Not sure on the mah rating of the portable battery. I personally have one that I use for my monitor on my drone. I ended up using it with my helix 5 for about 3 hours while updating and playing with the settings. It was only partially charged from using it with my drone. Worked pretty good and it is very lightweight compared to standard batteries. Biggest benefit is the 6 ft cord that comes with it. You can purchase an extra power cable for your unit from Amazon for about 13 bucks and add the connector for the 12v plug in and make it plug and play. The waterproof case on the battery comes in handy too with the carrying handle and the shoulder strap. It also comes with a wall charger, so just plug it in at the house to charge and go.
With regards to battery life, I would imagine that if you turned the screen brightness down on the unit it would extend the battery life quite a bit. Similar to a cell phone. If I keep my screen brightness down on my galaxy, my battery will last 2 days but if I turn it up, then I only get about 6 hours out of it.
I'm not saying this is a perfect solution, but I do know that it works and will possibly last all day. Especially with the smaller units. Do I think you will get a full day out of it with the TV screen units? Probably not, however I doubt that the kayakers and canoers are using these on their rig.