Most tablets these days can only charge via their wall charger. The USB ports simply don't provide enough current. You can "trickle charge" as leeshor mentioned but you will not be able to use your device and charge it at the same time and it will also take about twice as long to charge as using the included charger. The best policy for charging modern electronics is to use the included charger. If you need a second or a spare you need to make sure it supports charging at ~2A (usually they say works with iPad).