corsechris Posted October 18, 2023 Share Posted October 18, 2023 Some may recall I posted a while back about changing the 12V battery in my tin-top, a 2016 MB C350e. It was a bit of an epic for such a simple task given they buried it under the boot floor and some of the crash structure present there, hemming it in with a coolant pump and plumbing for the HV battery. So, next up we have the other 12V battery in the car. Why they need 2, given it has a chuffing great HV battery and a DC-DC converter is beyond me, but anyhoo.... And even more odd, the non-hybrid C class, among others, don't have an auxiliary battery any more, they have a complicated electronic module with 4 super capacitors in to provide this backup supply. The car was misbehaving due to this battery being declared faulty by the onboard diagnostics. Even with a fully charged HV battery, it wouldn't run on it, only operating in 100% ICE mode. At least it had the decency to politely tell us there was an 'Auxiliary Battery Malfunction', although only briefly. Good job Vickie spotted it when she was driving it yesterday. Without that clue things could have been much more expensive. So, where would you put a 12V battery? Under the passengers feet, obvs...... Remove trim panel over passengers feet, remove connection to courtesy light and passenger floor air feed temperature sensor. Remove left side trim piece. Remove carpet (I've seen folk just heaving this back out of the way, but it's a formed piece of carpet with huge chunks of padding bonded underneath and I didn't want to risk damaging it, so I took it out) Remove cover over auxiliary battery. Remove battery. Then replace what you just removed. Simples. Picture below is the passenger floor/footwell area. Battery is the black box with the red insulator on the top left corner. MB want £145 for a new one, no idea what they charge to fit it though. It's a bog-standard BTX-12 format AGM battery, so £45 gets you a decent one by next day from Tayna. Interestingly, I've seen various eBay sellers offering up the 'AUX14' format battery as a direct replacement. Bit naughty as that is physically too tall and unless you took a grinder to parts of the mounting, it would never fit properly. Then again, given it sits totally buried, I guess it could be forced into place and covered up quickly..... After fitting it, the car still wasn't playing ball. still not allowing EV mode, but after a short drive, it decided all was well again and is now back to normal. I have a dedicated diagnostic box for the car, but it isn't the full MB 'Xentry' system by a long stretch, so wasn't showing me any signs of an issue, nor any stored fault codes in any of the many, many (70) modules in the car, so I was relieved when it started to play nice again. No doubt MB would have happily relieved me of £300 to plug in their proprietary box-O-tricks to tick the 'new battery installed' box, had it been required. If/when the next battery fails (the HV one) that'll probably scrap the car. Quote Link to comment Share on other sites More sharing options...
dvd8n Posted October 18, 2023 Share Posted October 18, 2023 Simple jobs on cars can be stupidly complicated these days. It took an entire weekend to change two bulbs on my wife's MX5 last year. D 1 Quote Link to comment Share on other sites More sharing options...
Stuart Posted October 18, 2023 Share Posted October 18, 2023 I changed a headlight on our then 2 year old BMW X3 a couple of years ago after a pigeon flew into it and cracked the perspex. BMW wanted £650 + fitting. I bought a salvage one for £200 and fitted it myself. Had to take the whole front grill off and loosen the top of the wing to exchange it. My BL Mini used to be 3 screws. Guess that's progress...... 1 Quote Link to comment Share on other sites More sharing options...
corsechris Posted October 18, 2023 Author Share Posted October 18, 2023 It’s no surprise insurance premiums are so high is it. A minor front-end shunt could see a bill for £6k 1 Quote Link to comment Share on other sites More sharing options...
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