R230 Auxiliary Battery, is it knackered?


New Member
Apr 6, 2011
North East England
2007 R230 SL500
Hi all, Strange behaviour of my smart charger makes me think somethings wrong!. Sorry for the long blah blah, but,

I have experienced low power battery problems on previous cars, caused by not using the car over the winter. I got a CTEK XS 4003 smart charger which sorted it, and was big enough to cope with the 100a/h capacity on my last car.

I now have an R230 fitted with a 70 a/h auxilliary battery which I think is an AGM (its a merc one but the labels mostly torn off?) This was fitted to the car about 3 years ago, under warranty.

I connected up the CTEK at the end of last autumn and it took the battery to fully charged and it was left on float charge, changing to ripple 10 days in.

All was well 'till recently when the charger indicated fault/error mode. When reset it charges and when it gets to stage 6 to test the batterys' voltage it diverts into fault error. This suggests there may be a problem with the battery maintaing voltage?

The battery runs fine in the car and no error messages are displayed. I have checked the battery with a multi meter and the voltage is 12.2v. which drops to 11.8v with the ignition on engine off and the lights on.

The charger works fine with other batteries so is not faulty.

Is my battery on its way out or this perhaps because it is an AGM?. Any advice or opinions please

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