First, let's see if it's the battery.
Battery problems are by far the biggest cause of battery failure to charge.
Secondly, you can look at the problem of lithium battery protection board.
To solve the problem of the protection board, you can directly change a new one. If you do not change a new one, you must detect the specific fault of the protection board. MOS tube, IC, resistor, capacitor, fuse, PCB board, see which is the problem, just change that. If not, it is necessary to find the original manufacturer, so to have good development, it is best to find a strong protection board factory for long-term cooperation.
Finally, the battery cannot be charged, which may be a problem in welding and other work.
21v rechargeable battery can not be charged, this has two reasons,
The first is that the battery is broken, and if the battery is broken, it can't be charged,
The second reason is the problem of the charger, if the charger is broken or the model specification of the charger is not correct, it can not be charged.
You can find solutions according to your own problems