I had to go to the Apple Store earlier this evening to get my trackpad fixed (and they totally cranked out a fix in 10 minutes – thanks, guys!), and I overheard a Genius telling a couple complaining about the woman’s phone’s battery life that they could save battery life by force-quitting all their “running” apps in the multitasking bar.
I bit my tongue for as long as I could before blurting out, “Actually…”, and tried to give a high-level explanation of the issue. The Genius was fascinated, but the couple he was trying to help gave me blank stares, so I wanted to try and write this up in a slightly clearer fashion.
Basically, the short version is that Apple fakes multitasking. Unless your application is doing one of a few specified things*, Apple suspends it as soon as it goes into the background, and while it remains in memory, it’s using zero processing cycles, which means it is effectively using no battery life.
There’s a really awesome 15 minute video from Fraser Speirs where he actually goes through and shows real world usage of iOS on an iPad while explaining what’s happening in the memory monitor:
If you don’t feel like watching the whole thing, the basic gist is that as iOS suspends applications, the second that the application becomes suspended, CPU usage craters as the app does any cleanup it needs to in order to suspend, then goes to 0% and remains there until you reopen the app.
If you open a crapload of apps, the older ones will get garbage collected as the newer ones are opened, removing them from memory even as their icons remain in the multitasking list, facilitating relaunch. The only remnant of the app in the system at that point is its icon in the multitasking list.
One thing to watch out for: If you have a misbehaving app with background privileges, that can *absolutely* kill your battery. There was one bad update to the Weather Channel app that accidentally forgot to release the lock on the GPS, and drained my battery entirely in 3 hours flat. The next update fixed the issue, but I had to disable the app’s access to GPS until the update arrived.
If you notice that your GPS is constantly on (and you can learn more about the solid vs. hollow GPS indicators here), definitely take a look at your apps that are using location data, and see if they’re apps that *should* be constantly using location data, like a Navigation app or something like RunKeeper that’s actively recording your route.
Oh, and for the couple at the Apple store: The biggest battery drain is almost always the screen being on, and you mentioned you’re addicted to Candy Crush. Gonna go ahead and say you might just have your screen on more than you normally do.
* – I’m very interested to see how these same tests work on iOS 7 – there are some changes that loosen up multitasking restrictions for developers, and I’m interested to see if my theory that Apple’s kept them tight enough to make the battery life loss a wash with all the other optimizations they’ve made actually holds out.