You will need:
- two strong hands (or one weak hand plus a lever, e.g. flat blade screwdriver)
- replacement CR2025 3v Li-ion 'button' battery
- optional: 5-level mortice key
1. remove the corner ring (it'll snap the plastic corner off if you don't)
2. the lower section of the back panel lifts off in the direction shown (you could use a flat blade screwdriver, or twist a 50p, but I just used my index finger and a bit of elbow grease) - WARNING: do this above a desk, as the battery will probably fall out and you will want to put it back in the fob as quickly as possible, not much good if it slips down the edge of the driver's seat.
3. I imagine at this point you would be replacing with a brand new battery. Single Duracell CR2025 batteries with good lifespan are about £1 each delivered on eBay, but you can get 10 'unknown' brand for that price, and over 10 years will you notice much difference? Probably not. The original battery in mine (Panasonic) is nearly 10 years old and still working fine but I've replaced it as a matter of course. Li-ion batteries usually have 10 year "best before" shelf life, but they will clearly last much longer than that. 10 years would be my absolute maximum recommended for these fobs, unless you have a spare battery or fob in your pocket all the time.
Tip: Use a permanent marker to write the expiry date on the new battery's top surface, so you know when to next replace it!
4. Cover is replaced just as easily, just line up the ring corner edge first, and give it a good firm shove back into place using your thumbs.
5. Replacing the small ring is a lot more fiddly, I used a 5-lever mortice key to prise & hold the ring apart whilst I shoved it back onto the fob.
