Notebook battery discharges when I plugin connector

[starLED]

I have an older Acer notebook with a battery of 60% capacity health.

When the battery is 20% or less remaining, if I plugin adapter connector without adapter being plugged into wall socket, the battery discharges immediately to 0% and notebook shuts off.

Why is this happening?
Is this potentially dangerous to the notebook?

 

[aliarifat794]

The power management system in your laptop might be malfunctioning.

 

[starLED]

It happens also when the laptop is in sleep mode with 20% battery or less, and after plugin in connector and wakeup it shows 0%, meaning it discharged while not powered on.