I have a PowerBook G4 12" 1.25GHz 2004.
The G4 era battery cells can be replaced with new 18650 Li-ion cells. No software hack needed. and they are not like Intel books' batt with annoying "self-destroy" fuse. I really hate those ******!.
after the hardware work, no software "hacks" needed. But you need to calibrate the battery gauge, maybe 10 to 15 times. Because, within each cycle, the gauge IC only raise the capacity by 256 mAh at most.
There was a web page with a step-by-step guide. I lost the URL and can't find it any more. Maybe I should post my guide on ifixit.
Just ask some electronics DIY guy help you guys to do this. And, Good luck!
and, safety precautions:
1. There's no guarantee, at all.
2. be very very careful, don't mess with Lithium-ion batteries.
3. If you don't know what you about to do, DON'T do it!