I wanted to use rechargeable batteries and I wanted my pack to be small. AA battery seemed the logical choice. They are easy to find, just about everyone has a charger and if they don't, you just run down to the ESSO station and buy one (if you forgot yours or didn't think you would use up the 5 hours of battery life.)
My camera's external power supply has an output of 8.4 volts and 1.5 amps. Rechargeable batteries put out 1.2 volts. After some high powered calculations, I found that 7 rechargeable batteries would put out exactly 8.4 volts! What luck.
I was not worried about the current output of the batteries. They are 2500mAh and should provide plenty of current for my camera.
By looking at the plug on the external power supply, I was able to find the polarity being delivered to the camera. In my case, the barrel is negative and the inside is positive. This is important to know when you wire up your new plug and hook it up to the battery pack.
I also matched the plug from the external battery pack. It turns out that I have a size "H" coaxial.