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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  • FreckledTrout - Thursday, December 21, 2017 - link

    For my iPhone 7 its only a 29 step process on ifixit to replace the battery. Everyone should be doing it. LOL your post is humorous. Pretty much everyone who buys an iPhone should not be pulling it apart to replace the battery, ever.
  • sonny73n - Friday, December 22, 2017 - link

    I'm not worried about replacing the battery myself. I'm worried about Apple just implanted another software which probably always running in the background of my iPhone to monitor the battery current. But their main purpose is just to slower the older models out so people have to buy new ones.

  • Aloonatic - Thursday, December 21, 2017 - link

    AFAIK, the user that highlighted this also stated that the phone was still slow when connected to their mains charger, which is what I see a lot of Apply phone users do around the office (I don't see and Android users do the same but then many Android users don't use their phones so much) and as they are suing mains power the battery excuses should not exist, exposing this as the BS excuse that it is.

    If this had come out a few weeks ago, before Android 8.1 came out, I'd have said that Google were probably doing something similar as since the Oreo update my Nexus 5x was frustrating and almost impossible to use at times, but 8.1 appears to have fixed most of the issues that I've noticed.
  • Demi9OD - Thursday, December 21, 2017 - link

    I wonder if once you reach the battery threshold for slowing your CPU down, if replacing the battery or running it while plugged in can even reverse the throttling? Or is performance permanently crippled in software if you don't replace your battery soon enough?
  • mkaibear - Thursday, December 21, 2017 - link

    From the original thread it looks like once you replace the battery it goes back up to speed. Doesn't do it if you're running it from the mains though.
  • haukionkannel - Thursday, December 21, 2017 - link

    Jep. So if you change the battery, the speed is again just as new.
    The Apple did have two alternatives in here. Reduce the time phone is useable, (like most phone maker do) or reduce the speed to maintain long operation time.
    Hard choice in some situations. But as a customer I would like to make that decision, but Apple rule is to make all choices so that customer does not have to worry those.
    It has worked well to Apple, so most likely they keep on making these decisions, because their devices sell well, because end user don't have to decide anything, except the color of the phone.
  • sonny73n - Friday, December 22, 2017 - link


    Bullshit. I charged my iPhone 6s less than 50 times since I bought it brand new sealed in box. The battery is healthy but since the accidental update, the performance drops noticably.
  • Jon Tseng - Friday, December 22, 2017 - link

    "Simple solution: replaceable battery."

    I think what you meant to say is "replaceable battery and thicker and less sleek design"

    Unfortunately when it comes to basic design mechanics there is no free lunch...
  • lilmoe - Thursday, December 21, 2017 - link

    Well yea, what other incentive do Apple users have for upgrading?

    The thing is, this isn't anything new. This didn't start with this "battery issue" thing which clearly is a major design flaw in the battery, platform or both. iPhones slow down considerably with each major update and iPhone users will be more than happy to lie about it.

    Almost ALL iPhone users I know make it a life decision to update since they KNOW pretty darn well that it's going to slow their phone. In the matter of fact, the question I get the most from them is "will update x slow down my phone?"

    This isn't about newer software being more taxing and resource intensive. This is about deliberate obsolescence that EVERYONE has denied despite numerous accusations from iPhone users.

    I don't know, maybe this would focus more light on this really old issue and make tech blogs investigate this "dark secret" they've been trying to hide while falsely accusing other platforms of that same fault.
  • phillyboy - Thursday, December 21, 2017 - link

    Keep drinking that Kool-Aid, buddy. "Dark secret" lol

    My wife's original 5S phone, with the original battery, still works great and is still supported by Apple, something that the two-year-lifecycle Android phone vendors don't seem to give a shit about long term. Minus Google.

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