[JohnH]

OK Greg, I was trying not to do this but I will name names here, the two products I have tried are :

The PowerTech Lithium Powertank and the iFourway Lithium battery. Both were sourced from astronomy suppliers.
I am not aware of the chemistry - I assume LiFePO4- but could be wrong. Both were used only with the charges supplied.

Neither of these units produce more than 12.5V when fully charged and both showed a RAPID but small (0.5 to 0.7) drop in the first 30 mins of use and yes, after that, it is flat(ish) BUT this is of no use to me as the output is then down at 11.8V (and still dropping of course) or so at which point my NUC becomes glitchy. It seems when fully discharged these systems output as little as 9V.
The issue here is not to do with attached load (my rig's current draw is 3A max - but typically << 2A) while the capacity quoted is ok for my purpose (2-3 hrs in the field) it is not accessible in my case, I can only get only to about 25% discharge (according to the inbuilt level meter) before I start to get drop outs.

I was after an integrated, portable solution - I was hoping not to have to build my own but, perhaps sourcing the charger and battery plus relevant connectors etc is a better option. I take your point re capacity and perhaps 100Ah is a better target for me.

NOTE: I also have a small powerbank (probably the nasty LiCo) bought from Officeworks - and that allows me to select an output voltage and will drive my system happily for 3-4 times as long as either of the two options above despite being rated at only 20000mAh...