The internet got all kinds of angry at Apple recently. It turns out, Apple slows down the processor’s clock frequency on older iPhones. The internet took this as an opportunity to jump on “Evil Apple” for what it perceived as a marketing ploy to get people to upgrade.
But, the internet was wrong.
As it turns out, this was simply a case of good engineering by Apple’s engineers.
To understand why they would slow down the processor speed of old phones, you have to understand how lithium ion batteries work and have a basic understanding of processors.
Let’s start by taking a look at lithium ion battery technology.
Lithium Ion Battery Technology
Mobile devices use lithium ion batteries primarily because of their incredible energy density. They provide a lot of power and don’t take up much space.
We all want a battery that lasts a week. That is, until we have to carry it. Engineers must find a sweet spot somewhere between device usability and eternal battery life (known as “battery heaven”).
To understand Apple’s problem, you need to understand how lithium ion batteries (LIB) work. 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 cannot, and therefore get provided to the circuit. Note the electrons in the half-reactions for the cathode and anode:
Figure 1. Cathode half-reaction
Figure 2. Anode half-reaction
If there’s a path for the electrons, the chemical reaction will take place. If there’s not, the battery hangs in a balanced state and holds its charge.
But there’s a catch. LIBs age. 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, but they also lose the ability to generate high levels of current. The current production capability of an LIB is proportional 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 should consider is the current production capability. Typically, you know your required currents, so this is an easy call. Choose a battery that will give you enough current to power your design and enough extra headroom to be comfortable. But what happens a few years down the road? Your battery’s performance will degrade, but your device will still need the same amount of power.
The environment also takes a toll on LIBs. Like any proper chemical reaction, temperature is a factor. The colder an LIB, the slower the chemical reaction, the lower the peak current. Couple this with an old, degraded battery and you’re in for some trouble.
Eventually, you’re going to run out of extra power-generation capability. This is the problem Apple’s facing. Their older-model iPhones still require the same power that they did on day one, but their batteries aren’t holding up.
Now, Apple is slowing down your phone! Why? It’s actually for your own good. The reasoning comes down to how processors work.
Processors Need Power
Basically, processors are just an intelligently organized collection of transistors. When combined, they make up logic gates that form the core of modern processing (no pun intended). For gates to function properly, they need power. If a gate doesn’t get enough power, it’ll still work; it’ll just operate more slowly.
Figure 3. Low supply voltages mean increased propagation delay!
Heavy processing tasks require a lot of power, and mobile devices are designed to handle this load. But what happens if a device’s battery is degraded to the point that it can’t provide enough power?
Without sufficient power to the processor, the gates won’t operate as fast. Put simply, their propagation delay increases. Processors operate expecting a certain propagation delay. The timing of its operations depend on logic blocks making decisions within an expected number of clock cycles. Low power to a processor slows down logic blocks.
Then things break.
Then what? If you have a well-designed device, it’ll realize there’s an issue and simply perform an emergency shutdown. If your device isn’t so well designed, the electronics can be damaged.
Don’t take it from me, Apple says as much in their 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 actually 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 Deal with Old Batteries
Apple is attacking this issue on two fronts.
The first is clear from the Twitter example – a slow down of the phone’s CPU frequency. The old battery’s lower power capability means 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. Apple’s engineers are spreading out processor-heavy clock cycles to minimize the required battery power.
What Can You Do About It?
How do you avoid experiencing phone slow down?
First, take care of your battery. Don’t let it get too hot, especially if it’s fully charged. Also, don’t use off-brand chargers. A lower charging voltage is proven to prolong battery life, but it also takes longer to charge.
Second, expect to get a new battery every 350-500 charge cycles. They are not that expensive compared to a new device, and they can massively improve processing performance.
If you’re designing devices that use LIB, your users’ experience can hinge on battery management.
Make sure to have proper air flow around the battery. Also, think about whether or not the battery should be user-serviceable. Plan for battery degradation when selecting a battery and designing battery management circuitry.
Figure 4. Design with battery limitations in mind!
It Seems Apple Isn’t Evil
Though some people may wish it were true, this wasn’t an evil corporate marketing scheme. It’s just good engineering. A closer look at processor basics and lithium ion battery technology shows that Apple is simply doing what they have to do to improve their users’ uptime – a noble goal.