Robem

My wife's 2012 C250 W204 CGI fails to crank. Sometimes.
First off, disclosure, it still has the original AGM battery AND the P0304 error which I'm all over but just need the time to fix it.
One day, after for sitting for a handful of days, it failed to crank. I checked the battery voltage at 12.25v. Charged it up and it was ok for weeks of running to the local shops. Always cranked without hesitation. Then without warning it would fail to crank again.
It's done this a number of times now and each time I've checked the voltage before pulling out the battery charger and it sits between 12.2v and 12.4v.
I've loaded tested the battery and it passes as 'good'.
I've charged the battery and let it sit for a week, checking for voltage drop everyday. It sat around 12.45v for the week.
When it fails to crank, I sound the horn and it's fine. The headlights all seem fine too. So it's not like the battery is totally flat.
Thought it could be the starter motor but after charging it cranks straight away. Likewise when I jump start it (if it's parked on the street and I want to get it in the yard closer to the charger) with the emergency battery that lives in the garage. And this relatively little lead acid battery normal sits around 12.45v.
On the face of it, the battery tests as ok and I want to be sure it's past it before I spend over $500au on a new one.

cyberdanycool

Dear Robem, I don't trust expiration dates. For all of my vehicules, I do a full charge on each service for each lead-acid batteries (dont forget the little one in the trunk...). I use a Smart battery charger (around USD 120) with the reconditionning feature activated (work also for AGM). Then, I check my battery with a cheap tester (there is trillions of them on Amazon for USD 30) to be sure she have still the ability to deliver the amperage requested. For me, this is the only way to be sure I won't be stuck somewhere. Best regards.

Robem

I hear you.
I've got a 7 stage charger and all the gear but to no avail. The battery can be fine for weeks then suddenly not start the car.
This is our first Merc and first experience with an AGM battery. I was wondering if this behaviour was a Mercedes thing.

cyberdanycool

My c204 C350 (2011) never did that. I still have the OEM batteries on board. Dont know if they have been already changed, but for the next, I will probably buy aftermarket one, Bosch or Varta brands. The starter one will cost me equivalent AUD 280. AUD 150 for the auxiliary one.