Charging a phone battery on its own
A safe way is to buy a duplicate phone, but with no network contract. It will not be usable as a cell-phone, but it will charge duplicate batteries safely. Usually a green LED comes on when the battery is charged. You can take it out and put it into a "charged" bin where it will hold its charge for weeks or months, allowing you to charge up more than one spare battery safely. If your phone offers a 'charge percentage' display option, turn it ON.