There are some lingering myths about charging smartphones that are worth dispelling. Technology has come a long way since the 2000s — there’s no need to be as worried about the safety of your phone now as you would’ve back then. If anything, the overriding concern these days is prolonging your battery’s health, which can both make life easier and reduce the likelihood of an expensive replacement.
Is it safe to charge your smartphone overnight?
An easy answer that still warrants explanation
Yes, but it’s important to understand the details on this one. The fear you might have of leaving your phone charging unattended stems from the fact that there have, in fact, been fires caused by problems like overheating batteries and faulty chargers in the past. The risk isn’t zero. Incidents were rare a decade ago, however, and seem to be even rarer today, based on how little I hear about them.
Part of the reason for that may be an overall improvement in quality control, but phone makers have also gone out of their way to implement various safety measures. For a very long time, phones have automatically switched into a “trickle charging” mode after reaching 100%, letting the battery deplete slightly before adding any more power. They can also detect damaged charging accessories, typically demonstrated by low or fluctuating power levels. Phones may also refuse to charge if liquids are present, or controversially, if an accessory is uncertified — no matter whether it works in practice.
The risk isn’t zero. Incidents were rare a decade ago, however, and seem to be even rarer today, based on how little I hear about them.
One especially important safeguard is the use of internal temperature sensors. Modern phones will slow or outright halt charging when heat rises to dangerous levels. A battery or charger could hypothetically get out of control regardless, but your phone shouldn’t burst into flames just because you leave it plugged in overnight, or decide to mount it on your windshield during a hot summer roadtrip.
All phones have ideal operating temperatures. For an iPhone, for instance, the sweet spot is between 62 to 72F (16 to 22C). Apple warns that you shouldn’t use an iPhone when ambient temperatures exceed 95F (35C), but that’s less for safety than it is keeping a battery in good condition. In fact, there would be a lot of exploding iPhones in Texas if 95F was a serious fire hazard.
Is leaving a smartphone plugged in overnight bad for its battery?
Now things get complicated
Practically speaking, letting a new phone sit at 100% for hours every night probably won’t cause any issues for months, if not a year or longer. Depending on how much you’re paying attention — and how much money you have to spend on gadgets — you may not even notice any performance issues by the time you’re ready to upgrade. If you’re concerned about your battery maintaining its peak capacity, though, you should absolutely avoid keeping it at 100%, at least until it’s just about time to unplug. More on that in a minute.
There are a couple of issues at play here, the overriding one being that lithium-ion batteries tend to degrade fastest when they’re either empty or fully charged. Being empty is worse, since a device left in that state too long may never power up again — but regularly maxing it out is still going to shrink capacity. In two or three years, a phone that used to last a little over a day could end up running on fumes by the time you climb into bed. By the four- or five-year mark, it might be impossible to make it a whole day without a brief midday top-up.
The other factor is heat. Your phone will, naturally, need to run hotter for longer to reach 100%, particularly if you’re using a wireless charger, which is inherently less efficient than USB. That warmth should mostly dissipate once trickle charging begins, but any extra heat is going to accelerate degradation. Indeed, processor-intensive activities like 3D gaming may also contribute to the process.
How to keep your phone’s battery healthy
Not much to it
So how should you treat your battery if you want it to last a long time, but still have enough juice to make it from your morning shower to bedtime? If you’re going to charge every night, you’ll want to take advantage of iOS or Android’s optional limiting features.
Primarily these analyze your charging patterns, then automatically restrict how far your phone can charge until you’re expected to wake up. On an iPhone, for instance, you can go to Settings -> Battery -> Charging and toggle Optimized Battery Charging, which will cap your phone at 80% most of the night, but let it run up to 100% at the last minute. It doesn’t always get things right — that 6AM flight is going to throw things off — but it’s otherwise pretty dependable. You’ll find equivalent options on Android devices, such as Samsung’s Battery Protect, and Google’s Adaptive Charging (for Pixel phones).
Some phones will let you cap charging limits round-the-clock, but that tends to be more trouble than it’s worth. If you start the workday at 80%, there’s a good chance you’ll need to charge by dinner, and having to live like that may defeat the point of stretching out battery health.
If you’re going to charge every night, you’ll want to take advantage of iOS or Android’s optional limiting features.
80% is a magic number, you’ll notice. That’s because the optimal charge level for lithium-ion devices is between 20 and 80%. You can afford to go past those thresholds, but the more time your phone is in between, the better. It’s one of the reasons you should always turn on your device’s low-power/battery-saver mode as you approach the 20% mark.
There’s not much else you can do, but pay attention to temperature. You may be able to reduce degradation a little by pointing a fan at your charging station, or simply moving your phone into the shade if you have open blinds or curtains and the sun is still out. Keep other hot electronics as far away as possible. Heaters and radiators are the worst offenders, although a charging tablet or laptop may be an issue, particularly if your phone is sitting directly on top. If you want to use a MagSafe or Qi2 charging station, be sure to get a model that props your phone up in mid-air. Some have their own cooling fans to prevent trouble.
