W211 E200k battery charging question

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Active Member
Jul 4, 2019
Cork , Ireland
2005 E200 1.8 kompressor
Hi, just a quick question, I’m charging the main battery in the boot as car has been idle but soon to be in use, does charging the main battery also charge the auxiliary battery at the same time?
If system is solid, it should not. Aux battery is charged only shortly after engine start, IF BCM detects low aux circuit voltage, AND main circuit voltage is high enough. Otherwise isolation relay, like it says, keep aux circuit isoltaed from main circuit. If engine is not runnning, relay won't pull ever (if not 'welded').
The aux battery is disconnected when car is off so effectively has zero load on it and will last quite a while. 😉

I've got the diagram somewhere for this.
All you need to know about the W211 dual battery system..


  • 319 HO Dual Battery (ACB - ICC) 11-29-02.pdf
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