12V battery charging problems - no-start

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I thought this may help someone in the future as it helped me.


I have a 2015 B electric drive The dealer let the car sit for 3 months while ordering a part. During that time the 12 volt battery went down to Zero. when I got he car back within 2 weeks the car wouldn't start. I learned it I disconnected the battery fro 10 seconds and reconnected it it would start. I replaced the battery because its voltage was a little low, that was fine for a few days and the same thing started happening again. I believe the charging circuits were damaged by the actions of the dealer? Did anybody else have similar problems and Know what the solution is?

it can be more complex that replacing a MB 12v battery. dealer may have to reset several systems which went offline. did not happen to me with my Bs but twice with my ICE MBs
Here is my current experiment, I am using a 3 amp AGM battery charger to keep the 12 volt battery charged, charging it on opposite nights of the HV battery charging nights. so far so good. I am also going to search for a bad fuse. Otherwise I believe it was likely damage by the dealer when they tried to charge the 0 voltage battery.