I greatly appreciate your interest in my system. Because these days, I don't want to have to re-engineer it. But I may add a USB charger system for my rechargeable stuff like phone and tablet and e-cig instead of having all of these wall warts.
Power testing, real load control math.. I can do it, but I don't want to. ;)
/johnny
You want USB?
I have a female 12v accessory socket on my solar charging battery, coupled with a male plug with USB outlets <$10. Works great.
My advice to you is use the correct battery. You seem to have a good grasp on what you are doing.