Only issue i have is, it doesn't charge via usb unless its plugged in via wall..
The real issue is that recent tablets consume more energy than what the USB ports can provide. Check this
wikipedia page, and consider this:
The following "benchmark" (vaguely speaking) were done while driving, using the same power source (cigarette plug to USB converter, outputs 1.0 A):
- I had an HTC Hero (Single core, ~520 MHz) which could be charged with any work load on the phone (gps on with Maps/Navigation, while talking and using data while using the CPU at 100%). At worst it was simply charging slowly. Usually I had no issues with charging speed
- When I switched to a Galaxy S2 (Dual-core 1.2-1.4 GHz), I noticed I couldnt use Navigation (implying GPS here) and playback streamed MP3s over 3G anymore and expect the phone to be charged in the process. Over a ~1:15h drive, a fully charged phone lost ~15% if not more of battery charge.
My first big GS2 trip was one where I did the whole thing over Naviation with GPS in car, with local MP3 being played back. I had to pick up somebody when I had done 3 hours of driving, and I noticed that somehow I had around 50% battery left, and I made sure it was fully charged before leaving. Later the same day, I had more Navigation and GPS usage coming up (an additional 7 hours trip). So I used only Navigation/GPS, to attempt charging, and I could get it back from 30% to 70-80%.
Mind you, that was over a STOCK Samsung Android installation, maybe that wasnt optimized for battery consumption!
Even though I dont have any spec numbers and such, it's only normal that your quad-core tegra 3 can't be charged over USB. That being said, with the screen off and the tablet idle, you can still charge the tablet (like others mentioned), since apparently there is an additional, slower but less power hungry core being actively used while the tablet doesn't have alot of things to process.
Read this wikipedia article.
EDIT: I am also often in either fastboot or recovery mode on my TF300 these days, and after a few hours "idle" in either recovery or fastboot, I noticed the battery indicator was in the 75-80% range the next time I booted Android... and thats with the device plugged in the whole time! My guess is that recovery/fastboot doesn't use (or recognize) that additional low power core. Either way, sorry for my ramblings, and mods, sorry for my over 9000 edits
