Key has an RFID or similar chip. Older models the chip was a little box you could move between keys. The diamond model the ID is a soldered on chip.

The battery is almost never the problem. I refurbished a few of these and reflow the solder is what fixed them. For a while. Eventually i bought a new key and was the best $ ever spent on my e 53

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)