So I've got an iPhone 3GS that will be 2 years old come this July. I upgraded to 4.2.1 couple of months back & my battery life has just been horrible so I'm wondering whether there is a way to find out the 'battery health'? If I know the battery capacity has decreased to below 70% I'd wanna get a replacement, but if not I can wait a few months till I upgrade my phone and for now I can then go through the trouble of upgrading to 4.3.2, and of course rejailbreakkng etc. to see if that helps.

Any app on the app store / cydia that can estimate this kinda thing?