Why is it that l will charge my phone for a long time before it is ful

@ebenezerad44503 your A15 is a relatively new phone. You should not have to do anything to it but to get this fixed under warranty by Samsung. Hold them accountable for this and do not spend your money on something that they can fix.

The only thing I would do for now, is to make sure that the phone has all the updated software, since charging rate can be controlled by the firmware. From a hardware perspective, this could be a bad battery, bad charging board or even a bad power management IC