Where did you have it replaced? What year air?
In my experience the non OEM batteries really suck. Normally that is a bunch of BS I think, as I've had quality third party batteries for other electronics, but I've replaced batteries a few times in airs and the times I've used third party I end up pulling and returning after lackluster performance.
When you say half, do you mean half of what it was when new on a different OS? Something else to keep in mind.