Another way to keep the car alive while changing the battery is to hook up the front jumper ports (under the hood) to another car like you are jumpstarting.

Then remove the battery from under the seat taking extra care not to touch the battery positive cable to any body sheet metal (I use a very thick rag and immediately wrap it as it comes off the battery post)

This is because all mercedes will have a dead cigar lighter socket when the car is off - and you cannot leave the key in On position like on other cars to backfeed from the lighter - as the EIS is not mechanical switch but a computer module itself.

You can try the OBD port - it has a 12V live pin but I dont recall if its always hot or hot on key only.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)