A diode-bridge rectifier is what you want, you can get them at radioshack for about $3. All it does is convert AC power to DC power that a battery can use.
They're pretty easy to wire, it's 4 terminals. You plug one of the "~" terminals into the coil, and ground the other. The "+" and "-" terminals put out DC power. Remember that for stuff that's running on the DC system has to ground to the negative terminal, not the frame.