This Kastar battery charger with foldable flat pin for easy storage. The charger Input: AC 100~240V 50~60Hz; Output: DC 4.2V 600mA. LED indicate: Red light for charging; Green light for standby or ...

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 ...

like in this clever mash-up by [bethiboothi] that takes advantage of the fact that the popular TP4056 lithium-ion battery charger module happens to be almost the exact same size of the ESP-01.