Just replaced battery for first time in my 41 month old C250CGI with 72500km. Was fine until about 36 months (end of warranty period), then failed to start on 3 occasions in last 5 months. On third occasion I relented and replaced battery and the roadside service guy said he really wasn't happy to sell me one as he said the genuine Mercedes ones were closely matched to the finicky controlling software and processors in the car's electrical circuitry. OK, but this meant a visit to MB Sunshine Coast, and $582 for the battery, $145 in labour and about $50 in sundries. Not a cheap exercise! At least I had two excellent coffees, got to look over a couple of CLAs in the showroom while they tried quite hard to sell me one (CLA45AMG on 12 month wait, then Agility finance strongly offered at $1500/month for 60 months, taking it 3 years past end of warranty ... As I drove away I reflected on the price of my current battery in the 6.9 - $235 from Autobarn and fitted by me in about 10 minutes.
For the labour charge on the C, they also elected to install updated software which they said would give them better assurance about electrical diagnostics in future and interestingly, a reduced tendency to battery discharge by accident.
If you have a modern Mercedes this is typical. But once out of warranty, $750+ for a battery I haven't even seen as it's under its plastic cover, is a relatively small cost. Makes you think very carefully about keeping these cars more than about 4 years, even though to date, this has been my only problem and I love the car. It's still on its original Continental SportContact 3 tyres too, though the back ones are now at legal minimum tread. I need to save the mortgage for new XWXs for the 6.9 as I don't want whitewall Maxxis and want to keep originality with the now dainty 14" bundts.
All good fun. Had a great drive in the Blattenhauser today - purred like a kitten. 35 years young. Will the C be around in 31 years from now?