Instead of a laptop battery, find one of those external li-ion portable DVD player batteries. I used one for a while before replacing it with a larger pack from batteryspace. The battery was designed to provide extra power for those portable DVD players. It had a built-in charge controller, so all you had to do was plug in a wall-wart transformer to charge it up. It featured an LED battery level indicator. It was rated for 9V @5400mAh. I got it for less than $40.