battery won't fully charge

hi,

i recently replaced the battery in an iPhone 3GS. With the battery replaced, it will not hold its charge for any longer than a few hours. Also when on the charger, it won't charge past 79%. The battery i bought was off ebay and it was cheap. is it the battery? or what else could it be? I'm confused why it won't charge past 79%. it would fully charge before i had the battery replaced, but it would run out in less than a day.

i hope somebody can help!!!