this is a really good helpful website that I came across on the net after searching for information on how to replace the NiCad battery inside my dead Game Gear battery pack that I got from an ebay auction that including another battery pack (a working PowerBack), 3 games and a Game Gear (no sound but I fixed it with a new capacitor) all for $8 bucks.
http://www.jonathanen.com/pages/Help/Replace.html
the only modification/ or update I'd make to the information given above would be where to obtain the battery. He recommends the Sanyo RC battery, but that battery is discontinued and can't be found anywhere today.
But I found this site (see below) that offers same type of battery and even provides customizable connectors for the wire from the battery during check out, which will eliminates the task at the end of the above guide of having to rewire the new battery.
http://www.onlybatterypacks.com/
I placed a order today, and can't wait till I get my new NiCad battery. If it all works well I'm going to have 6 hours of gaming instead the original factory default of 30 minutes to a hour.![]()