Charge the batteries in wireless accessories
The Magic Keyboard, Magic Mouse, and Magic Trackpad each have a built-in rechargeable battery.
Check the battery level. Click the Control Center icon 
in the menu bar, click the Bluetooth icon 
, then choose the device.
Charge the battery. Connect your Magic Keyboard, Magic Mouse, or Magic Trackpad to your Mac with a USB-C to Lightning Cable or Lightning to USB Cable.
Tip: To charge the battery faster, be sure your input device is turned on when you connect it to your Mac mini.
Learn more. See Disposal and recycling information or visit Apple Lithium-ion Batteries.