Make sure the charging speed is normal or fast, not slow. Slow charging does not supply enough power for the openSPOT3 to operate, so it will drain the battery while the openSPOT3 is turned on.
In order to make sure the charge speed is normal or fast: use the supplied wall charge adapter, or connect the openSPOT3 to a standard USB port of a computer, also try flipping the USB-C connector of the cable upside down. Make sure you use the supplied USB cable, or another one which is good quality, otherwise the charge speed will be slow.
You can check the charge speed on the Status page, or on the power LED after plugging in the device:
http://manuals.sharkrf.com/openspot3/en/led-states.html#power-led