I finally got round to changing the batteries in both key fobs today. The recommendation is to replace them every 3 years, otherwise you have to go through a whole load of kerfuffle to pair the fob back to the car. The instructions in the handbook are not entirely clear and I confused myself a bit by trying to skip read it, but its actually dead simple.
- A CR2032 button battery, LR recommend Panasonic.
- A small screwdriver to separate the two sides of the fob.
- Your Land Rover!
- Sit in the drivers seat with the drivers door closed. Keep it closed for the duration of the battery replacement*.
- Turn the ignition to position II, return to position 0 and remove the key.
- Gently split the key fob casing into its two parts, taking care not to damage the rubber seal.
- Slide the old battery out of the clip, avoid touching the circuit board or other components.
- Insert new battery. The manual says don't leave any finger prints on it which can cause premature discharge.
- Push the two halves of the fob back together.
- Operate the key fob at least 4 times to re-synchronise it with the car.
- Turn the ignition back to position II and you're done.
*The immobiliser has a passive re-arm function which activates within 30 seconds if the drivers door is opened. If you don't open the driver's door this period is extended to 5 minutes.
If you only have one fob and the immobiliser re-arms during the battery change you have to go through the re-synchronisation rigmarole, which involves turning the key in the ignition numerous times and opening/closing the drivers door repeatedly.