Battery drainage beyond its charge

Hi all,

Need somebody smarter than me for this issue. I have a 6s plus with a battery draining issue and have replaced the battery multiple times as well as changing the charging port however the same issue persists to plague me. What happens is when the battery depletes the phone will not take a charge no battery sign nothing just dead. I figure the phone has to be draining power elsewhere for the battery to deplete after phone switches off. The phone charges fine when on and recognises a charger etc but when phone is left to die it no longer charges and the only way to get by the issue is to get a new battery. Done hard reset and dfu restore with no change..

Any info greatly appreciated.. Thanks for reading.