BMW Key FOB Battery Replacement and Car Association

The X5 came with two keys, both of which physically operate the locks and ignition, but only one of which communicated with the car remotely. Today I successfully managed to bring it back to life in two phases.

First of all, I’ve replaced the battery. This involves cracking the key open, desolding the old battery, soldering in a new one, then gluing the key shut again. The first part – cracking the key open – is the hardest.

Initially I thought I could do it with a small flat head screwdriver, but all that did was damage the key plastic. It turns out these things are glued together, so I needed to take a Stanley knife and firmly cut through around one edge. Then I could get a screwdriver in and pop the unit open.

The soldering part was straight forward. I used some generic glue to bond the key part together and left it gently squeezed in a vice for half an hour to go off.

Having done all this, I was pretty gutted to discover that the key didn’t work. I wondered if I needed to re-associate the key and the car in some way. In fact, thinking about it now, maybe that’s all I needed to do anyway, but a new battery is a good thing either way.

I found a variety of instructions online, and I think this was the best of them, but I’ll document my process here for clarity.

The main gotcha was that I managed to associate the second key just fine, but I then realised I had unwittingly disassociated the other key. So here’s what I did to sort it all out.

  1. Put a key (either) into the ignition. Move to position 1. Wait a second. Remove the key.
  2. Pick up a key. Hold down the ‘unlock’ button. While it is held, press the ‘lock’ button firmly three times. Release the ‘unlock’ button. The car will lock and unlock.
  3. Repeat step (2) with the other key.
  4. Insert either key into the ignition, move to position 1, remove the key.

Having done that, both keys now work perfectly.

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