In reply to Heike:
> But how do you open the back without tools?
It's easy. If it's an 'unscrew' type you go to Maplins and you buy a watch-back remover for £6 (a three-legged adjustable spanner with varying sizes of bits). If it's a pop-off type they will also have the little pointy 'cuticle' tidier that does that job. Maybe some jewellers screwdrivers too - all of it is only a few pounds.
Choose the right 'bits' for your new spanner and adjust it to fit the watch back. Hold the watch firmly in a tea towel (stops pointy bits digging into your hand) and uncrew it. Take the back off carefully and note where the rubber seal sits if there is one. Take a note of which way the battery sits (posiive up or down) and remove it. There may be a small screw holding it in place but frequently not. Obtain a new battery - Maplins (although their batteries are crap IMHO) or from a decent independent jeweller (better). Should be less than £5.
Put the new battery in. Make sure that the seal is in place. Take a cotton earbud and put a tiny wipe of vaseline around the thread on the back (not on the thread in the case). Refit the back and then tighten it with your spanner. I go as tight as I can, with the limiting factor being my ability to hold it tight in the tea towel. At this point, if the tool slips, the tea towel stops you unscrewing a slice of your hand.
Take a note of the battery size and keep it with your spanner, so next time you can buy the battery before you take the back off.
I decided to do it myself when I got fed up paying a fortune for a watch to be pressure tested, then had to take the back off myself in a very smart jewellers because they weren't strong enough to do it themselves, and then I asked a friendly jeweller about any hidden pitfalls (which is where the vaseline thing comes from).
I'm happy doing very expensive watches and cheap ones. My casio altimeter one is a PITA - too fiddly, and on that basis I'm actually happier doing it myself than giving it the Saturday boy.
Battery life tends to be longer than new watch warranty, so that's not an issue. if it is, buy the same battery type off the internet. My wife sees no value in climbing kit but does like shiny things, so I do her watches as well as mine.
If your eyesight isn't that brilliant a magnifying glass makes things easier - I have some tweezers with a lens attached.
Post edited at 16:25