Will slow charging extend iPhone battery life?

I use my iPhone with the app Sleep Cycle every night. It tracks my sleep quality. Because of that, I have to keep the iPhone on charger every night. My iPhone is always fully charged every morning. I have always been trying to keep as long life for iPhone battery as possible since I bought my first iPhone back in 2008. Based on my reading over the years most batteries will last longer if they are charged slow, so I use the old Apple USB Charger 5W every night, instead of the more powerfull 20W (I think it is 20 W or is it 18?) USB-C Charger that came with my iPhone 11 Plus. Any thoughts?

All of your assumptions are true. I’m not sure what other thoughts you are looking for.

For those who buy a new phone frequently, it probably doesn’t matter. Those that use their phones a lot during the day, may not be able to get by with one overnight charge and will want a fast charge or two to keep up. The most important factor in battery life is the number of full charge, but test have shown that fast charging does have a impact.


Agreed, but I would also point out that the number of charge cycles will probably impact battery life a lot more than the charge speed.

For example, my wife and I have similar model iPhones, with batteries that were replaced at about the same time and we both use 5W chargers. Mine holds more of a charge than hers because she typically runs the battery down to near-empty every day (gaming, social media, etc.) while I don’t (most of my gaming on the phone is done while it’s attached to a power brick).

I would say that, yes, you can maximize battery life using a slower charger, but your battery usage will have a greater impact. And you should expect to need a battery replacement after 2-3 years, no matter what your usage pattern may be.


I was curious to see if anybody had some insight in the Optimized Battery Charging. (iOS 13: Settings > Battery > Battery Health > Enable Optimised Battery Charging). According to my way of thinking, I would like the charging to finish at 0530 using a trickle of current during the night since my pattern is to wake between 0600 and 0730. If the optimization worked like that the power of the charger would be unimportant. If this is even technically possible to do I do not know.

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With optimized battery charging, the phone uses machine learning to understand when you most often wake up in the morning. When it is working, it will do a normal charge to 80%, hold it until an hour or two before it thinks you normally wake, and then complete the rest of the charge. If you look at the battery graph when you wake you will see this.

The last 10 to 20 percent of a charge are what stresses a battery most, and holding at 80 with a trickle charge rather than trickling at a a full charge should stress the battery less long-term.

I should also note that fast charging with faster than a 1 or 2 watt charger will fast charge only to a certain percentage (I’m not sure what the real amount is, but it could be 80%, it could be lower) and then the phone slows the charge to a lower wattage - again, to put less stress on the battery. This is one of the reasons why I think that using fast chargers really doesn’t have a major effect on long-term battery health compared with slower charging.


Just so you know, there’s an entire discussion already on that exact topic. Short answer: optimized battery charging should do exactly what you want.


Basically, anything you can do to reduce heat will help your battery’s longevity, so slow charging using a lightning cable will be much better than fast charging or inductive charging.

Also, I’ve heard that - depending on your daily power needs - it’s better to charge up to about 80% and not drain your battery all the way down. Reducing the “pressure” in the battery keeps it much healthier.

My iPhone 11 Pro Max’s battery health is hovering around 99% after almost a year of use, and though I don’t strictly follow the guidelines I’ve given I do try to follow them when it’s convenient.

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Is it better to charge to 80% and drain to 50% repeatedly than it is to charge to 100% and drain to 70% repeatedly? Certainly it’s much easier to charge to 100% than to 80%; would it be worth the effort to avoid charging to 100%?

Is that another way of asking if charging from 70% to 100% generates more heat than charging from 50% to 80%?

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As I understand it, Li-ion battery packs are stressed more at close to full charge than they are at less than full charge, so experts suggest that charging up to 80% and small discharges and charge cycles should be better for long-term battery health than charging fully and discharging to and charging from 70. I’m not sure that anybody has quantified what the difference is for iPhones; whether the long-term battery health would be worth the hassle of paying such close attention.

Then again, I’ve heard of people who almost always use their phones when they are plugged in and after a couple of years of this they still had full battery health.

My feeling is that I’m just going to use my phone and charge it when it’s convenient to me. I have a 31 month old iPhone X at 91% health so I feel that I’ve done alright.

Yeah, two big factors are lifestyle and battery capacity of your device.

I’m retired now, and my iPhone 11 Pro Max has scads of battery life so I charge the device when it gets below 50% (and I notice), and don’t need 100% charge to get through my day.

I’m pretty much always near a desktop, and when I’m out it’s just to shop and I don’t spend lots of time in stores so I do plug in to the car for navigation and CarPlay so it gets charged there too.

If you have a device with less capacity and are away from chargers for long periods of time, you may have less choice in the matter.

Thanks, all, for the useful information.

I’m most disappointed to learn that the “Optimized” charging I had heard about first a few weeks ago is actually not optimized. I turned it on assuming (never assume!) that it would optimize the life of my battery by stopping the charge at 80%.

Perhaps I can train myself to, before stumbling toward the bathroom in the morning, first unplug the charger, to stop the so-called “optimized” from pushing on to 100%.

Perhaps Apple will add that as an option…

That’s not what optimized battery charging does. It is not supposed to stop charging at 80% or at any other percentage.

The purpose is to use some machine learning to determine when it can reasonably expect to be connected to a charger for an extended amount of time (e.g. at night at home, or in your office), disabling the “fast charge” mode in those situations:

The idea is that in these situations, there is enough time for a slow charge to fully charge the device, so there is no need for fast charging. But that’s all it does.


True, but there are some windows laptops that allow the battery to stop charging at 80% for people who mostly plug in and only occasionally use the device off power, in order to increase the longevity of the battery pack. I can see why Apple doesn’t want this setting on the iPhone - the idea is to allow people to have a full battery when they do a long term charge - but there may be some people who would benefit from this behavior. Still, there is such a thing as too many options, and the people who would want and benefit from this probably aren’t worth adding the option to settings.

Optimized charging does not change the rate of charging, it simply holds the battery at 80% until it’s necessary to finish charging to 100% by the usual time you remove it from the charger.

Tesla allows drivers of their electric cars to specify the max charge and they recommend 80%-90% for daily use. On those days when a full charge is desired, bump it up to 100% and plug it in. After a day or two, if you forget to set it back to the “daily max” setting, the car will remind you to lower it.

Seems like an easy thing for Apple to do if they wanted to. If you don’t burn up your battery each day, why not reduce the stress on the batteries by allowing you to specify a lower daily charge?

For quite a while now I’ve been reading that Apple and Tesla have each been on track to begin making their own batteries in the not distant future. Elon Musk has been very vocal about it:

Apple isn’t as far along in the game, but they usually keep their mouths shut until the end result is ready for debut:

Well, when a replacement battery for your “mobile device” costs $30,000 then you really, really want it to last 10-15 years at least. I limit my charge to 70% most days and charge with a slow (7,000 watt) charger at home. I only charge above 90% with fast (150,000 watt) chargers when I’m traveling.

When it comes to phones, though, I really think most people would be better served by just plugging it in when they can and not stressing about it. I’ve been working with batteries since the 1970s and obsess a bit about longevity (I have an iPhone 6s Plus in daily heavy use that still has 88% maximum capacity) but, seriously, I’d be happier if I just let it go and replaced the battery or the phone every couple of years.


That’s my understanding … though I’ve never measured anything.

My practice is to charge it when it’s below 50% and disconnect it when it reaches at least 80%.

My wife will be replacing her iPhone 7 which has something like 80% battery health with my 11 Pro Max, but she’s always running around with the 7 connected to a battery brick - I gave my daughter and wife battery bricks last Christmas.

My daughter’s getting a 12 128GB, and I’m ordering a 12 Pro Max when they become available.

I want some carrier flexibility, and I’m frankly a bit sick of 7 generation phones and their flakey batteries. If I’m reading it right, I should save about $45/month switching to T-Mobile from AT&T.

Of course, even if that’s true there is no payback from the cost savings - it would probably take 56 months to make up the cost of the new phones from cheaper plans even if we ignore the present value of the 11 Pro Max - but there should be a definite quality-of-life improvement over those rickety old phones.

Hopefully now that we know a bit more about battery health we can preserve it more effectively.

Certainly, iPhone batteries are much cheaper than Tesla batteries but there’s no reason Apple couldn’t take some stress off the battery by allowing users to set a lower charge limit, if they want. I never use more than about 60% to 70% of my phone’s battery in a day, so there’s no reason to charge it to 100% each night if the lower limit would extend its life.

Apple was never too big on giving users a ton of options. I bet they figure at $69 for a new battery on a device that costs on the order of ~$1k they don’t need to offer settings for exactly how and when to charge.

Just use it till the battery’s toast and then get it replaced. I doubt they’d put it exactly that way though, doesn’t sound terribly green to me. Still, I’m pretty sure that’s essentially how they see it.

Personally, I’m OK with that. The problem I have with swapping a battery after 2 years of use or so is not the $69, it’s that I have to get one of these super rare appointments in the middle of my work day, and then it’s not at all clear that they’ll just quickly do the swap in the back and return my phone 20 min later so I can get back to work. There are these great stories of people being told they need to leave it over night and stuff. :man_facepalming:

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