there are a number of threads (just check the tag for battery and battery drain) that discuss this issue. a reboot on diablo will burn up about a full days worth of juice (normally the tablet burns about .5%/hour which translates to about 10-15% battery loss due to rebooting which is why it is suggested not to reboot your tablet. it is more power efficient going to sleep than to turn off.
I recently had an issue where i thought there was a firmware problem with the battery level. it turned out to be a bad battery (mine was 13 months old)