2017 C43 W205 Draining new Start Battery

[Eirik]

Installed myself a new Starter battery (fully charged) yesterday and 5 hours later my MB app told me that the battery was critical low and needed to be charged
Any idea or top tip on what can cause this and if there are something I can check myself.
I have an iCarsoft if that could be used in any way?
The spec for the new battery is: Exide START-STOP AGM - EK800, 80Ah, 800A
I guess handing it over to a MB garage here in Norway for fault finding will be expensive (hourly rate £135,-)

 

[ChrisPDuck]

That's a heavy drain - something like 10A maybe? That seems like a lot for one of the regular vehicle consumers to be responsible for. Alternator perhaps? Failed diodes and/or a faulty regulator can cause such a drain. Do you have a multimeter? Is the alternator wiring accessible on your car?

 

[Eirik]

I have a multimeter and was thinking of test every fuse to see if i can find something.
Also i will try to connect the iCarsoft and see if a fault comes up
The battery is on charge now and hopefully i will start tomorrow evening looking into it.
So every tip and suggestions are welcome

 

[ChrisPDuck]

Fuses are a good place to start. If you don't find the fault that way, you could maybe check the alternator diode pack, as per this video:

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You don't have to remove the alternator to do this, but you do have to remove the battery cable from it to do this test, so disconnect the battery before touching that terminal. In fact, disconnecting the battery might be enough, but I think it's possible that might leave other diodes in the circuit somewhere, so best to disconnect the wire from the alternator I think.