C300 Battery Critical Warning

[DavyBee]

I have a 68 plate C300 Coupe and since it's not used so much I was getting the battery critical warning through the MercedesMe app. In February I
had the battery replaced by Halfords with a 70Ah AGI unit but again it wasn't used much so started getting the critical battery warnings again in June. It was serviced in July and I asked MB whether the battery was ok. They checked and said the battery was in good order but it won't charge by driving only short distances and infrequently so suggested a charger and conditioner. I bought the 2 amp version of the NOCO Genius charger as I wasn't too worried about the speed of charging. It never seemed to charge the battery beyond 50% so I contacted NOCO who said I should get the 5 amp version as there may be a drain which is bigger than the charger can supply. I tried with the 5 amp version and the same happens. I connected a multimeter and it's showing 12.1 volts. But I also checked for parasitic drain and on the 10A setting, is showing 12.3 amps. Apparently it should be no more than 50mA.
I'd appreciate any suggestions to get the battery fully charged.

 

[Blobcat]

Hello and welcome,

I’ve 8 Optimate/Ctek battery conditioners in use on my “fleet” all keep them fully charged. I recommend connecting yours and leaving it on.

12.3 a draw is huge, are you sure everything was switched off?

 

[mioba]

To get the battery fully charged -
sort the drain.
You answered your own question.

 

[star]

Did you let the car go to sleep, it can take quite a few minutes to do so.

 

[DavyBee]

Thanks for the replies. I thought the 12.3 drain was huge so maybe I didn't measure it correctly as presumably with a drain that big the battery would be completely flat in no time? There doesn't seem to be anything switched on, the doors are locked and closed. Any way to check the drain is correct and to identify the reason?

 

[s5tuart]

"But I also checked for parasitic drain and on the 10A setting, is showing 12.3 amps."

12.3 Amps would induce smoke!
I would re check that measurement!

 

[DavyBee]

Thanks this is what I did but will check it again. Would it be an idea to disconnect the battery and charge it?

 

[s5tuart]

Thanks this is what I did but will check it again. Would it be an idea to disconnect the battery and charge it?

If it's something you've not tried, then yes.
That'll double check your connections too

 

[DavyBee]

Update: I disconnected the battery and connected the charger. Previously the charger cycled between 25-50% occasionally to 75% but overnight back to 25%. After less than 10 mins charging with the battery disconnected, it's now at 100% and optimising. How can this be?? That it's almost fully charged from less than 25% in such a short time?

 

[ajlsl600]

Seems likely something not going to sleep. Now u know batt will fully charge, stick it back on ( follow benz instruction on that) then check in am. If near dead again ur looking why. If still OK Yr prob not. So much stuff now req good batt.

 

[DavyBee]

Thanks will check tomorrow morning

 

[SL63 Mark]

But I also checked for parasitic drain and on the 10A setting, is showing 12.3 amps. Apparently it should be no more than 50mA.

Maybe it was 1.23 amps. Still very high.

 

[DavyBee]

I think so too so I'll get it checked out

 

[star]

Just had one in last week, initial draw was 12.7 amps, it then dropped to .5 amp after 8mins. Still too high.

 

[DavyBee]

Did you find out what was causing it?

 

[star]

Yes, it’s not going to sleep, part on order to confirm diagnosis.

 

[rich.g.williams]

Update: I disconnected the battery and connected the charger. Previously the charger cycled between 25-50% occasionally to 75% but overnight back to 25%. After less than 10 mins charging with the battery disconnected, it's now at 100% and optimising. How can this be?? That it's almost fully charged from less than 25% in such a short time?

You are talking about your own charger and how it is configured to view the world. Have to know about batteries even then it's hard to know their state of charge between empty and full.

 

[rich.g.williams]

"But I also checked for parasitic drain and on the 10A setting, is showing 12.3 amps."

12.3 Amps would induce smoke!
I would re check that measurement!

Here is another and very good video about parasitic drain:-

Battery drain testing - good video


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