Due to space constraints, Apple is able to fit a 16.3 Wh battery into the iPad mini, whereas the iPad 2 enjoys a 25 Wh power source. You'd think the tablet with the smaller battery would charge faster, but that's not the case. It takes almost 45 minutes longer to charge an iPad mini to 90% than an iPad 2. The mini takes 30 minutes longer than the iPad 2 to get to 100%, too. Why? Charging is a careful balancing act between convenience and the battery's health.
This leaves the iPad mini at a severe disadvantage though, because the Nexus 7 reaches its full charge significantly faster. Even Amazon beats Apple this time around.
The inability to charge while plugged into your PC or Mac via USB is an annoying omission shared by most Android-based tablets. The Nexus 7 and Kindles are exceptions. Fortunately, all iOS-based devices can charge as they sync.
Apple's iPad mini again takes longer to charge than the iPad 2, though. The mini's only redemption is that the Nexus 7 and Kindles both take significantly longer to hit 90%, and then 100% charge this way.