I keep one with the battery all the time. When I car camp or travel I plug the cigarette lighter cable into the lighter outlet of the battery to trickle charge it during the day (I do the same in the car to and from to charge it faster). I keep the other one in my backpack (they're surprisingly light) when I go hiking. I connect an adapter that converts 12V to a powered USB socket and then an Energizer USB battery charger or any of my toys which can be charged off of USB power (most of them).

They're expensive but I didn't pay retail so I considered it to be worth the expense at the time.

I've been working on custom making a cable so I can connect both at once and double the power. 24 Watts is pretty respectable for a solar panel you can almost pocket.