ArduPilot has a built-in power regulator, so you can power it from a separate battery at anything from 5-15v. To do this, you have switch the solder jumper from the default RC power to battery power. This means unsoldering the solder jumper that the board ships with and resoldering it on the other side. These pictures show the difference (apologies from my crappy soldering on the second one):
One thing to keep in mind if you make this change: Unlike the standard setting, if you use BATT input it will only power the ArduPilot board; it will not power your RC receiver or servos. They must have their own power supply, such as a receiver battery pack or ESC for an electric plane. This is designed for people who fly gas planes (no ESC) and want the RC system on a separate power circuit so the autopilot can't drain that battery.
If, however, you want to to power both the RC system and the ArduPilot board from the same source, leave the board in its default setting and connect the battery straight to the RC receiver; the receiver will then power the ArduPilot board.