I have a nice Ozito reciprocating saw that has a LiOn battery pack standard, but I also want to reuse the collection of good quality LiPo (Lithium Polymer) battery packs I have for model planes and helicopters. I've created an adapter to breakout the tools DC contacts neatly to a 3D printed plate that can mount a regular LiPo pack with velcro.
I use 4S packs which are about 16V and the tool runs hard and strong on them, probably because they have less volt drop than the stock LiOn packs.
I've also used LiPo packs to resurrect obsolete NiCd type cordless drills from Bosch and Metabo and they can give good quality tools a new lease of life.
For 12V tools I use 3S Lipo packs, and for 14.4V - 18V I use 4S packs.
(Do it at your own risk of course, as there will be no fault protection from a LiPo pack like the OEM pack may have)
LiPo packs can deliver a lot of current and need to be used correctly when charging and discharging, and I remove the battery from the tool when in storage.