Here some alternative ways to charge the tablet...

Left a $100 solar panel (8V / 0.5A) with a nokia car charger and adapter from nokia plug to n810. On a sunny day it takes the same time to charge than on the mains.
Right a cranking charger. Also takes the same time but 3 hrs cranking is a bit tedious ... but for emergengy runout is is ok for a few minutes.....

Is just for fun (or on the way when the battery is dead, particularly cranking ...)