How does the battery life of the iPhone 3GS compare to the iPhone 3G in "real-world" tests? Is the battery replaceable?

Please note that the iPhone 3G has been discontinued. As of the June 7, 2010 introduction of the iPhone 4, Apple continues to offer the iPhone 3GS as a "low end" model.

Apple formally reports that the iPhone 3GS offers superior battery life than the iPhone 3G in some functions.

Battery life for using the 3G network for voice and data is the same for both models and standby time is the same as well. However, Apple reports significantly improved battery life for wi-fi Internet use, music playback, and video playback:

However, as official battery life numbers from manufacturers tend to be optimistic, third-party objective tests provide a better estimate of battery life in the "real-world" under actual usage conditions.

In an effort to stretch the battery life as much as possible -- using it only for calls over the older 2G network -- C|Net reported that the iPhone 3GS delivered more runtime and was closer to the Apple provided estimate than its predecessor:

In our initial tests, the iPhone 3GS' battery lasted longer than its predecessor's. We could go longer during a day of heavy use before having to recharge. In a straight talk time test on EDGE, we managed almost 11.5 hours of battery life, which is impressive considering the iPhone 3G delivered 8.75 hours of 2G talk time. Keep in mind, though, that real-world use will be a better judge of the iPhone 3GS' endurance. The large color display, frequently switching between different applications, and heavy 3G use will drain the battery faster than just making a call.

The WSJ's Walt Mossberg found the battery life to be improved for basic "day-to-day" use:

On my 3G iPhone, I usually could make it through the day, but it was often a close call, with the battery indicator winding up in the red. By contrast, the new model did much better, never hitting the red zone and rarely requiring interim charging at the office or in the car, even though, because I was testing it, I was pounding it much harder than usual, making more voice calls, playing lots of videos and music, trying numerous apps, constantly downloading email from two accounts, and syncing two calendars over the air.

The Engadget blog -- likely to be "pounding" the iPhone 3GS harder than Mossberg in its own real-world tests -- had different results:

Apple claimed that the battery life on the 3GS is somehow better than what we've previously seen on the 3G, but in our tests, we could barely tell the difference between the two handsets. . . In our tests -- real world, admittedly non-scientific tests -- we hardly noticed a difference in battery life between the new and old model. The point is, you're still very much in the same boat when it comes to general use -- but on the plus side, there's no noticeable deficit in power with the new version.

The always objective iLounge reported essentially the same findings as the other reviewers but found the outcome to be far less acceptable:

From our standpoint, the single biggest problem with the iPhone 3GS as a telephone is its battery life. When used on 3G networks either as a phone or data device, it achieves only a meager 5 hours of calling time on a full charge -- the same as the iPhone 3G -- and even less if you're using features such as Bluetooth or GPS at the same time. In a mixed-mode test. . . the iPhone 3GS's battery lasted for 4 hours and 44 minutes of actual use, with an additional 17 hours on standby. . . iPhone 3GS's poor battery life thus makes it a bad choice for road warriors who have heavy calling needs.

Third-party teardowns from iFixit and RapidRepair revealed that the battery in the iPhone 3GS -- like the iPhone 3G -- is not soldered, so replacement is possible for a technically-inclined user, but the battery is still not "swappable" or easy for a user of average skill to replace.

Ultimately, the consensus from most "real-world" usage tests is the iPhone 3GS battery life is modestly better than the iPhone 3G, but still not fantastic. Any improvement in battery life is welcome, and the iPhone 3GS should meet the needs of most routine "day-to-day" users, but heavy users may continue to wish that Apple had made the battery swappable so one wouldn't have to consider bulky external batteries.