Over the last week there's been increasing discussion and evidence of Apple's supposed introduction of CPU throttling mechanisms directly related to the battery wear level on Apple iPhones. The story started out with a report on Reddit of a user experiencing slow performance on an iPhone 6S and subsequent discovery that the performance restored to its full potential after a battery replacement.

Graph credit: John Poole, GeekBench 4 Blog

The report prompted GeekBench author John Poole to make use of the GeekBench benchmark submission database to plot performance of individual devices against iOS versions and discover a clear trend of ever increasing distribution towards lower performance points with newer OS versions.

Matthew Panzarino from TechCrunch was able to get an official statement from Apple when enquiring about the problem:

Our goal is to deliver the best experience for customers, which includes overall performance and prolonging the life of their devices. 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.

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.

Apple’s official statement sheds some light into the issue but comes short of an actual technical explanation of why the power management is failing. The company claims that the issue at heart is because of the battery degradation and its inability to supply sufficient current at a stable voltage.

Graph credit: Electronic Component News

Capacity and supply voltage of a battery decreases over time as a function of charge cycles and charging behaviour (Higher charging currents causing more degradation per cycle). This causes the total useable battery capacity before the cut-off voltage to decrease.

The problem facing the iPhones as Apple explains it is however two-fold; the issue at hand happens only during load spikes in which the battery isn’t able to maintain a high enough voltage for the PMIC to reliably be able to use as a source. 

Graph credit: Electronic Component News

SoC blocks such as CPUs and GPUs can have very short transitions from idle to load causing steep transients and load spikes going above the +10W ranges. As batteries degrade over time and the cell impedance also rises also in function of the state of charge and temperature, the current flow becomes restricted and the cell is no longer able to satisfy the power requirement at a high enough operating voltage.

Apple’s approach to the issue is to limit the peak power spikes by reducing the CPU frequencies over time as a function of battery wear. This solution however raises several questions; how does Apple decide the throttling behaviour and was the core fault an engineering fault or design choice?

As demonstrated in John Poole’s dataset Apple is throttling the CPU frequency in several stages. What exactly defines the thresholds to reach these stages can be either a rather simplistic counter taking into account the charge cycles of a given battery, or (and more hopefully) a more dynamic method that would be able to take advantage of the battery PMIC’s capabilities of determining battery wear. Given however that it seems that the OS is limiting performance even at high state of charges, such as fully charges batteries, it seems that the limitation implementation is unfortunately of the more simplistic type.

The second question comes to regard to as why it came to the issue in the first place as this should be a universal issue affecting a greater number of smartphones, not just Apple iPhones.

The first unique characteristic separating Apple iPhones from other smartphones is that Apple is using a custom CPU architecture that differs a lot from those of other vendors. It’s plausible that the architecture is able to power down and power up in a much more aggressive fashion compared to other designs and as such has stricter power regulation demands. If this is the case then another question rises is if this is indeed just a transient load issue why the power delivery system was not designed sufficiently robust enough to cope with such loads at more advanced levels of battery wear? While cold temperature and advanced battery wear are understandable conditions under which a device might not be able to sustain its normal operating conditions, the state of charge of a battery under otherwise normal conditions should be taken into account during the design of a device (Battery, SoC, PMIC, decoupling capacitors) and its operating tolerances.

If the assumptions above hold true then logically the issue would also be more prevalent in the smaller iPhone as opposed to the iPhone Plus models as the latter’s larger battery capacity would allow for greater discharge rates at a given stable voltage. This explanation might also be one of many factors as to why flagship Android and other devices don’t seem to exhibit this issue, as they come with much larger battery cells.

While much of the latter part of this piece is just my personal conjecture as to the deeper causes of the problem, it is clear that this is a larger issue for Apple that has no simple solution (beyond replacing the battery). How this affects more recent devices such as the iPhone 8 and iPhone X, or verifying if indeed the Plus variants would be less prone to the problem is something that will require a lot of testing, collaboration and data collection over longer periods of time.

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  • lilmoe - Thursday, December 21, 2017 - link

    Do they also notify the user about the drop in performance......?
  • philehidiot - Thursday, December 21, 2017 - link

    Maybe they should twist this properly. How about saying "our phones are built to a high standard and are very long lasting but we are held back by limitations in battery technology. Therefore, if you choose to keep your Apple phone running for a long time (and given the investment as well you might), you should see it like a car. It's an expensive investment designed to run and run but there are parts which will need servicing and replacing as a matter of course but most phones don't last as long as ours so you never normally run into this problem. You wouldn't choose to replace your entire car just because the tyres were worn and with our well built phones you don't have to replace the whole phone just because of a worn battery".

    This is of course including a fair amount of marketing bullshit but equally it is relatively true. iPhones are high end products and they do endure so it's easily spun into a decent narrative that disses other companies and promotes theirs.
  • vanilla_gorilla - Thursday, December 21, 2017 - link

    I'm an iPhone user, my phone runs fine, I think most of these comments are hyperbole -- but I agree with this. If the phone has detected that the battery has lost enough life that it is throttling performance it should notify the user.
  • zorxd - Thursday, December 21, 2017 - link

    why didn't they inform the user about the existence of such "feature"?
  • shabby - Thursday, December 21, 2017 - link

    Because apple knows best... this should at least be a toggle option somewhere in the settings menu.
  • Jon Tseng - Friday, December 22, 2017 - link

    Because they don't tell users about every feature. Juat same as they didn't tell X users about the feature that mutes the ringtone when you're looking at it. It's just not their style.

    They design a system to work as best as they want and let people have it on a take it or leave it basis.
  • lilmoe - Thursday, December 21, 2017 - link

    This pisses me off because I've seen so many iPhone users buy new iPhones because their 1 or 2 year old iPhones become slow and laggy to the point where OCD starts kicking in.

    Apple's processors should be very capable of handling ALL modern software and eye candy. Or, so they are advertise by all....
  • Stuka87 - Thursday, December 21, 2017 - link

    Those iPhone users are obviously hunting for a reason to upgrade. An iPhone does not feel "laggy" after only two years. 3-4 years, sure, then the OS starts to outrun the performance of the phone. And this issue has nothing to do with the SoC anyway.

    The issue is with the battery aging. And I have had this EXACT issue happen, where the phone randomly shuts down when CPU usage spikes. Replacing the battery does fix it. This OS update always would have prevented the shutdowns.
  • pennyfan87 - Thursday, December 21, 2017 - link

    I blame Anand.
  • philehidiot - Thursday, December 21, 2017 - link

    Ha, now that's funny!

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