iPhone 3GS battery oddities

Question

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:

1. The rapid discharge first happened when the phone was 9 months old,
2. The phone is now 18 months old,
3. The battery behaves normally and holds a charge just fine, and...
4. The rapid discharge happens only every two months or so, over the course of a day or two

Answer 1

No, the iPhone does not self-condition. Apple has a guide for conditioning laptop batteries, yet I can't find a version for iOS devices (still looking).

There is a general tips page on maintaining great battery life, as well as general terms about battery replacement.

Personally: This sounds like a bad battery. It sounds like it just plainly is not holding a charge after a certain discharge point. Remember that iOS multitasking is not true multitasking. Barring streaming audio (actually persistent audio in general), there is (almost) NOTHING that your phone can constantly do for more than 20 minutes at a time.

I've streamed audio for hours (hours being... maybe 5?) during the day at ~40% and it did get under 20% by the time I was done, this was both on an iPad, and roughly equivalent on an iPhone 4. Connected WiFi, 3G (on the iPhone) enabled but not being the active radio in use, of course.