get another battery..
for sure sounds like that one is beaten...
turn off your device and plug it in the charger..
charge it fully..
if the prob still persists ...it's the batt.
the os may "never" been reflashed...which could be adding to the prob.
but with a proper new battery...[even a recently purchased original official oem bp-5l that is old.. may be in fabulous shape...I have 8 official original bp-5l's 5 of them came with the n800's I bought...three didn't ...all of them I have been using ...some for a looonnng time now...half a decade + ...and none of them are below 94% to this day...good track record...]

once with a battery that will charge and keep said charge properly...
I would suggest immediate reinstallation of the os.
then of course clone the base [latest and last os.. with no tweaking...] diablo 2008 for n800 [from the internal "on device"] to a sd card in the internal sd card slot...then from that cloned base os on the internal card...install cssu, telescope, asui, ...diablo turbo [the most recent uber tweak for the n8x0...] and then you will find your battery and your system can be optimized to the ~nth degree ...
you'll be laughing...