Hey guys,

So ever since I bought my car from the previous owner, I would get the low battery message after 3-4 days of the car sitting. In about a week, the car would be completely dead. The car only gets driven usually on the weekend...

The main battery is new, as stated by the prior owner.
I replaced the auxiliary battery a couple of months ago.

The car does have keyless go....

I have read every other thread of parasitic draw, but never got a good baseline of how long before you get the error message.

My father's E320 w211 would last 3 weeks without being touched and it has the original battery....but has no keyless go, nav, pano if that makes a difference

Thanks!!!
Chris

 

I just went through a similar issue and will share what I discovered :

(1) Your draw in "MB sleep mode" should be around 20mA or 0.020 amps. I was told the MB spec is less than 50mA, but have not confirmed this. On most other OEMs it is less than 30 mA.

(2) If you have a datalogger plugged into the OBD2 port - disconnect. On my car my OT2 was drawing 0.4 amps even when in "OT2 sleep mode".

(3) Apparently the BCM (battery control modules) can go bad and cause a parasitic draw, as well as the keyless go - so I would start with the fuses that are on these circuits first.

(4) If you have any aftermarket electronics installed, verify these are not the problem.

(5) If you still have not found the source, you will need to go circuit by circuit (removing fuses) to find your problem.

To test the draw, open the trunk and get access to the battery and make sure all other doors are closed. Put your keys (since you have keyless go as far away as possible so as to ensure they are not keeping the system awake. Let the car sit for 30 minutes after opening the trunk.

Loosen the clamp on the negative side of the battery, but do not remove the cable yet. Take an amp meter and connect it in series with the negative terminal of the battery (post) and the clamp on the negative terminal wire. Raise the cable up off the post and read the current draw (this will be your parasitic draw and should be below 50 mA or 0.050 amps). Remove fuses until your problem is found.