Since getting my iPhone I've been charging it every night for fear of being on a long road trip and running out of juice. But just before docking it on my nightstand last night I had an epiphany. I have an iPod charger in my car, so why should I be charging it every night? Although there's no risk of partial charging causing a memory effect in Lithium
Ion Polymer batteries, they still can lose 5-10 percent of their storage capacity every year from the time of manufacture.
My reason for pausing at the charger last night was different: I didn't want to use up any of the finite amount of "charging cycles" in my iPhone's battery. When it eventually dies an iPhone out-of-warranty battery exchange will set you back US$85.95 and takes three days. A loaner iPhone is available for US$25. It turns out that the 400 charging cycle cap is somewhat of an urban legend. Macworld's Jason Snell spoke with Apple Vice President of Worldwide iPod Product Marketing Greg Joswiak to clear up the confusion.
“After 400 complete cycles, the iPhone’s battery still has 80 percent of its charged capacity,” Joswiak said. “And by a complete charge cycle, I mean completely draining the battery, a full chemical cycle.” In other words, using a little battery and then putting your iPhone back in its dock doesn’t count as a charge cycle. If you use a quarter of your iPhone’s battery and then re-charge it, Joswiak said, that’s the equivalent of a quarter of a charge cycle. “If you top it off, you’re not wasting a charge cycle,” Joswiak said.
There have also been reports of iPhone batteries not fully charging. Apple has admitted to a bug stating that it is merely cosmetic: "Your battery is fully charged, but the UI (User Interface) is just not correctly reflecting this." They go on to say that the issue will be addressed in a firmware update.
Apple's knowledge base article iPod's battery doesn't charge is an excellent troubleshooting resource and further information is available in iPhone Service: Frequently Asked Questions. Apple's iPhone battery page and Tips and tricks to get the most out of your iPod's battery are also loaded with tips.
In (somewhat) related news Energizer is planning to release an external iPhone (and iPhone?) battery in October. Energizer's Energi to Go (US$30) external iPod battery will provide up to 46 hours of playing time with two AA Lithium e2 batteries. Who wants to bet that it's retrofit to work with iPhone?