Key battery losing power/dead? (Black fat key, not silver triangle key)
The replacements are very cheap (I found an 8-pack on ebay for $4.00 shipped) -- CR1220 Lithium Battery.
Just use a knife to pop open the back of the batter (there's a rectangle with a little spot where you can put the knife to pop it off). Unscrew the two phillips screws (need a pretty small screw driver), and just replace the battery. Make sure you put it in right side up