The battery that came with my desire drained what seemed to be fairly quickly. Apps such as JuiceDefender helped, but I'd still be doing well to make it last ~12 hours with the amount of usage I was putting it through. I bought a HTC branded spare battery, it has the same P/N on the back as the original, but it seems to last much longer.. at least 25% and maybe closer to 50%. It's been unplugged 6.5 hours and is still at 66% remaining, with a fair amount of web browsing, app installing/removing/updating, and general playing with it sporadically throughout the day. This suggests the new battery could last up to 19 hours! The S/N of the battery that came with the phone is 2SBA1038012676, the new one is 2SBA103K008523. Anyone who thinks their battery is under performing should consider buying a spare - it may have better performance, and even if it doesn't, you'll still have a spare of equal capacity which will help.