I recently discovered Apple slows down the processor’s clock speed on older iPhones. Immediately, the public rose in anger at what could only be a marketing stunt to sell newer phones.
But, the public was wrong. As it turns out, this is simply a case of good engineering on the part of Apple.
To understand why they would slow down the processor speed of old phones, you should understand how lithium-ion batteries work and have a basic understanding of processors. Let’s start with Li-ion battery technology.
Lithium-Ion Battery Technology
Mobile devices use Li-ion batteries primarily because of their incredible energy density. They provide a lot of power and don’t take up much space. This is crucial for modern designs. For example, an iPad 5’s footprint is over 60% battery!** So, the weight and design of the tablet rely heavily on battery size.
We all want a battery that lasts a week—that is, until we must carry it. Engineers try to find a sweet spot somewhere between device usability and eternal battery life (aka “battery heaven”).
To understand the Apple problem, you should understand lithium-ion battery (LIB) limitations. LIBs use a chemical reaction in which the anode (lithium-doped cobalt oxide) passes lithium ions to the cathode (graphite) through a special barrier. The ions, using an electrolyte as a conductor, can pass through this barrier. Electrons do not pass through, and therefore get provided to the circuit. Note the electrons in the half-reactions for the cathode and anode:
If there’s a path for the electrons, the chemical reaction will take place. If not, the battery is essentially in a balanced state and holds its charge.
But here’s the catch: LIBs age. LIBs’ electrodes don’t go bald (thanks, middle-age), but they do get slower. A test from Battery University showed that LIBs experience a capacity drop of up to 20% after 250 charge cycles.
Not only do LIBs lose capacity, they lose the ability to generate high levels of current. The current production capability of a LIB is directly related to how fast its chemical reaction takes place. The faster the reaction, the higher the current.
When choosing a battery for your product, one spec you must consider is the current production capability. Typically, you know your required currents, so this is a pretty easy decision. You choose a battery that will give you enough current to power your design and enough extra headroom to be comfortable. What happens years down the road? The battery performance will degrade, but your device will still require the same amount of power.
Environmental conditions also take a toll. Like any good chemical reaction, temperature is a factor. The colder the LIB, the slower the chemical reaction. A slower reaction rate means a lower peak current. Couple this with an old, degraded battery, and you’re in for some trouble.
At some point, you’re going to run out of extra power-generation capability and have a problem. This is where Apple stands. Its older-model phones still require the same power (or maybe more?) as on ship day.
Some poor hipster waited in line for that phone, sold it to you when the next model came out, and you’re the poor fella that got stuck with an old battery. Now, Apple is slowing down your phone! Why? It’s for your own good.
The reason boils down to the way processors work.
Processors Require Power
At their core (no pun intended), processors are simply an intelligently organized collection of transistors. When combined, they make up logic gates that form the backbone of modern processing. For logic gates to function properly, they need power. If a gate doesn’t get enough power, it’ll often still work—just slower (Fig. 1).
1. Low supply voltages lead to increased propagation delay.
Heavy processing tasks, for example mobile gaming, require lots of power. Take a VR game, for example. It will need advanced video processing, audio processing, cellular radio communications, 3D rendering, and accelerometer algorithm processing—all in real time. Fortunately, phones are designed to handle this type of load.
But what happens when the battery degrades and can’t provide the necessary power?
Without sufficient power to the processor, the gates don’t operate as fast. There’s an increase in their propagation delay. But, processors operate with an anticipated propagation delay. The timing of operations depends on logic blocks coming to decisions within an expected number of clock cycles. Low power to a processor can slow down a logic block.
Then things break.
What happens if things break? If you have a well-designed device, it’ll realize there’s a problem and simply crash on you. If the device isn’t so well-designed, the electronics can be damaged.
Don’t take it from me, Apple says so in its statement on this matter:
"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."
So, what are Apple engineers doing to “prolong the life of their devices?” They’re slowing down the CPU frequency if the phone detects an insufficient battery voltage.
An iPhone owner on Twitter documented their iPhone 6’s CPU jumping from 600 MHz up to 1400 MHz after a battery replacement.
How iPhones Handle a Degraded Battery
Clearly, Apple is tackling this issue on two fronts. The first is made evident by the example on Twitter—slowing the phone’s CPU frequency. This is the “slam on the brakes” approach. The degraded battery’s lower power-generation capability leads to a larger propagation delay in the processor. Slowing the CPU frequency ensures that there’s enough buffer time to cover a non-ideal propagation delay.
The second is addressed in Apple’s statement—it’s the “work smarter” approach. Essentially, Apple engineers are working to spread out processor-heavy clock cycles to minimize the power required from the battery.
Using both approaches in tandem should help people with older-model iPhones, but that’s little consolation if your phone is running at 600 MHz.
What You Can Do About It?
How do you avoid this issue? First, take care of your battery. Don’t let it get too hot, especially if it’s fully charged or being used heavily. Furthermore, don’t use knock-off chargers. A lower charging voltage is proven to prolong battery life, but it also takes longer to charge.
Second, plan on getting a new battery every 350-500 charge cycles. They’re not that expensive, and can vastly improve device performance.
2. Design with batteries in mind!
If you’re designing devices that use a LIB, your users’ experience can hinge on battery management (Fig. 2).
Make sure to have excellent air flow around the battery. Think about whether the battery should be user-serviceable. Plan for battery degradation in the battery you’ve selected and in the battery-management circuitry. Don’t be afraid to follow Apple’s lead and match your device’s performance to its battery state. It’s much better for your device to run a little more slowly than for it to go up in smoke or randomly crash!
It Turns Out Apple Isn’t Evil
Though some people may wish it were true, this isn’t an evil marketing scheme. It’s just good engineering. A closer look at processor basics and lithium-ion battery technology tells us that Apple is simply doing what must be done to improve its users’ uptime—something we should all strive for.
I’d love to hear your thoughts on the issue. Let me know on Twitter(@Keysight_Daniel) or the Keysight Labs YouTube Channel!
**iPad 5 dimensions: 6.6 × 9.4 in. = 62.0 in.2
iPad 5 battery dimensions: 5.12 × 7.4 in. = 37.8 in.2
37.8/62.0 = 61.0%
Yes, I know I left out depth, but you get the point.