Leaving devices plugged in after full charge. Harmful to the device?

[Trush Jodie]

My younger brother won't let his smartphone plugged to a power source for a minute after full charge. He said it is harmful to the device. Is it truly harmful to the device if the device stays plugged in after full charge?

 

[eLdavis]

Most modern smartphones now have safety charges, so even when the battery is full and still plugged, the phone automatically stops collecting power.