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Power complex

May 13, 2014 by Robin Englebright   

At our app swap cafe today we shared some top tips for iOS devices, Beth previously shared a tip that charging your iPhone in Airplane mode made it charge faster… However at the event Beth had a bit of follow up – that the tip might have been an urban myth.

Later the topic of closing apps on iPads as a way of prolonging battery life came up, which I was pretty sure didn’t make much difference.

So I thought I’d investigate and maybe find the truth.

An iPhone 5S has a 3.8v 1560 mAh Lithium ion battery, and can charge at up-to 1 amp, limited by the charge control circuits in the phone, battery condition, battery temperature and the rating of the charger attached… so the very fastest it could charge is about 1.5hrs (1.56 aH/ 1 A) However we probably won’t have optimal conditions, so for our example lets assume we can charge the device in two hours.

Now the question becomes can we significantly drain the battery over two hours?

Obviously if you sit around watching Netflix on the phone during the two hours the battery won’t charge as fast, and I’m assuming putting the device in airplane mode isn’t just a way of preventing user activity, so to test a likely situation I rashly assume that the phone won’t be used when charging.

In this case the draw on the battery will come from active apps, and those running in the background, which touches on our second “myth” that closing apps will prolong battery life.

The developer guidelines for iOS indicate there are 5 states for an app, of these two have potential to use power.
If the app is “Active”, it’s in the foreground AND recieving events,

or

If the app is executing code in the “Background”.

Apps usually only briefly pass through the Background activity state to close threads when switching between apps, unless they have permission to run in the background.

However iOS is dead smart at managing its resources and is happy to move inactive apps to a suspended state, which keeps them in memory but not executing any code, and if memory runs low the system may close suspended apps to provide resources to the foreground app.

The upshot of this automatic app management is that iOS will suspend apps that aren’t actively being used, so the power draw from these apps is negligible.

The only apps that may be drawing power are active Background apps, and the only ones affected by switching to airplane mode are those that use the the wireless, carrier connection and bluetooth.

The behaviour of apps in Background can be set in:

Settings > General > Background App Refresh.

and indeed here it does state that turning off background refresh may help preserve battery life. I was a bit surprised to find I had 25 apps using background refresh.

iPhone Background app settings

So in summary switching to airplane mode may well make your device charge faster if you have lots of apps set to Background refresh, but probably not by much.

Closing apps manually is going to free up memory, but if they are suspended will not change power consumption.

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1 Comment »

  1. Craig Wakefield says:

    I heard the same rumour rob, and when I investigated it online I found a number of people had tested it e.g. http://www.tuaw.com/2014/02/07/does-your-iphone-really-charge-faster-in-airplane-mode/. The conclusion was pretty much that it was faster, but marginal. Interestingly no one mentioned the role of individual app usage….. Nice work!

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