You can salvage lithium 18650 ... invest in a charger that is capable of running some capacity tests against the cell. Or if you’re a bit more adventurous, you can build this “Battery Health ...

Over at Hackaday.io [AndyHull] popped open some cheap LiPo battery ... to four 18650 LiPo cells connected in parallel. The controller also boosts the voltage to 5 volts for one or two USB ports ...