I had a similar problem with my 18 months old Samsung Wave about 10 months ago, which I wanted to hand down to my daughter who was heading off travelling. We bought a compatible ( but not particularly cheap) battery off the internet and it worked well for only about 3 months, then started needing daily, or more, charging, then it expanded until the cover would no longer fit on the phone.
I have now bought a replacement Samsung battery, which so far appears fine.
I appreciate that this does not answer your question as I didn't buy it from that company but I am a little wary of generic batteries now.