Does iOS 4 draw more juice than iOS 3? Short answer: yes, says Aaron Vronko, CEO of Rapid Repair, an iPhone and iPod repair shop.
Since most people don't have a new iPhone 4, which comes with a bigger battery, most iOS 4 users are running it on an iPhone 3GS or 3G. This means you're probably experiencing less battery life if you upgraded operating systems to iOS 4.
Keep in mind that an iPhone's main battery drain comes from the 3G radio and broadband chip, LCD screen, CPU cycles and, to a lesser extent, main memory (also called Mobile DDR). The battery drain stems from iOS 4's multi-tasking feature and how it taxes main memory and the CPU.
Here's the problem: Background apps are loaded into main memory where they're restricted to a couple of APIs for background processing. "It's not drawing a ton of power, but system memory has to be refreshed by the CPU constantly," which means background apps tap the battery, Vronko explains.
How much battery drain? Vronko says the multi-tasking feature means that most of the 256MB of Mobile DDR in an iPhone 3GS on iOS 4 will be occupied. Thus multi-tasking can consume up to 10 percent of the total charge over the course of eight hours.
Compare this with the iOS 3, where only a few proprietary Apple apps ran in the background. For most iPhones, a portion of the 256MB of Mobile DDR wasn't being used (not withstanding a big game app running in the foreground requiring, say, 200MB of memory). This means that memory was consuming only 5 or 6 percent of the total charge over eight hours.
"With only one main application running, memory usage would have been low enough that the OS could have frequently turned off a quarter or even half of the memory, saving critical power," says Vronko. Given iOS 4's use of memory, iOS 4 probably drains 10 to 15 minutes more than iOS 3 per charge, he says.
Multi-tasking apps also require extra processing. While the small number of APIs and processor time available to apps running in the background minimizes the increase in processor load, "it can't prevent it completely," Vronko says. "I expect that the battery impact from Apple's frugal flavor of multi-tasking is probably still 4 to 5 percent. While that's probably significantly less than the hit taken on a more open platform like Android, it will probably still account for another 10 to 15 minutes per charge."
All tallied, if you combine iOS 4's multi-tasking feature that fills up memory more often with the additional CPU cycles from multi-tasking apps, "iOS 4 is likely costing 20 to 30 minutes more than iOS 3 on each charge," Vronko says.
iPhone owners might argue that they don't use the multi-tasking feature much, and thus are free from this battery drain.
Unfortunately, you are multi-tasking and don't even know it. For instance, whenever you call up a new app, the last app gets automatically thrown into the task switcher bar (double-clicking the home button brings up this bar) where it's kept in main memory and taxing the CPU.
Once the main memory fills up, iOS 4 will take apps out of system memory but keep them in the task switcher bar. "When you see that long list of 20 apps in your task switcher, probably only around the first three or four apps are still in system memory," says Vronko. (You can also force-quit apps in the task switcher bar just like you delete apps in screens.)
How to Save More Power
To make up for this battery loss, Vronko says, Apple clearly worked out power efficiencies in other areas--most notably, the way the iPhone uses the 3G radio and processor. The 3G radio and processor can consume an incredible amount of power. An iPhone battery life goes from 8 hours when browsing over Wi-Fi to 5 hours over 3G, Vronko says.
Vronko figures Apple tweaked iOS 4's management of the 3G radio and processor, such as being quicker to shut off the 3G radio when apps aren't using it, defaulting to lower transmission power when appropriate and getting the 3G processor to enter idle mode more quickly.
Apple also probably improved its auto backlight setting, says Vronko. Go to Settings ->Brightness, and you'll find a button to turn on auto brightness. Vronko thinks Apple improved the iPhone's ability to sense ambient light and set a more appropriate backlight. By reducing the backlight by, say, 10 percent, you'll save several percentage points in battery life per charge.
Vronko recommends iPhone users keep auto-brightness on. It's just one of many tips for saving battery life. For instance, in addition to disabling power-hungry features, Vronko also advises iPhone owners to plug in their iPhones into power outlets as much as they can. "Whenever you're using the battery, you're slowly killing the overall battery life," he says.
Every time you go through a charge cycle on your iPhone, you'll permanently lose anywhere from 30 seconds to a minute of battery capacity. Typically, you'll get 250 to 500 charge cycles before a lithium ion battery has outlived its usefulness, Vronko says.
Lastly, be mindful of the ways an iPhone battery gets run down: 3G, LCD screen, CPU and main memory. When using these features, don't be surprised when your battery indicator drops into the red. "Streaming video over 3G might be the biggest battery killer," Vronko says.