I have been fixing iPhones for almost a year now, and from my experience i have found 3G/S batteries to be VERY dodgy. Regardless of who you buy the battery off, for some reason there is a high percentage of faulty batteries. Whether it be the logic board's damaging them upon installation (static discharge or other electrical causes) or purely because they are made from a poor design, its all about the luck of the draw.
Make sure that when you replace the battery again next time that you are not around objects that can create static (carpet or other friction surfaces), or you can invest in an anti-static wrist strap that earths you, that will do the job too :)
Anti-Static Wrist Strap - Anti Static Wrist Strap
My advice is to send back to the iFixit team and request for a replacement. They shouldn't have any problem with that :)