Somewhat ridiculously, the description here only says 20mm (diameter) battery, but not the specific model (ie: thickness). According to Sparkfun's site, this takes a CR2032.
E
Eric
Too useful to stop buying these.
I really just want a through-hole 2032/2025 clip to solder onto PCBs. These never really fit that well into my projects but having the switch included makes them too convenient to stop. It also seems that LilyPad makes these "wearable" items quite robust.