It could be a poor quality replacement battery. To be certain, use a battery utility, such as [http://www.coconut-flavour.com/coconutbattery/|coconutBattery] (for Mac) or [http://www.3u.com/|3uTools] (for Windows). It will tell you what the health of the battery is. Anything less than 80% of design capacity will require replacement. After testing the battery, I would contact [http://help.ifixit.com|iFixit Support] to see about a replacement.
It is possible that you have an issue on the logic board so if another replacement battery behaves the same way, that is where I would look for the problem.