Sahkgye MPPT Solar Charge Controller 4.2V 3.7V 18650 Lithium Battery Charger Module with Function SD05CRMA

What shocked me was the size of the thing, I thought the packet was empty, it was sooooo tiny, less than the size of your fingernail. It seems to work when connected to an 18650 cell and a decent panel, only time will tell.