My 3GS showed a greatly improved battery life after a couple full calibration cycles. If the battery calibration is off, the phone might be telling you that it has a full charge when it only has a 75% charge, and that it's empty when the battery still has 25% left. Charge it an hour or two past full and it will now know what full the full level really is. Let it run down past the 10% warning till it shuts off (can't hurt the phone or the battery because of the new battery protection circuitry) and it will learn how low empty really is. The real actual battery life won't change, but you will be able to use your phone for twice as long, because the battery meter and warnings will be calibrated more correctly. With Li-Ion devices, I might do this about once or twice a year (the battery doesn't age fast enough for once a month to make any difference.) ymmv.