external battery charging option for Asus EP121 tablet pc

Hello all,

A friend posed me with a problem as to how to extend his field time with his Asus EP121 tablet pc. It does not have removable batteries so the option appeared to be to come up with an external battery pack to power the tablet when the internal battery got low on charge.
The output of the DC charger is listed as 19.5 V DC / 3.08A.
A suitable option to me appears to be a Dewalt cordless tool battery pack. The unit in particular is the DCB200 20V MAX lithium ion battery pack, spec'd at 20V at 3.0AH.
Aside from having to come up with an interface between the connector on the battery pack to the jack on the tablet, is this idea advisable or viable?
Thanks,
Gutbucket

gutbucket@bell.net