battery charging bars go to halfway then battery disapears

I bought this thing off an area classifieds and the lady told me that it needed a new wall charger however between the 2 that came with (wall charger and car charger) she said the car charger worked the fastest! I got it home fired it up and set it up just how I wanted it, got a few new apps, played games on it for 4 days before the battery even came close to dying so I plugged it in and right away I noticed the "battery charging" icon that comes on the screen showing it's charging, goes like 2.5 bars and disappears! So I bought a new charger, plugged it in and its been charging for an hour and it's doing the exact same thing and will not turn on! Did I buy a lemon? Could the battery maybe need replaced? I'm out of ideas! Can someone please help?