If you have another battery that uses 18650 battery, you can test the battery in that light. Otherwise please follow the steps below to charge the battery inside the MH25GT.
1 Remove the pocket clip if it is installed.
2. Connect the Micro USB plug of the charging cable to the light
3. Connect the USB plug of the charging cable to a powered USB adapter or a computer USB port if you are not sure whether the adapter is good or not.
Observe the red indicator next to the charging port on the light
4. If the indicator flashes rapidly, press the power button on the tail cap as if you are trying to turn on the light. (there will be no beam output after you click)
5. If the indicator blinks slowly (once per second), the light is charging the battery. You can let it charge.
In short, the power switch needs to be turned on to activate the charging
6. A full charging takes about 6 hours. When it is completed, the light will terminate the charging process automatically and the red indicator shall become steady.