Found a quickfix for my problem:

Bought a tiny USB hub with connector for external 5V power.
The hub is powered by the apc mobile power pack. The N810 can then be charged from the tiny usb hub.
The hub is enough keep the APC mobile power li-ion battery running and the N810 charging.
I wonder how much energy im wasting this way ;-)
Side effect: I can now use 3 usb devices with the N810 which draw more power than the N810 can provide in usb host mode.
Also I can simultaneously charge my digital camera, mobile phone and all those other electronic gadgets that I find in my pockets each day.

Some people mention that the apc mobile power stops delivering power 15 minutes after pressing the button. I can't confirm that. The mini usb hub runs until the battery is drained.