Whats wrong with this phone ?

My brother in law has a TCL phone it's been dead for 2 days we tried to charge it for him but the battery goes from 50 down to 1 % then shuts down. We try several different cords but none works. He had his phone since March he has a tablet so hes hardly ever on it just to listen to his music. What's going on with it ? How can we fix this ?