USB Power
Two simple ways, not clean, but simple:
1. Do the 12v always on mod and put a usb charger into the lighter plug.
2. If it's in a locked garage, leave the key in, turn to accessory position and plug into the usb port.
I did this with my cell phone recently, when I went into the garage later to check on it my phone was charged and the battery for my quad copter were ready.