Well the phone is about 5 years old by this point so it is reasonable to assume that the battery has been very overworked. Lithium Ion batteries such as the one in your phone only last 2-3 years and your battery is (probably) about 5 years old. You should just replace the battery. Take a look at [[Samsung Galaxy Battery Draining Fast|this|new_window=true]] troubleshooting page to try to fix it without replacing the battery