Just a quick tidbit, also - the 12v battery I use is a 12Ah LiFePO4. It should have around 140-150wh of usable power. I had it charging for about 2 hours yesterday at a picnic shelter on the trail. It took 20 some watt hours in that time (no where near full charge). Then I used it for the rest of the last bit of the ride to power my tablet and charge my bike computer. I put it on the charger last night when I got home and its taken 88 watt hours over 10 hours so I'd say thats fully charged.
I don't know what the % drain was at the time it went on the charger, or what the % charge was yesterday when I stopped at the shelter, but going off the 88wh and the voltage around 12.5v I know I was pretty low. The battery cuts off (self-protection turns the terminals off) around 11.5 or 11.7v I think, but the real kicker to it is the lithium batteries hold voltage up all the way to the last 10-15% or so of charge. When the voltage starts dropping it tanks quite fast. So I am not sure how much usable power was there with the 12.5v or so I was left with. I will have to experiment with that some time and see - pull out 80-100wh and then see what I can do with it. It was still running the tablet and bike computer, and those were around 5 watts combined, but for how long would it do that? Possibly heading in to 2 days hunkered down in camp with this rain that likely doesn't amount to a whole lot of being able to run electronics - like the ham radio which I would have definitely wanted to be using, and did use, on my off days. The lower the state of charge on the battery the less current draw it can hold, also. So there may be 30wh left in the battery, but if you try pulling it at more than a few mA of current the voltage would likely sag and trip the protection circuit. In fact, I think that happened a few times. I taped the terminals up on the battery because I had them come off inside my bag a time or two. Yet, the power still cut off a few times. I can tell when it happens - I have a power meter on the stem that shows the voltage, amperage, wattage, and accumulated wh - you can see it in the picture of my loaded bike when I left the last camp in the last post of mine above (small screen between the bar and steer tube). If its not on there isn't any power and nothing is charging.