Charging through USB will depend on the charging unit used. Some car chargers will not charge a tablet, neither will some USB's plugged into a computer despite the fact it will charge a phone. For example the Apple charger outputs 2000mA whereas some plugs will only output 500mA or less which may not be enough to power the IPad. My wife has an IPad2 which seems to last forever and she is constantly on WiFi, whereas my daughter uses an IPad4 and is playing games all the time, so tends to charge every day using the supplied charger.
Love is better than anger. Hope is better than fear. Optimism is better than despair. So let us be loving, hopeful, and optimistic. And we will change the world. - Jack Layton (1950-2011)
Blackberry PlayBook 64GB Nokia 1020, Microsoft Surface PRO 2