I recently bought an N-series after-market charger because my Ebay-special N810 came without the charger and the seller has been real slow about sending it to me! Anyway, the charger is only be able to do about 400ma and often cycles on and off (I think it is the N810 internal charging circuit doing that because the external charger's voltage drops down to low for it). Anyway, the net effect is that the charger is able to charge the batteries in several hours, or run the device, but not both at the same time (When using the N810 while plugged in, the battery charges at a very very slow rate)

So it is a good idea to purchase a charger that can actually do the rated 890 ma current rate.