If your device don't charge for some hours and if you try to swich on with your charge connected and the red light still on.
Remove all the screws and the Rubber cover to found the battery wires, with care measure the voltaje of your battery, if that is less than 2 volts you need a new battery.
The first thing to order a new battery or build it by your self is check if the PCB board is working.
At this point you must disconnect your battery from the PCB board, connect the USB and measure if you got 7,5 Volts. If that measure is correct you can replace the battery an all will be work at the end.
You can buy a new battery at amazon, ebay or aliexpress but the most cheapest solution is build it by your self, I replace it for only 8 Euro.
If your PCB board don't have the correct voltaje at the output without any battery connected, I reccomend you buy a new deviene.
If you need some help with the battery build just contact me for more help.