This is a plug housing which fits for a NIU battery to make your own (fast) charger.
You can use 3.5mm gold plugs and glue them in.
I'm using a 45€ 48V 15A 720W power supply and have set it to 52V, so the battery will not be charged to 100%. I don't want to risk the expensive battery.
When far from full, the PS charges with 21A.
Be sure, that you know what you're doing, don't kill your expensive battery and don't burn down your home!