Laptop battery packs used to use standardized metric battery cells (tool batteries often still do.) Those aren't very space efficient though. Still. The cells

Laptop battery packs used to use standardized metric battery cells (tool batteries often still do.) Those aren't very space efficient though. Still. The cells they do use are all going to be the same voltage since the chemistry is the same. In theory you could swap them out with a set of same size or smaller lithium cells.