I have a couple of battery packs from Costco.
They have a claimed capacity of 7500mAh, a claim that I think needs a healthy dose of salt consumed with it.

I think the claims are all a little exaggerated, the Lithium battery for my GS 'only' has 4Ah ie 4000mAh and is rather larger than these portable units.

A while back I was looking at getting something akin to the XP1 mentioned above having seen similar at the bike show. Anyway looking around the net produced claims of capacities up to 57Ah in something the size of a pack of fags - thats bigger than the battery in my lads car!

Anyway to the OP's dilemma, battery pack (of proven quality) charged on bike during day, charge the comms/phone at night