Apple has admitted it does intentionally 'smooth out' the performance of its smartphones when a battery gets too old.
A consumer watchdog that evaluates electronic gadgets' performance sparked an outcry after it found that processing power of older models is limited once battery life begins to suffer.
Many claimed it was an attempt to force users to upgrade to the latest model.
Today, Apple said the software update is actually prolonging the life of the phone and stopping it from shutting down as the batteries age and becomes less effective.
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Apple intentionally throttles the performance of its smartphones when a battery gets too old, a new study has claimed, including the iPhone 6 and 7, once battery life begins to suffer. This image shows CEO Tim Cook at the launch of the iPhone 6 on September 9, 2015
However, the firm has come under fire for not being more transparent about the software update, or explaining to users when it happens.
The shutdowns are caused when iPhones hit peaks of processor power that the battery was unable to power - causing them to simply shut down.
Apple saidits software 'smoothed out' these peaks, allowing the phones to continue operating.
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'Our goal is to deliver the best experience for customers, which includes overall performance and prolonging the life of their devices,' Apple said in a statement.
'Lithium-ion batteries become less capable of supplying peak current demands when in cold conditions, 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.'
Geekbench developer John Poole decided to analyse the results of a number of iPhone 6 and 7 models running various versions of iOS.He found an update issued to fix a ' sudden shutdown' issue wit the iPhone 6 was to blame for poor performance
Apple also said it is expanding the programme to more handsets.
'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.'
The drop in performance came to light in benchmark tests, from Toronto-based firm Geekbench.
It found that on certain versions the Apple operating system, phones with older batteries achieved much lower performance scores.
Battery capacity is expected to decrease as they age, but processor performance should stay the same.
However, users with older iPhones with lower-than-expected Geekbench scores have reported that replacing the battery increases their score, as well as the performance of the phone.
Apple said it recently extended the feature to iPhone 7 with iOS 11.2, and said it plans to add support for other products in the future
Geekbench developer John Poole decided to analyse the results of a number of iPhone 6 and 7 models running various versions of iOS.
He found that an update issued to fix a 'sudden shutdown' issue, when users found their iPhone 6 was turning off when its battery reached half charge, was to blame.
This resulted in the poor performance results observed.
Writing in a blog post, Mr Poole said: 'It appears the problem is widespread, and will only get worse as phones, and their batteries, continue to age.
So it's true Apple intentionally slow down old iPhones. Proof: My iPhone 6 was bought 3years ago and recently got really slow. APP 'CPU DasherX' shows iPhone CPU is under clocked running at 600MHz. After a iPhone battery replacement. CPU speed resumed to factory setting 1400MHz. pic.twitter.com/pML3y0Jkp2— Sam_Si (@sam_siruomu) December 20, 2017
'I believe, as do others, that Apple introduced a change to limit performance when battery condition decreases past a certain point.
'If the performance drop is due to the 'sudden shutdown' fix, users will experience reduced performance without notification.
'Users expect either full performance, or reduced performance with a notification that their phone is in low-power mode.
'This fix creates a third, unexpected state.'
Without this warning Mr Poole believes that the fix will cause users to believe their phone is slow so they should replace it, rather than the battery which is actually at fault.
'This will likely feed into the 'planned obsolescence' narrative,' he added.
Apple's iPhone 6s battery fiasco caused some iPhone 6s handsets to shut down without warning.The issue only affected a 'relatively small batch' of the devices manufactured in 2014
Every time Apple launches a new handset people seemingly flock to Google to ask why their current iPhone or iOS is slow.
This trend has been seen every year since Apple released the iPhone 3G in 2008.
Explanations for the slowness range from Apple's iOS causing problems on older devices to the firm deliberately slowing down old phones to make people buy the new handsets.
This latter explanation is known as planned, or built-in, obsolescence.
This is the idea that manufacturer's deliberately make their products in such a way that they become out-of-date sooner.
By doing this, the companies can encourage customers to buy the latest model of a certain product.
This also stimulates demand for products because people return again and again.
Alternatively, the so-called 'slow iPhone phenomenon' may be psychological, as people believe their phone is being sluggish even if its not.
In a blog post Felix Richter, from analytics firm Statista,said:'Many of us know the feeling: one day we're perfectly happy with the smartphone we've been using for the past 12 months, the next day, after we've seen a colleague showing off his new gadget, ours suddenly feels inadequate, bulky and slow.
'The good news is we're not alone.'
He continued there are two possible explanations for these spikes.
The first is that it is a psychological phenomenon caused when people convince themselves their old phone is slow in comparison to the latest devices, and this could help them justify buying a new one.
Google Trends data (pictured) shows that searches for 'slow iOS' and 'slow iPhone' significantly peaked on the days after each was released. Explanations range from Apple's iOS causing problems to the firm deliberately slowing down old phones to make people upgrade
The second explanation, which Mr Richter said is much more plausible, is down to the operating system not being compatible with the hardware on older phones.
New operating systems are often designed to work most effectively with the more advanced processing power and RAM seen in the newer models meaning older models can struggle to keep up.
In 2014, a study by Harvard University PhD student Laura Trucco appeared to back up conspiracy theories that Apple deliberately slows down older models of its iPhones to encourage users to buy a new release.
The study analysed worldwide searches for 'iPhone slow' and compared those results with similar searches for the term 'Samsung Galaxy slow'.
Interestingly, it discovered the term was unaffected by new releases from Samsung, but this may be due to the fact Android updates aren't rolled out at the same time like iOS ones are.
While some MailOnline readers said at the time they hadn't noticed a slow down, others claimed Apple sabotages older phones through software updates.
'This is common knowledge,' one reader wrote.
'If you want to keep your iPhone running at the same pace do not do the software upgrade that comes out within six months of a new iPhone release,'
Sendhil Mullainathan, a professor of economics at Harvard, described the results of the study as 'striking'.
But added it does not prove Apple has done anything wrong.
No matter how suggestive, he said, the data alone doesn't allow anyone to determine conclusively whether their phone is any slower.
There are other explanations for why an older model iPhone may slow down, he claims.
For instance, the latest version of the Apple operating system, iOS, is always tailored to the newest device and may therefore not work as efficiently on older models.
'Hearing about a new release makes you contemplate getting a new and faster phone,' he said. 'And you suddenly notice how slow your old phone is.'
Dom Ferkin, managing director of UK-based iOS experts, Creation Application, agrees.
He previously told MailOnline he doesn't believe Apple is doing this intentionally.
'On every hardware release they tend to upgrade the chips and they are faster every time they are released,' he said.
'Each year they release a new iOS. If you're running an iOS 7 on a 5 chip, for example, it's comparable to running Windows XP on a Windows 95 machine.
'It's just enough to annoy the users, but it's needed if you want the slew of new features that Apple releases each year.'