Hy all, have a problem here. Long story short. I had a flat battery in the trunk for electrical equipment. I opened car manually start normal, went to garage and checked rear battery it was hot, guess battery shorted inside. Pull ot out and replaced with a new one. Went for a couple hours drive in highway, after about 1.5hour drive stopped for a smoke, came back to a car and it didn't start looks like starter battery flat, jumped positive wire from a starter battery to trunk battery, started right away. Finished my trip, shutdown the engine and tried to start no luck. Charged the front battery manually and been driving for a one week until it got flat again. Connect MB Star diagnostic tool, went to actuations and activated battery coupling relay, connect multimeter at this point it was charging, but as soon as i turned off star it opened again. Was reading around and found that starter battery is charged by some other module next to the trunk battery. Some dc/dc charging converter. Can someone point out what to check next.
Also maybe someone could check for me please, who has mb like mine with 2 battery's after you start the car what is charging for your starter battery in the engine bay?

 

The purpose of the small auxiliary battery is to provide current for shifting the transmission out of park should the main battery fail. This small battery does not have the power nor is it used to start the engine.

 

It is probably just a bad battery..... You would be surprised how many of those batteries we do at the shop. And they have a mind of their own....... Sometimes they start and other times they won't. I have never had and issue with a W221 and the charging of the starter battery.

 

2007 has a starter battery in the front, only from 2009 i think they have this very small battery.

 

I replaced the battery to brand new, still the same.
Maybe someone can check with multimeter how much power is charging after start of the car?

 

Hi,
The 2007 model has two batteries a starter in the bonnet area (not a tiny battery in the interior console area) and a consumer battery in the boot behind the back seat vertical panel (not in the boot floor)
Refer attached.
Grant.