It's the standard half-AA battery.
A LC2 will NOT work without one or with a low-voltage one. Believe me, I know this from experience.
What mine did was something like this:
Turn on.
Wait for about 30 seconds, booting normally.
Bong and reboot.
Bong and reboot.
Bong and reboot.
As far as I know, it wouldn't stop ever if I left it on. Any further hard-resets would cause it to go straight to the bong-reboot cycle.
NOTE: When replacing the battery, take the old one out and leave it overnight(or 2 nights) and then put the new one in and leave that overnight. Then you can boot.