I don't suggest you getting this. I had that battery for my HTC Incredible and it also fit the EVO I have the Thunderbolt and the 1800 mAh battery does NOT fit my Thunderbolt. If it did, I wouldn't be selling them with my Incredible.
I was really hoping it would, but it doesn't. So take my word on this and don't get this.
I hope you will heed my words and not get this. It will not fit the Thunderbolt.
If you are wanting a 3rd party battery for it then go to HTC Express and you will see for yourself.
Also, it only gives about an hour or two more battery life.