I always have a big stash of 18650 cells, all from dead or used and working laptop battery packs.
Got a 18650 charging board charged all batteries up (took me a while got like 40 batteries, mixed capacity, and brands). Did store them in a safe place, checked voltage for voltage drop and cleaned out the dead ones after a few months.
So I have only spend like 2 dollar on a charging board and some electricity for charging the batteries. Green for the environment and my wallet !