Portable battery using this adaptor spec
Why try and "home-brew" something? 5V out is about as standard as it's possible to get, nowadays. Get a Lithium-Ion power-bank, something like this 26000mAh unit.

It has 4.8A max output current, and 26000mAh will do 4A for 6.5h (if that really is the capacity, these things tend to be exaggerated somewhat.)

You'd need to make a custom charger cable, paralleling the 5V &amp; 0V lines between the ports, so as not to overrate the USB connectors, but as you're talking about making your own, I'm sure that won't be a problem.