The factor that contributes to battery wear is full charge discharge cycles. It’s a fact of the chemistry of the battery. You’ll notice that most devices arrive out of the box charged to about 50%, which is the most chemically stable level for the battery.
Keeping the charge as close as possible to 50% (max 80% and min 20%) will likely make your battery last longer, but by how much is debatable.
If I were a manufacturer I would kill the percentage display, limit the charge to 80% but display it as full and display a 20% charge as empty. But if I did that, I suppose I would reduce profits from battery replacement and device upgrades!
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A post was split to a new topic: MacBook Pro battery setting that limits charge to 80% not working
I presume Apple has collected more than enough telemetry from our devices to publish a comprehensive paper on the subject. It would be a delightful project for a competent statistician.
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If only it would be that simple. Modern Lithium rechargeable batteries are affected by several factors, including:
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Age. Even if it has been sitting disconnected since manufacture, it will lose capacity over time. After 2-3 years, the loss will be significant.
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Charge cycles. As you wrote, this is a big deal. Modern batteries claim to support more cycles than ones designed a decade ago, but all will lose capacity as they experience more drain/charge cycles.
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Temperature. If it gets too hot, lifespan will be reduced. Not just obvious things like leaving a phone on a car’s dashboard in summer, but also the heat of a CPU running at capacity (e.g. a long gaming session) or a display driven to high brightness (e.g. outdoors on a sunny day) will likely have an impact.
Fast charging can also create heat. But generally not enough to cause overheating unless the device is trying to fast-charge a battery that is close to full.
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Deep discharge. If a Lithium battery drains all the way to nothing, it will be impossible to recharge it. Protection circuitry will prevent it from draining that low - the circuitry will power-off your phone when the batter gets low enough for this to be a concern, but if it drains to shutdown and you don’t charge it soon afterward, the natural self-discharge rate may reduce it down to the point that the protection circuitry will prevent all charging - to avoid the risk of fire from trying to charge a battery that can’t be charged.
But even if the level doesn’t drop below the “no charge” threshold, running low on a regular basis can shorten overall capacity.
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Charging to 100% all the time. Many claim that merely allowing it to charge to 100%, even at slow rates that don’t cause overheating, is bad. I’m not convinced about this one. If it has an impact, I think it’s going to be less than the items I mentioned above.
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Storage when fully-charged or drained. The recommendation is that a battery should be at around 80% when disconnected for long-term storage in order to maximize shelf life. Too low may result in self-discharge below the “no-charge” point if it remains in storage for too long. Others here have written about a chemical reason why storage at 100% is bad, but I don’t really understand that part.
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Great post, but I’ll note that Apple recommends long-term storage at about half-charge - as close to 50% as you can.
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I won’t try to argue here, but note that Apple is talking about storing a device. The 80% figure I heard is meant for when a battery is disconnected from any device. For example, when a manufacturer ships batteries to a factory for assembly or if you have an (older) device with removable batteries, and you remove it from the device for storage.
I assume that there is a difference between the two situations.