My old battery had something like 8,000 cycles on it so I needed to replace it.
It didn't go very well. The new battery wouldn't fit. I ended up cutting away some of the battery case to make it fit better, but its still not perfect. At least now its flush with the rest of the case.
Also the corners on the new battery are slightly different then the corners on the case. The new battery is technically older then my computer. So i don't know if the two are related.
Make sure the battery you're buying isn't older then your computer.