iPhone dead within 3hours of new battery, won't charge or turn on

I bought a refurbed iPhone 6 in May 2015. About 6 weeks ago I replaced the battery because I was only getting a day out of it and it eventually died and wouldn't charge or turn on. The same thing happened a few days ago, I got another new battery (thinking maybe the last one was dodgy) today and within 3 hours went from 100% to 10% then died and won't charge or turn on.

I can't just be the battery, it must be a fault with the phone draining the battery that quickly and not charging or turning on

It's out of warranty and I suspect the seller won't be overly helpful given the time that has lapsed.

Any ideas helpful people?!