Very many battery packs are built using standard size cells inside the plastic package. When cells go bad, it is often possible to replace the cells with new one either as DIY project or have some repair company to do that. Usually my option one is to find replacement battery for devices, and if it is unavailable or terribly expensive, consider putting in new cells. Building the new battery pack needs some knowledge and right tools (small spot welder and/or soldering iron).
Cordless Drill Battery Pack Rebuild for $20 or Repair for $0: Nicd Battery Pack Rebuild
Warning: Trying to fix battery without knowledge to do it right can lead to more damaged battery pack, damaged device being powered and possibly start a fire