iPhone 6 massive battery drain and heat


I just bought an iPhone 6 from internet and i’ve been scamed

The phone is getting its battery drained from 100% to 10% in 4-5 hours (on standby), 2 hours(usage) plus the heat on its back.

I’ve tried another battery same thing drain + heat

The phone is charging normaly even when the battery is fully drained. Do you have any idea how to fix that? Thank you in advance