Won't charge while turned on and drops charge

I have had my OnePlus 2 phone for two years now. I haven't had any problems with it until recently. It was charging fine but now it won't charge right. I plug it in to charge and the charging light will come on but then it stops charging. The only way I can get it to charge at all is by turning it off and letting it charge that way. But once I have it charged to 100% and go to use it, it drops dramatically. Like for example it'll be at 100% and if I let it sit there and not touch it it drops down to 80% within 2 minutes. At first I thought it was the charger so I got another but it still did the same thing. I haven't bought a new cord as of yet.