All lithium ion batteries, whether it’s in your phone, or in your laptop, degrade with use and over time. That is an unavoidable fact of lithium technology today.
This means if you discharge it, and then re-charge it, it’s going to degrade. This also means if you leave it on a shelf for a year, it’s doing to degrade. Also heat damages batteries, so leave it in a hot car one day, it’s going to degrade like you wouldn’t believe. Basically lithium ion batteries are constantly conspiring to expire on you, and you either accept that fact, or you become a slave to your battery and cater to all its exacting needs.
So the question of “what can I do to maximize the battery capacity of the phone over time?” does have an answer, but it's something like: use your phone during the day until it’s at about 40–60% charge, don’t charge it, then turn it off and put it in the fridge overnight. This seems a little odd, but there's good reasoning behind it. And to understand that, we first have to get our facts straight about lithium ion batteries. And in doing so, well learn about the best practices for maximizing battery life too.
So let’s get better informed and dispel some of the nonsense out there about this topic, shall we?
What does battery degradation look like anyway?
Take a look at this chart. It’s taken from tests these guys did on some lithium ion cells from LG, by fully charging, and then heavily discharging the battery. Each time the cell was “cycled”, the capacity was measured. So this chart shows how the clear and inevitable dwindling of cell capacity with each charge and discharge.
Admittedly, this was a heavy duty cell undergoing some pretty heavy cycling, which would be much more wearing than what a phone battery would experience, but it’s a nice graph of actual measurement of degradation over time
An often used convention for the overall life of the battery is the “80% capacity cycle life”, or the number of cycles until a cell drops to 80% of its initial capacity. In the above case the initial capacity was 2500mAh, and the cycle life would be how long it takes to drop to 2000mAh capacity, or 200 cycles.
How many cycles does the iPhone get?
On Apple phones and laptops, actual cycle counts can be found using an app (or for laptops, it’s available in system information):
(source: Battery Percentage app on iTunes)
For laptops, see here: Determining battery cycle count for Mac notebooks), it shows that some of the newer laptops get a good 1000 cycles, which would indicate that Apple are actually using a different cell chemistry for their laptops than they do phones.
Apple states the cycle life for the iPhone (they don’t specify which one exactly, so we assume all of the later models) as:
Your battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles. The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers a battery service for $79, plus $6.95 shipping, subject to local tax.
(source: Apple: Batteries - Service and Recycling)
Apple defines a “charge cycle” like this: that is to say, counting only discharges, and taking the equivalent of 100% of the battery’s capacity, regardless of whether it comes as one chunk, or over several charge sessions. (Note: this later on, you’ll see why this is incorrect, and the exact state of the battery matters
(source: Apple: Batteries - Why Lithium-ion?)
You can work out what your battery life should be if you take cycle life at face value and ignore all other effects: let’s take a couple of scenarios:
- You come home after school/work and your battery is on 30%, and then you plug it in and leave it plugged in overnight. You’ve used 70% of a cycle per day, which works out to about 2 years for your phone drops to 80% capacity.
- You use your phone heavily in the morning down to 50%, plug it in to charge back up to 100%, then use it again in the afternoon and evening down to 20%. You’ve used 130% of a cycle per day, which works out to about 1 year for your phone drops to 80% capacity
- You’re an avid Pokemon Go player, and you head out for Go sessions three times a day, discharging your phone down to 20% each time. You’ve use 240% of a cycle per day, so that works out to be about 7 months before you drop to 80% capacity (in this case you should probably get a portable battery pack)
So does that mean there’s no degradation when I charge?
Not so. This whole charge-cycle deal is more about making it easy for you to estimate how durable your battery is going to be; and for Apple it’s to help them enforce warranty exchanges. It’s only an approximation, or rule-of-thumb for what happens in reality.
For a better estimate of degradation, you need to also look at how much you charge and discharge a lithium ion cell. So you can’t just count how much energy comes out of the battery like Apple do for their warranty above, you need to factor in the fact that lithium ion (and in fact most other battery chemistries) will wear faster the more you discharge it.
In that respect, batteries are a bit like muscles - you can walk for miles at an easy amble, but lifting weights will leave you tired out, even if you’re actually spending the same amount of energy.
Take a look at this chart from the tech datasheet of a lithium ion battery manufacturer. It’s maps how deeply you discharge the battery (Depth of discharge: if you discharge your phone from full to 90%, that counts as a 10% DOD, it’s the opposite of SOC, state of charge, which is how we normally think about batteries).
It show shows that if you drain this particular cell all the way from 100% to 60%, and then charge it up again, you’re going to get about 10 000 cycles out of it. But if you only drained it from 100% to 80%, you’re going to get ten times more cycles out of it (but each cycle is only worth half the energy, but then again, that’s still 5 times more cycles per unit energy output).
This means: by more frequently recharging, and not letting the battery drain, even though each cycle puts out less energy, so you’ll need to recharge twice as often, your battery will degrade 5 times slower! While the above chart may not be representative of an iPhone battery, the principles will be the same: charge more often, and you’ll get more cycle life out of the battery even after accounting for the reduced energy used per cycle.
Apple’s 500 cycle life is therefore only an approximation, Apple probably did some studies or collected data about charge habits of their users, and then found a cycle life that matches the average DOD and other charge characteristics. This also means that if you do extra-deep DOD discharges, you’ll probably end up with fewer cycles than if you did light DOD discharges, which would make you more likely to be eligible for Apple’s warranty battery replacement.
Does that mean I should leave it plugged in, and the battery will last forever?
Actually, there’s yet another factor at play here. While we can confidently say that leaving the phone on charge is better for battery life than letting it discharge and then recharging it, that doesn’t necessarily mean the battery will be unaffected.
Lithium ion batteries degrade anyway, even when not in use, known as “calendar aging”, and unfortunately the degradation depends on how much energy is stored. In that respect it’s a bit like not going to the toilet often enough - if you hold it in, it’s going to be bad for you.
Here’s a chart from the venerable Battery University, a great and informative website if you want to learn more about batteries
(source: BU-702: How to Store Batteries)
Looking on the right two columns (note, cell phone batteries are probably a type of lithium ion called lithium cobalt oxide - LiCoO2 chemistry, though it’s hard to say these days since Apple and others have probably done some custom battery work to improve cycle life). The left column is for what happens when you store a battery for 1 year at 40% charge; the right column is for 100% charge.
At room temperature, 25°C, you can see that a battery left at 100% charge will have a 20% reduction in capacity even if unused; while a battery left at the ideal storage charge of 40% will have a measly 4% reduction in capacity.
What this means for your phone is: even though you’re using it throughout the day, this calendar ageing is happening in the background. If you left your phone plugged in all year round, then you’d expect calendar ageing at a similar level to the right column, or 20% capacity reduction. Realistically, the phone might spend a third of the time at 100%, and the rest of the time at some degree of discharge, resulting in calendar ageing of maybe 10% a year. (though the figures above would be for some typical batteries, we don’t have the data for Apples battery technology, maybe they’ve done something fancy to improve calendar life, or maybe they haven’t and want you to replace your phone more often).
Uh huh, so what do I do now?
So to conclude, do you leave your phone charging all the time, or do you unplug it when it’s done? Well like I said, the optimum method would be to let your phone discharge to 40–60% to reach this ideal storage charge (which would minimize calendar ageing), don’t recharge it (minimizing cycles), and keep it in the fridge overnight (minimize thermal degradation).
But frankly, you’re damned if you do, and damned if you don’t, so you might as well use your phone in whichever way seems most convenient for you, and when your battery gets too low, either consider an upgrade, or get a battery replacement (Apple do it for $79, but 3rd parties will do it cheaper, you can also do it yourself).
Bonus:
If you were truly anxious about optimization, let’s do some back-of-the-envelope calculations: let’s assume that the calendar ageing is 20% after 1 year if left at 100%. Therefore, let’s assume that the difference between leaving it plugged in, and unplugging it is 8-hours of extra time spent at 100% battery. Which means a third of the above value: leaving it plugged in all the time results in 6.6% extra calendar ageing from being plugged in
But, leaving it plugged in also saves on your battery cycles, so that offsets this extra calendar ageing by reducing the number of battery cycles. Let’s say you’re using the phone while it’s plugged in, and you would have normally drained the battery from 100% to 50%, let’s assume this is roughly in-line with what Apple considers a “cycle”, and therefore your equivalent saving is 0.5 of a cycle a day, or 183 cycles a year. If 500 cycles is 80% or 20% degradation, then 183/500 is about 7% degradation from extra cycles from not plugging it in
So, ballpark, if you’re using the phone while it’s plugged in, then you’re causing 6.6% extra calendar ageing, but also saving 7% cycle ageing. For a sum of a whole 0.4% difference between leaving it plugged in while in use, and unplugging it.
Of course, I’ve made a huge number of assumptions and estimates, I’ve only briefly mentioned the impact of temperature; I’ve not gone into how different top-up charge strategies that the charge controller might use for a phone plugged in might have an effect; there are a plethora of other factors that you’d need to take into account for a comprehensive explanation of what’s going on. Furthermore, your phone usage pattern might not match the assumptions I’m making, which would render some of these calculations irrelevant for you. Nevertheless, I hope I’ve adequately demonstrated how futile it is the exercise to try to optimize for phone battery life.
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