Yarg.
No ...not fun.
Hmm...I honestly wouldn't bother with battery resus. at this particular time...there are still originals and compats that are much cheaper.
However in the future it may be cost efficient to rejuv. it.

Right now under $10 (including shipping to Canada from the States....price may be more or less depending on specific locations Internationally....so it isn't too bad for a single official batt.) on ebay for a official nokia bp-4l. However you do have to watch for knockoffs. with the official nokia sticker...
So don't go throwing that official Nokia batt away even tho the charge is diminished.
They are still the best thing to use in a n810 and they are starting to get scarce...when the time is right ....get it rejuvenated.
In the meantime I would grab a few official or compat bp-4l batts ... since some sellers offer a deal on multiples . Never hurts to have a few back-up batteries ..ie. one charging while using the other....or esp. for travel. Good thing to have a few.