Grabbed on of these from ebay to see how it might perform.  23000mAh (yep..23,000) with a max output of 19v/3A (12v and 16v and USB also options).  I expect it will be good for my 817 (max 2A on TX) but will also be able to hold my laptop up during a contest without having to resort to a generator.   Lots of different size connectors and cable options.  The only combination missing is Andersons, so looks like a trip to Jaycar for a suitable power connector & then attached a pair of power poles.