Can you leave a battery charger on overnight

There are a lot of questions about cell phone batteries. Should you leave your smartphones plugged in overnight? Is it bad for the phone? Bad for your safety? What's the right thing to do?

In fact, how much should you charge your phone? When's the right time to plug in? Should it go down to zero every time? Up to 100%? How do you get the longest life out of the battery inside a smartphone? Does it really matter if you're only going to keep the handset around for a couple of years before an upgrade?

The debate goes well beyond the worry of moderate harm to a device, as some people have fears of "overloading" a smartphone battery. That worry seems relatively justified, as it was only a few years ago that Samsung's Galaxy Note 7s were bursting into flame due to battery issues. But as we've explained before, unless a device has some serious manufacturing defects as that phone did, the fire-in-your-pocket (or on the nightstand) aspect is unlikely.

The problem is that some of the research and opinions out there are diametrically opposed. We've narrowed down the right and wrong things to do as best we can. We present to you the myths and truths of iPhone or Android phone charging, in particular when plugging in overnight.

Charging My iPhone Overnight Will Overload the Battery: FALSE

The one thing all the experts agree upon is that smartphones are smart enough that they do not let an overload happen. Extra protection chips inside make sure that can't happen in a tablet or smartphone or even a laptop. Once the internal lithium-ion battery hits 100% of its capacity, charging stops. If you leave the smartphone plugged in overnight, it is going to use a bit of energy constantly trickling new juice to the battery every time it falls to 99%. That is eating into your phone's lifespan (see below).

The Best Thing to Do:

Don't worry about this too much. 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 constant trickle-charging. Or plug your phone into a smart plug that's on a schedule to turn off.

Potential problems that could be encountered while charging overnight:

Is It Hot in Here?

Trickle charges can generate some heat. Many experts recommend taking a phone out of the case to charge overnight. But that's not always feasible with a complicated protective case.

At the very least, do NOT stack a bunch of crap like books or other devices on top of a charging device. And for the love of Jobs, do not put it under your pillow. Do any of the above, and you can expect the phone to get hot—not necessarily enough for spontaneous combustion, but enough to damage the battery (see below).

(Photo: Shutterstock/spyarm)

If you are afraid of fire, some in the UK recommend leaving the charging device on a dish or saucer(Opens in a new window) while plugged in, or putting it on something metal that is more likely to dissipate heat, the way a heatsink does on the chips inside a PC. That's not much of an option if you use a wireless charging pad, so don't sweat it.

Bad Cables

If you're using a knock-off cable that isn't from the manufacturer, or at least "certified" in some way (iPhone Lightning cables should be MFi certified(Opens in a new window), for example), it could be a problem. The cord and connectors may not be up to the necessary specifications for the phone or tablet. Don't skimp by buying chintzy cables.

I Should Freeze My Phone to Prevent Battery Problems: FALSE

Lithium-ion batteries hate two things: extreme cold and extreme heat. With cold, repeatedly charging a smartphone in sub-freezing temps can create a permanent plating of metallic lithium on the battery anode, according to BatteryUniversity(Opens in a new window). You can't fix that problem; doing it too much is only going to kill the battery faster.

(Photo: iStock/sankai )

The battery is not alone in hating heat: all the internals of any smartphone dislike warmth. It's a computer in there, and computers and hot air are mortal enemies going back decades. Leave your black-screened smartphone sitting in the sun as you laze by the pool someday, and don't be surprised when it throws a warning that it needs to cool off. In that case, put it under the towel. In the summer, keep it off the dash of the car, preferably in the shade.

Apple specifically says charging iPhones over 95 degrees F (35 degrees C) will do permanent damage to the battery; expect the same with any modern smartphone.

The Best Thing to 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 Before Charging: FALSE

Running a smartphone until it's dead—a full discharge—every time is not the way to go with modern lithium-ion batteries. Don't even let it get that close to 0%. That wears out a lithium-ion battery even faster than normal. Partial discharge is the way to go.

(Photo: GettyImages- Priscila Zambotto)

Batteries are on borrowed time from the get-go. The insides are in a constant state of decay that can't be helped. Over time, the materials inside are simply going to hold less and less power. If you've got an older iPhone still in use and wonder why it's only got a charge for a few hours compared to the almost full day (or two) you got when it was new, that's why. Capacity diminishes over time.

Only go out of your way to drain a smartphone battery to zero to recalibrate(Opens in a new window) the internal sensor that displays your phone's battery level. It's hardly guaranteed—in fact, many people don't think it works at all(Opens in a new window)—but it's recommended by some, especially if you've got a phone that hits 10% (or even 20% or 30%), then abruptly dies.

Even if 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 believe this is worth doing. Then give it a reset for good measure.

The Best Thing to Do:

Plug the phone in before it asks you to enter a low-power mode(Opens in a new window); iOS will ask you to turn that on when you hit 20% power. Plug it in when the phone is between 30-40%. Phones will get to 80% quickly if you're doing a fast charge. Pull the plug at 80-90%, as going to full 100% when using a high-voltage charger can put some strain on the battery. Keep the phone battery charge between 30-80% to increase its lifespan.

Apple claims that with fast charging the iPhone, batteries can increase 50% in only 30 minutes. That requires a USB-C power adapter, which in turn means owning a special USB-C-to-Lightning cable(Opens in a new window), or using a higher voltage charger like the one from an iPad or even a MacBook.

My Battery Develops a 'Memory': FALSE

Developing a "memory" was a problem with older nickel-cadmium (NiCad) batteries(Opens in a new window). That's where the whole "discharge the battery entirely" thing came from. As we said, it'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. Your smartphone battery over its lifetime degrades enough that in the same amount of time charging, a new phone could hit a full charge, while an older phone might only get to around 82%. BatteryUniversity (Opens in a new window)calls it "old man syndrome."

(Image: Shutterstock/Jirsak)

Another way to look at it is that newer batteries are just hungrier to suck up all that power.

Apple claims that "Apple lithium-ion batteries are designed to hold at least 80% of their original capacity for a high number of charge cycles," but it admits that the amount differs from product to product.

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Apple iPhone batteries also support "fast charging," so they'll get to 80% pretty quickly. After 80, you'll see the capacity increase slowly, some of which is to prevent heat buildup—which extends battery life. But guess what? Fast charging isn't great for a lithium-ion battery, either. It corrodes even faster(Opens in a new window).

Older iPhones came with a 5-watt (that's 5 volts at 1.1 amps) charger block. It works, but of course, you can charge faster with a 10W charger, which has an output of 5 volts at 2.1amps—that's the kind of charger that comes with an iPad. If you stick to the Qi-based wireless charging, keep in mind that most support 7.5W, with wireless fast charging now available.

The Best Thing to Do:

Stop worrying about "memory." If you are going to charge overnight, don't fast charge. Use a slow charge. That means your charger should be lower voltage.

Phone Batteries Only Last a Few Years: FALSE-ish

Phone batteries measure their lifespan in "charge cycles." That means every time you discharge up 100% of the capacity, it's one cycle count, but it doesn't mean you went all the way to zero.

For example, if your phone is at 80%, you go down to 30% (that's half the battery capacity), and you charge it back to 80 and use that 50% up again, that's one cycle. You could use 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, not necessarily 400 to 500 times the phone is actually plugged in to charge.)

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 that's when the battery lifespan goes downhill even faster. Here's Apple's graphic(Opens in a new window) trying to explain it:

(Image: Apple)

While your phone battery doesn't have a "memory" that makes the capacity worse and worse, that limited lifespan means you may want to swap in a new battery.

Back in 2017, Apple admitted to secretly slowing down batteries on older iPhones in the name of "overall performance and prolonging the life of...devices." After an uproar, Apple offered battery replacements for these phones at a discount for a while. With "Right to Repair" becoming a bigger deal, in 2022 Apple launched a self-service repair store option that includes battery change options.

You can try it, but battery replacements are generally best done by a professional. Few new smartphones have a truly user-swappable battery. Opening up the guts of your smartphone, even if it doesn't void the warranty, isn't for the faint of heart.

Why are there almost no smartphones 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 smartphone user in the US keeps a phone for a while—one survey (Opens in a new window)showed 26% only update phones 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 it running. The phone makers aren't exactly fans of right-to-repair options.

The Takeaway Here:

If you plan to swap phones every year or two, charge it any way you want, as often as you want, and don't worry about the diminished capacity. However, if you want to stretch things out, use best practices for lithium-Ion batteries as described above. It may help. Or, just get a new battery installed every couple of years. It's much cheaper than a brand new phone.

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