Mercedes SL55 auxiliary battery charging issue

[Southcoaster]

I’m wondering if I can call upon the expertise on this forum to assist me with a non charging issue on my vehicle...The auxiliary battery in the boot has been connected to a battery conditioner for the past few months and has been effective at keeping both batteries fully charged,however when I went to the car today,both batteries are flat and the conditioner is displaying an error code and will not charge.I borrowed another charger which will also not charge the battery.
My question is,why is the battery now deciding not too charge? any ideas most welcome.

[anonymous-user]

These are notoriously fussy batteries for some reason. My guess is the battery is a goner, probably a cell has gone down. It's interesting that both batteries are flat and that indicates to me there is an issue with one or other of them. Does the main battery accept charge OK (when disconnected from the car).

I would check both of them at an auto electricians, but I suspect the auxiliary battery has died.

A new one is about £120 from Merc so it's not that horrendous, but make sure that is the issue first.

[Monkeylegend]

I keep mine on an intelligent trickle charger connected to the auxiliary battery, and put a new starter battery on about 16 months ago. All was ok until a couple of months ago when the starter battery suddenly went dead and wouldn't take even a direct charge.

Battery replaced under warranty and all is ok again.

These cars are very battery sensitive and maybe it was a dud battery or maybe they don't like being on constant charge. Time will tell if the new starter battery goes the same way. They are only £90 ish though so not a major problem.

[7795]

I had an SL55 back in 2003 and from memory, i had the similar issues as you op. It was under warranty and I think it was an alternator fault/alternator fuse that was faulty...

I also had 2 set of driveshafts replaces, untold air shock absorbers, and the roof creaked and creaked. All done under warranty though; thank God,

[Southcoaster]

Thanks for the aforementioned advices.The problem was itentified as caused by two issues,first the auxiliary battery (in the boot) had failed,it was only two years old,so replaced by Mercedes under warranty,secondly the starter battery(under the bonnet) had also failed,as this was the original,as it had lasted 13 years I wasn’t that surprised! That was replaced for £114.

I am aware that some of these cars have issues but this is my second one a 2005...2006 model which I have owned for 9 years and apart from services (by the main dealer) and tyres,nothing else has gone wrong yet!!!!

[XRS]

Had the auxiliary battery issue on my wife's S205 C250. A month out of warranty. Turns out both this and the starter battery needed replacement, for which the dealer eventually decided to pay for almost all of the cost.
Seems they haven't quite mastered these issues, even today.

[Monkeylegend]

Some Mercs do seem to drain their batteries rather too quickly if they are standing around to much. The SL seems to be one of the main culprits and it will flash up a host of warning messages in the process.

Having said that my W212 E class is now 7 years old, done many miles and the original battery still seems to be ok, fingers crossed.