So here it goes. I changed both rear (main) and front (aux) battery cuz I had red warning on dash. That didnt help, so I ran the thest with multimeter and readings were like this: rear- 12,8 (14,15) and aux 12,9 (12,9) so as far as I can see aux is not charging. Took the readings after day and they were the same so no need battery drain either. That could mean aux battery relay has to be changed. Went to MB stealership and checked if it has done recall on BCM (software update) and that was done by previous owner so I could swipe out BCM... Now here comes the tricky part. When I get the red warning, I cant start the car (sometimes it starts after 30 turns), and if I get the warning, it stays on untill I clear the errors by disconnecting battery (tried driving the car for all day without turning it off and didnt help). While I have warning on, all the readings with multimeter stays the same within normal. When I jumpstart the car, it goes up just fine but with warning on.

So how can it be, that the readings are good with exception aux battery's not charging but holding the charge 12,9 (which is strange), but warning still comes up and cuz of that my car doesnt start.

I cleared all errors and could drive for 2 days and now all this nonsense is back. And for last two weeks I'm digging through forums and trying to figure out how can aux battery affect engine start (it has nothing to do with that).

Any suggestions?