I found it pretty rare, and these are 22$ rechargeable batteries, I will need a charger for them.
And please link me to a trustable site, not one of these suspicious chinese ones that nobody have heard of
Why wouldn't you know when it's full? ... it will still work perfectly fine. Just be careful the magnets/wires don't short while the charger is on or you'll probably blow it up. Might be best to solder the magnets with the 2 same poles facing each other so they don't try stick together.