BTW, you might want to know before you open the case that the battery you are replacing is labeled "ML1220/2E2E0X", which is a small watch-style cmos button battery soldered to a wire pigtail, so it's ready to plug in.
They can be found on [http://www.ebay.com/sch/i.html?_nkw=ml1220+battery|ebay] or [http://www.amazon.com/dp/B005J1UC30|Amazon]; if you're paying more than $5 including shipping, you're being ripped off.
Another option is un-solder the original, find a generic ML1220 battery, and then solder the old pigtail connector to it, using shrink-wrap tubing to seal things back up.
[This battery is probably identical to the very common CR1220, both being 3V lithium button batteries. Dunno. Switch to a different model if you don't mind the uncertainty and risk.]