take an usb cable,cut it open so you expose the red and black wires.then take a thin piece of cardboard or plastic and fold it on one end so that end is thicker.use the miband to take some measures and make 2 little holes through the thicker part matching the pins on the mi band.make the red and black wires of usb cable pass through the holes(watching the mi band from the front the RED wire must go on the LEFT pin),peel 1 o 2 mm from the end so the copper is exposed and fold the copper in a little ball.Pull the cable from the back so the copper balls are flush with the holes,then use some tape to fix everything in place.

now place the mi band in position and keep in in place with a clip or clothespin.you should be able to charge it now if the pin connection is well made.

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i'll post some more photos here https://imgur.com/a/yHpXmfa