Hi, I have a similar issue to you - I can't really use the AC charger as the pins are loose on my Pandora and so it mostly doesn't charge (the red light doesn't come on). Why can't you use an AC charger - do you have some hardware fault, or haven't you got an official AC charger?
Anyways, because of the above I am forced to charge via mini usb as well. I have quite a few different mini usb cables and a few usb -> wall chargers. What I did was examine each of the usb->ac chargers to see which output the biggest amps - the 5 chargers I had varied from 850mah to 2amps. Obviously I picked the 2amp one (which is an official Nexus 7 one). I am now going to do some tests on the 3 mini usb->usb cables I have to see which one is best - using this combination I hope to charge via the mini-usb as fast as I can. However, not matter which combination I try I always get the red light on - so maybe you have a hardware issue (or your cables are knackered) - when you open the unit (and you're in the full gui) do you see a little white or yellow line beside the charging %. If it is white - it is charging, if it is yellow - it is not. Also, you can hover over this and it will tell if it is charging