Apple's iPhones slowed to tackle ageing batteries
Apple has confirmed the suspicions of many iPhone owners by revealing it does deliberately slow down some models of the iPhone as they age.
Many customers have long suspected that Apple slows down older iPhones to encourage people to upgrade.
The company has now said it does slow down some models as they age, but only because the phones' battery performance diminishes over time.
Apple said it wanted to "prolong the life" of customers' devices.
The practice was confirmed after a customer shared performance tests on Reddit, suggesting their iPhone 6S had slowed down considerably as it had aged but had suddenly sped up again after the battery had been replaced.
"I used my brother's iPhone 6 Plus, and his was faster than mine? This is when I knew something was wrong," wrote TeckFire.
Technology website Geekbench then analysed several iPhones running different versions of the iOS operating system and found some of them did indeed appear to have been deliberately slowed down.
What was Apple's response?
Apple has now confirmed that it made changes to iOS to manage ageing lithium-ion batteries in some devices, since the batteries' performance diminishes over time.
"Lithium-ion batteries become less capable of supplying peak current demands when in cold conditions, [when they] have a low battery charge or as they age over time, which can result in the device unexpectedly shutting down to protect its electronic components," the company said.
"Last year, we released a feature for iPhone 6, iPhone 6s and iPhone SE to smooth out the instantaneous peaks only when needed to prevent the device from unexpectedly shutting down during these conditions.
"We've now extended that feature to iPhone 7 with iOS 11.2, and plan to add support for other products in the future.
"Our goal is to deliver the best experience for customers."
Why do lithium-ion batteries degrade?
Lithium batteries degrade over time because of what happens during the charging and discharging cycle.
During both those events, lithium ions migrate through the material forming the battery.
Studies using electron microscopes have shown that each time the ions do this they make tiny changes to the physical structure of that electrolyte.
The effect is like "rust creeping unevenly across steel", according to one scientist who has studied the phenomenon.
The changes effectively erode the material so it can hold less of a charge and can hamper its ability to provide a steady power supply.
Higher voltages make the erosion happen more quickly, as do higher temperatures.
Should Apple have told customers?
"By choosing to implement this quietly, it appears more nefarious than it really is. That doesn't engender trust," wrote developer and blogger Nick Heer.
"Apple has long been very good about managing expectations… this is an instance where they blew it. Needlessly, I think."
Replacing an old battery in one of the affected models should return the phone to its former speed. Doing so costs £79 in the UK and $79 (£59) in the US.
"They should be more transparent about it," said Chris Green from the tech consultancy Bright Bee.
"You're taking away performance that somebody has paid for. If you're going to slow down the phone over time, you should explain why it is happening, so people understand it is ultimately for their benefit.
"But I do see where they're coming from. By slowing the phone, it does help mitigate the problem of the diminishing battery."
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Analysis by Rory Cellan-Jones, technology correspondent
It is a combination of urban myth and conspiracy theory - the idea that Apple builds planned obsolescence into the iPhone, slowing down older models to make you splash out on an upgrade.
Now it turns out to be true - the slowing down part, if not the motive behind it.
Apple's explanation that it is all about managing the performance of ageing batteries seems quite reasonable. After all, the fact that the lithium-ion batteries that power mobile phones degrade over time is well known and owners of other leading smartphones also complain of poor battery life after a few years.
But what will frustrate and anger many iPhone users is Apple's lack of transparency. The software tweak at the heart of this story happened last year and rumours about the deliberate downgrading of performance have been bubbling for weeks. But it was only a couple of days after a software developer showed exactly what was happening that Apple finally came out and explained.
Here is a company that is - as any journalist can attest - fanatical about controlling the message about its products. It has also benefitted from the devotion over the years of customers who are more like fans. But maybe it is time for Apple to show a bit more honesty in that relationship.