(Credit: Irina Shatilova/Shutterstock)
Even decades after mobile phones became ubiquitous, many questions and competing theories still remain around phone batteries. People still routinely ask: Should you leave your phone plugged in overnight? Is charging to 100% bad? Is it even safe?
This discussion goes well beyond the worry about moderate harm to the phone, as some people fear "overloading" the battery. After all, years ago, Samsung's Galaxy Note 7 smartphones were bursting into flames. But as we've explained before, unless a device has some serious manufacturing defects (as Samsung's phone did), a fire in your pocket (or on the nightstand) is unlikely.
The real issue is that battery advice is often contradictory. So we dug into the latest, most reliable research to separate fact from fiction, break down the biggest smartphone charging myths, and give you real, actionable advice.
Charging My iPhone Overnight Overloads the Battery: False
One thing all the experts agree on is that most devices are smart enough today to avoid overloading. Extra protective chips inside ensure that can't happen in a tablet, a phone, or even a laptop. Once the internal lithium-ion battery hits 100% of its capacity, charging stops.
With older phones, if you leave them plugged in overnight, they will use a bit of energy by constantly trickle-charging the battery whenever it falls to 99%. That is eating into your phone's lifespan (see below). Newer phones and OSes compensate for this; since iOS 13, Apple has shipped phones with Optimized Battery Charging turned on, so iPhones will usually not charge past 80% when connected to a charger for an extended period (like, say, overnight). As of iPhone 15, you can (and should) set the phone to never charge above 80% (go to Settings > Battery > Charging); if it's on a charger, it won't even try to recharge until it reaches at least 75%.
The Best Thing to Do
Don't worry about overnight charging. Plug the phone in or place it on the wireless charger when you go to sleep. If you wake up in the night, unplug it or move it to prevent continuous trickle charging. You could also plug your phone into a smart plug that's set to turn off on a schedule. And make sure that the Optimized Battery Charging option is turned on.
Potential Problems With Charging Phones Overnight
Trickle charges can generate some heat. Many experts recommend removing a phone from its case to charge it overnight. But that's not always feasible with a complicated protective case.
At the very least, do not stack anything—books or other devices, for instance—on top of a charging device. And for the love of Jobs, do not put your phone under your pillow. Do any of the above, and you can expect the phone to get hot—not necessarily hot enough for spontaneous combustion, but warm enough to damage the battery (see below).
(Credit: Shutterstock/spyarm)If you're afraid of fire, leave the charging device on a dish or saucer while it's plugged in, or place it on something metal that is more likely to dissipate heat, like a heatsink for the chips inside a PC. That's not really an option if you use a wireless charging pad, so don't sweat it.
Bad Cables
If you're charging with a knockoff cable that isn't made by the device's manufacturer or isn't at least "certified" in some way (iPhone cables should be MFi certified, for example), that could be a problem. The cord and connectors may not be up to spec for your phone or tablet. Don't skimp on buying chintzy cables. That said, as of iPhone 15, even Apple supports USB-C charging cables, which you may already have from using with other devices.
Therefore, don't think you have to buy chargers made by your phone manufacturer. Most handsets these days support universal standards. One such standard is USB Power Delivery (PD)—all major modern phones from Apple, Samsung, and Google support it. You just need a compatible cable to connect the PD charger to the phone.
I Should Freeze My Phone to Prevent Battery Problems: False
Lithium-ion batteries hate two things: extreme cold and extreme heat. Repeatedly charging a phone in sub-freezing temps can create a permanent plating of metallic lithium on the battery anode, according to BatteryUniversity. You can't fix that problem; it's simply going to kill the battery faster.
(Credit: iStock/sankai )Your phone battery is not alone in hating heat; all the device's internal components feel the same. Your phone is a computer, and computers and hot air have been mortal enemies for decades. Leave your black-screened phone sitting in the sun as you laze by the pool, but don't be surprised when it throws a warning that it needs to cool off. Instead, give it some shade. In the summer, keep it off the car dashboard.
Apple specifically says that charging iPhones in an environment hotter than 95 degrees F (35 degrees C) can permanently damage the battery. Expect the same with any modern phone.
Things You Should Never Do
Don't charge a phone when it's too cold or hot. And don't put your phone in the freezer.
My Battery Should Always Drop to Zero Power Before I Charge It: False
Running a phone until it's dead—a full discharge—is not the way to go with modern lithium-ion batteries. Try not to let it get close to 0%. That wears out a lithium-ion battery faster than normal. Partial discharge is the way to go.
(Credit: GettyImages/Priscila Zambotto)Batteries are on borrowed time from the get-go. The insides are in a constant state of decay, beyond help. Over time, the materials inside will simply hold less and less power. If you've got an older iPhone still in use and wonder why it only lasts a few hours, compared with the almost full day (or two) you got when it was new, that's why. Capacity diminishes over time.
Drain a phone battery to zero only when you want to recalibrate the internal sensor that displays your phone's battery level. Success here is hardly guaranteed—in fact, many people don't think it works at all—but it's recommended by some, especially when a phone reading a 10% charge (or even 20% or 30%) abruptly dies.
Note that even when you do use the phone all the way to auto shutdown, that may not mean the battery is actually at 0%. Leave the phone alone for a few hours if you want to try recalibrating. Then give it a reset for good measure.
The Best Thing to Do
Plug the phone in before it asks you to enter low-power mode; iOS will prompt you to turn it on when you hit 20% power. Plug the phone in when it's between 30% and 40%. If you can't turn on Optimized Battery Charging, pull the plug at 80% to 90%; going to 100% with a high-voltage charger can put some strain on the battery. That last 20% is causing the most chemical stress.
Keep the phone's battery charged between 30% and 80% to increase its lifespan. Eighty always seems to be the magic number when it comes to lithium-ion batteries.
Apple claims that with fast charging, iPhone batteries can charge to 50% in just 30 minutes. That requires a USB-C power adapter, which on older phones means using a special USB-C-to-Lightning cable or a higher-voltage charger such as the one from an iPad or even a MacBook.
Don't fast-charge your phone if it doesn't support it. That's just another thing that will strain the battery. But it's hard to find a phone these days that doesn't support fast charging. iPhones have all had the aforementioned USB PD since the iPhone 8 came out in 2017.
For more, read Does Fast Charging Ruin Your Phone Battery?
My Battery Develops a 'Memory': False
Phones developing a "memory" was a problem with older nickel-cadmium (NiCad) batteries. That's where the whole "discharge the battery entirely" dictum came from. As we've said, that's not necessary with lithium-ion batteries.
So why do lithium-ion batteries not seem to last as long as they age? It's not about "memory." It's about capacity. Over its lifetime, your phone battery degrades enough that, with the same charging time, a new phone could reach a full charge, while an older phone might only reach 82% or so. BatteryUniversity calls it "old man syndrome."
(Credit: Shutterstock/Jirsak)Another way to look at this: Newer batteries are simply hungrier to suck up all that power.
Apple claims that its "lithium-ion batteries are designed to hold at least 80% of their original capacity for a high number of charge cycles"—there's that 80% figure again—but it admits that the percentage varies by product.
New iPhone batteries' fast charging gets to 80% pretty quickly. After 80%, capacity increases more slowly, partly to prevent heat buildup and thereby extend battery life. But guess what? Fast charging isn't great for a lithium-ion, either. It makes it corrode even faster.
Older iPhones came with a 5-watt charger block. It works even with today's phones, but you can charge faster with a 10W or 12W charger (like you get with an iPad). iPhones from the 13 to 15 can charge between 25W to 27W; iPhone 16 peak is 30W, while the iPhone 17 series is 40W.
Apple doesn't even include charger blocks with phones anymore, but the company sells a USB-C 40W Dynamic Power Adapter with 60W Max for $39 and a 35W dual-port adapter for $59. Or you could buy a third-party USB-C charger.
If you use Qi-based wireless charging, fast charging is now available. Apple's own MagSafe chargers, available for the last three generations of iPhones, can charge at 25W.
The Best Thing to Do
Stop worrying about battery "memory." If you're going to charge overnight, don't fast-charge. Your charger can be a lower voltage, such as the 5W charger that came with your pre-2017 phone. Better yet: Get a MagSafe charger that holds your phone upright in landscape mode—on an iPhone, that can turn your phone into an instant overnight alarm clock with the StandBy mode.
(Credit: Apple)Phone Batteries Last Only a Few Years: Sorta False
A phone battery measures its lifespan in "charge cycles." That means that every time you fully discharge the phone's battery, it counts as one cycle. But it doesn't mean you went all the way to zero.
For example, say your phone is at 80%. You go down to 30% (that's 50% down, aka half the battery capacity). If you charge it back to 80%, you use that 50% up again—that's half a cycle. You could use up 75% one day, 25% the next; again, that's one cycle. Expect iPhones to have a lifespan of 400 to 500 charge cycles—but again, that's not necessarily because the phone is actually plugged in 400 to 500 times.
If the phone's capacity has eroded enough, you may have to do that 50%-charge-and-use a couple of times a day, and the battery lifespan will go downhill even faster. Here's Apple's graphic trying to explain it:
(Credit: Apple)While your phone's battery doesn't have a "memory" that causes capacity to shrink, its limited lifespan means you may want to replace it.
Back in 2017, Apple admitted that it secretly slowed down the performance of older iPhones in the name of "overall performance and prolonging the life of...devices." Following an uproar, the company offered discounted battery replacements for a while. With "Right to Repair" becoming a bigger deal, Apple in 2022 launched a self-service repair store that includes battery-change options. You can try it yourself, but battery replacements are generally best done by a professional.
Few new phones have a truly user-swappable battery. Opening up the guts of your phone, even if it doesn't void the warranty, isn't for the faint of heart.
Why are there almost no phones with swappable batteries? Most lithium-ion batteries perform effectively for around two to three years, and that's when the manufacturers would prefer you upgrade to a new phone. Why bother making it easy to swap a battery?
Now that the average phone user in the US keeps a phone for a while—one 2022 survey showed 26% update phones only every two years, a further 28% wait three years, and a quarter even wait four years—there's far less incentive for phone makers to help you keep your phone running; they aren't exactly fans of right-to-repair options.
This situation might get easier in a few years, as the European Union now has a law that forces Apple and other phone makers to manufacture all phones with user-replaceable batteries by 2027.
Other Disproven Myths
Sometimes new battery myths crop up on social media and video-sharing sites. Here are a couple mentioned on TikTok and Instagram that you should take with a healthy heaping of salt, courtesy of musicMagpie:
1. Swipe Up on Apps to Save Battery
This tried-and-true way to close iPhone apps is among the more useless things you can do with your phone. It helps with rebooting the app itself, perhaps if it's unresponsive, but it's not going to save you memory on the phone, and most of all, it's not going to help your battery. Opening and closing apps over and over is actually going to use up more energy than just leaving them running in the background.
2. Reduce the White Point for 'Unlimited Battery'
Utter nonsense: The white point of your phone, which you can change under Accessibility, adjusts the overall screen color. It's handy for those who look at the phone in the dark a lot. Reducing the phone's actual brightness may extend battery life because the pixels receive less power. A little. But nothing makes the power unlimited except leaving it plugged in full-time.
The Takeaway
If you plan to swap out your phone every year or two, charge it however you want, as often as you want, and don't worry about diminished capacity. But if you want to stretch out your phone's useful life, pay attention to best practices for lithium-ion batteries, as described above. Or you could just get a new battery installed every couple of years, which is much less expensive than a brand-new phone.