I bought a 3GS in August 2009. After about 9 months I started seeing occasional precipitous drops in battery level overnight (i.e. from 50% to "dead", as in all you see is the red "charge me" icon). This would happen every few weeks, even if I made sure nothing was running in background before going to bed. Since upgrading to iOS 4 and multitasking it's happened once, but in general iOS4 seems to have improved the standby time of the device slightly. Usually I see a 1-2% drop in charge level overnight.
The question: Does the iPhone run its own battery conditioning cycles when it decides it needs to? If not, what would cause this effect?
EDIT: Clearly the battery is not going bad:
- The rapid discharge first happened when the phone was 9 months old,
- The phone is now 18 months old,
- The battery behaves normally and holds a charge just fine, and...
- The rapid discharge happens only every two months or so, over the course of a day or two