Ipad 4 battery completely draining overnight

My iPad 4 battery has started draining overnight even when it has around 50% battery before I go to sleep. I normally leave my iPad on charge overnight but recently I haven't been doing that and twice now I have woken up to a dead iPad which won't turn on until I charge it. I make sure to close any background apps before locking and leaving it for the night, and I dont turn it off either. The other night I left my iPad with about 70% battery and decided it didn't need charging overnight, but when I woke up it had drained itself of battery. It's worth noting that I bought this iPad from eBay and it had been refurbished and only recently started doing this. Additionally, I woke up this morning to a dead iPad and put it on charge at around 9am before going to work and upon coming home from work my iPad was only at 94%, meaning it had taken around 7 hours to charge and it still wasn't at 100%. Can anyone offer me an explanation or any tips on what I can do to stop my iPad from draining overnight and to help it charge quicker? Thank you!