I have read posts about this warning. How can you tell which battery needs replacing? I have a 06 E350 4 Matic. I only get the code when the car sits a day or two. It starts perfectly.. I reset the code and it wont show up again until it sits for a day or two.

 

if the car starts fine, the battery in the trunk is probably weak. the starter/small battery under the hood is for the starter.

 

I thought the main battery was for starting and the small one was back up for safety electronics?

 

Are you sure. I am inclined to agree with Jay..

 

Yes you are correct, the aux battery is there basically help back up the electronics if the main battery is weak. My mistake, some cars use the battery under the hood for the starter. But its a bigger battery. This is not the case in this 06 E350.