We have a Hoover Linx cordless vacuum that En got when she worked at TTI (the folks that now make Hoover and Dirt Devil). This was around five to six years ago now. The Li-ion battery was starting to no longer hold as much of a charge as it had in the past.
Four security Torx screws later I found it was populated with five 18650 batteries. This is a very standard size for batteries so I ordered some from eBay — good LG ones that I know will perform well.
Total cost: $20 + the T10 security bit I buggered up. ($1?)
The biggest annoyance was popping the spot-welded connections. Other than that, it was pretty smooth sailing.
Video coming in the future BTW.