Hi WSPS
As far as I remember it had a back up battery and we just pluged into a 12 volt powersupply on the aircraft, it was over 3 years ago.
The base of the system was was curved like a knee board with a velcro elastic, so it sat quite secure on your thigh.
I would have thought the Avmap lV would have improved on the lll that said I wouldn't buy an Avmap now, I've been useing a garmin 295 for the last few years which I'm really happy with.