Why not just re-cell the battery pack?

I am thinking either battery refill dot com or doit your self. I couple if older nicad's from remote control cars or something, solder and time.

Shouldn't be too hard. Heck I'd even draw up a schematic, calculate resistance, charge times etc. Then do a nice PCB and move the controller board. Etc. You get the idea.

If I really cared I test it out in one of my breadboard sim programs.

It can be done though.