Hello,
I have done many searches but my issue seems to be a little unique. Any suggestions would be appreciated.
The other day the red battery light came on. I went to the trunk to see some smoke coming from positive side of battery (3 months old). I initially thought cable was loose. The rear battery and front battery are both in the 12's when car is not running. When running the rear is a steady 13.6 at idle but the front is only 12.1 or so. So seems the alternator is functioning. I then thought the BCM. The front battery seems to maintain a charge but a low charge. I did try to hook up communication via a snap on reader and would not read.
Any suggestions would be greatly appreciated. Thanks!

 

How old is the front battery and is it an AGM? The AGM needs a little higher voltage to charge than a wet lead battery. So both batteries should be AGM type. If you see smoke there has to something bad and sounds like high resistance. Are terminals corroded? You have to remove the terminals properly and clean the posts and clamps.

I believe that the MB manuals say nothing about how to remove the batteries for service, charging. Ticks me off. The conventional wisdom is to remove the negative terminal cable of the rear battery first, then the front battery. Now the car has no power and should be safe to work on. I leave the positive on unless I am replacing the battery.

Now here is where it gets interesting! If you need to replace the front battery, or rear, which is the first cable you will connect to the new battery? I always did the positive, but the battery installer says the negative.

Who is right?

 

WIS is pretty clear on this - the starter (front) battery is only to be disconnected or reconnected after the consumer(rear) battery has been disconnected. Negative terminals are ALWAYS disconnected first. The main reason for "negative first" is in case you are turning your wrench and contact the body (ground), it won't create a short circuit.

 

Thank you both for taking the time to help me with my issue. I will check the age of the front battery to day and report back. The rear is definitely still charging. And it does seem the front is getting a charge, it's just low. Thank you again!