to sleep. Then your draw should be very low, 125 mv or less. So if the car periodically turns on (maybe faulty module) and wakes up the car, the higher draw starts over again until it sleeps again. and then wakes up again over and over and over again. Lots of Youtube videos showing how to find the parasitic draw. When the draw is constant, this job is much easier; when it only happens sometimes, much harder to discover the source of the problem. I recall seeing a video where the rear wiper module would turn on periodically thru the night and the battery was low by morning. I think this video helps explain.

 

I had one car with a parasitic drain and it turned out to be the electric trunk latch. These drains can be a nightmare to sort. I wish you good luck with the fix.

 

I have too, my 2003 would leave the front passenger interior door handle light on, my 2007 would flash the left mirror turn signal in the garage at night intermittently, I was never able to figure out either, dealer either. twice in the last five years or so the puddle light passenger side rear view mirror would be on in my '13, haven't figured that out either. none were a constant drain however.

 

Change your AUX battery as you are getting alarm for it. It could be your whole problem as if the battery is bad it could be pulling current and drain the main battery. And you don’t waste anything with changing it even if it didn’t solve your problem as it obviously needs to be replaced anyway.

Around $60 at dealer, cheaper ones available from other sources. Located in driver side end of dash.

 

OP, very nice job of explaining the problem and listing the issues.

As KEY08 said, tracking down parasitic drain is difficult and time consuming. I haven't had the issue on my E550 thankfully.

Keep us posted with what you find, and good luck getting this solved.