I just got an Anker Astro3 10000mAh battery pack. I plan to use it while camping this summer, mainly to charge a phone, but occasionally to charge my tablet.
It is intended to charge 5V USB devices, and has two USB ports. It also has a 12 volt port. The adapters for the 12 volt port, of course, do not work with USB or the Transformer. So I made an adapter, and I am good to go now for charging the tablet.
The battery pack charges from a 12 volt wall charger. I have also made an adapter to charge it from the 12 volts on my motorcycle. So I will be charging it from the motorcycle during the day when riding, then using it to charge my devices in the evening at the camp. I've already tried it a couple of times, and it worked well.
I tried to charge the battery pack using a solar panel, but the solar panel was far too wimpy. It was like trying to run a house air conditioner from a penlight battery. It may make you feel good to try it, but it isn't going to do much.
So far so good. This is very close to what I need.
The issues I have had to work around are:
1) I want to also charge an in helmet bluetooth intercom from the USB port. But the charge current is so low, the battery pack auto powers off, thinking nothing is plugged in. Not a big deal because I can plug in my phone at the same time. That keeps it turned on until everything is charged, then it auto shuts down.
2) When I charge the Asus Tablet, it doesn't appear to auto shut down. Just the opposite of #1, it looks like the load stays high enough to never shut down when charging the tablet. This may not really be true, as I have only tried it a couple of times. Also, it isn't good for much more then one and a half charges of the tablet, as expected.
3) You need to figure out your own harness to get the 12 volts to the tablet.
4) No charge cable for a car. I mean this is rated for an input of like 10 to 15 volts, or some such. This thing is GREAT for charging portable devices. Charging it from a car is as simple as a cable, but there isn't one.
Over all this charger is a winner. At right around $55 US, I recommend it, if you can fabricate a plug for charging the Transformer.