There also is the high likelihood that the issue is most certainly the battery.
There hasn't...to my knowledge...been any new genuine n810 or n800 batteries made in many years.
It has been a long time since I could find even original used batteries .
There are however many cheap attempts at n8x0 batteries.
Some well meaning attempts ...some not.
So be aware when looking for "new" batteries.

There is a solution to this ...inevitability .
Using a n old dead battery...using the casing ...rather... casing, even a cheap knock off battery casing.
And carefully removing the guts of it, one can hook up wiring to the insides of the battery posts, that lead out of a drilled and rubber gasketed hole in the removable back access (since leaving the battery cover off the device means it will not boot )
The wiring leading to a " step down" that is "tuneable " ..and from there to an electric outlet.
Thereby making ones device "future - proof"