Smartphone Charging Continuously Smartphone Charging Wrong Way

If you're unsure whether there's a "right" way to charge your phone—or whether charging it too long, too often, or too fast can damage the battery—you're not alone. I've been writing about phones and tech since 2011, and before that I was an iPhone sales specialist at an Apple Store. Even with that experience under my belt, it has never been totally clear to me if being careful about how often I recharge my phone actually extends the life of the battery enough to make a difference, or if it's just another hassle in a world with far too many of them.

Some people just plug their phone into a charger (or toss it on a wireless charging pad) whenever power is available. Others fastidiously keep their battery between 40% and 80%, never allowing a full charge, under the belief that the battery will last longer as a result. Personally, I keep my iPhone on a Qi wireless charger on my desk all day while I'm at work, and I juice it up overnight, as well.

After speaking with battery researchers and the reuse experts at iFixit, reviewing studies on phone replacement trends, and analyzing some user data from Wirecutter staffers, we've found that although micromanaging your phone's battery can likely extend its life to a small degree, the results might not be worth the inconvenience in the long run.

What the science says

Charging your battery causes its performance to degrade over time, no matter how you do it. Smartphones are powered by lithium-ion batteries, which work by moving charge carriers (in this case, lithium ions) from one electrode to another. The ions move in one direction when charging and the other when discharging.

Moving those ions puts stress on the electrodes and leads to reduced battery life, according to Hans de Vries, senior scientist at Signify (formerly Philips Lighting), where he co-authored the research paper "Increasing the cycle life of lithium ion cells by partial state of charge cycling," which appeared in the journal Microelectronics Reliability. "The lithium ion needs some space in the electrodes and the electrode has to make this space, and because of the stress … the electrodes will gradually degrade and that is also then a loss of capacity in the battery," de Vries said in an interview.

That is especially true when you're topping off a battery the last few percentage points. Former Wirecutter senior staff writer Kevin Purdy—now of iFixit, a site that documents how to repair common electronics and other household items—suggested the analogy of a sponge. "It's pretty easy to fill a sponge from dry to mostly saturated. But trying to force a nearly saturated sponge to absorb the very last drops of liquid requires pressure, and likely leaves more liquid pooled on the surface," he said. "That 'pooling' is the SEI (solid electrolytic interface) buildup on a battery. SEI buildup reduces the overall capacity of a battery."

Charging your battery to full capacity less often, and not letting it run totally dry, can extend its life—somewhat. Putting less stress on the electrodes results in less degradation, and ultimately higher capacity for a longer period of time. "It is possible to prolong the battery life by not completely charging and not completely discharging. So we'll say stay between 20% and 80% or so," de Vries told us. The battery lifetime is "inversely proportional to the amount of lithium ions that you put in the electrodes."

This is one reason Apple offers optimized battery charging on its iPhones, keeping the charge beneath 80% until you need the battery topped off. Android doesn't have a similar system-level algorithm, but individual manufacturers such as OnePlus and Asus have introduced their own optimization features.

An iPhone lying face down on a concrete surface, with a white fast charging power source next to it. — Photo: Michael Murtaugh

Heat is another factor that negatively affects battery life. "Heat is the worst enemy of batteries," according to Battery University, a repository of battery science information maintained by battery-testing company Cadex. "Lithium-ion performs well at elevated temperatures but prolonged exposure to heat reduces longevity."

According to iFixit's Purdy, heat is particularly a problem when you're wirelessly charging. "Depending on a number of factors—alignment, sophistication of charging base, phone cases, interference—your charger can end up delivering as low as half of the current it draws into your phone," he said. "Where current meets resistance, there is heat."

The people behind the wireless charging standard are dismissive of these concerns, however. "We are not aware of any negative impact of prolonged wireless charging," said Menno Treffers, founder and chairman of the Wireless Power Consortium, the body that maintains the Qi wireless charging standard. "[Qi] makes it possible for the phone to switch the charger in standby mode when the phone's battery is full."

Treffers even suggested that frequent top-ups, which are common with wireless charging, may actually extend battery life. "According to research we have seen, battery lifetime actually increases by 4x when the depth of discharge—or amount that the battery is drained—is limited to 50%, rather than 100%," he told us. "In other words, by continually topping up the phone battery during the day, as you might do with wireless charging, and not letting your phone battery dip below 50%, you will actually increase the lifespan of your battery."

What the manufacturers say

The major phone manufacturers declined to provide any recommendations for specific charging techniques when we asked, but they do offer vague tips on their websites.

Apple says that you should "charge your Apple lithium-ion battery whenever you want" and goes on to advise that "[t]here's no need to let it discharge 100% before recharging." On a different page the company notes that you should avoid extreme temperatures (especially over 95 degrees Fahrenheit) and remove cases that might cause your iPhone to overheat while it's charging, but Apple doesn't state when you should or shouldn't charge or suggest any optimal charging thresholds.
Google's recommendation is similarly straightforward: "Charge as much or as little as needed. You don't need to teach your phone how much capacity the battery has by going from full to zero, or zero to full, charge."
Samsung advises charging regularly and keeping the battery above 50%. The company also states that leaving your phone connected while it's fully charged may lower the battery life.

Even if constant charging affects battery life, will you ever notice?

So, charging your phone all the time and letting it run dry are habits that could shorten its battery life. But are they liable to have enough of an effect to make a practical difference before you upgrade to a new model?

Two-year wireless service contracts may be a thing of the past, but modern installment plans usually still require two years to pay off a phone, meaning people who don't buy their phones outright are likely to keep them at least that long. A notable exception is Apple's iPhone Upgrade Program, which promises "a new iPhone every year." But even with the advent of such programs, recent data suggests that phone replacement cycles are lengthening rather than getting shorter. A 2019 study found that Americans now keep their smartphones for an average of nearly three years. Those who prefer Apple may keep their iPhones even longer—up to four years, according to one analyst's report.

If you don't upgrade regularly and don't follow ideal charging practices, it stands to reason that you may find your phone's battery life lacking over time. However, other factors—such as how much you use your phone in general—likely have a much larger impact on battery longevity than charging behavior. That's because lithium-ion batteries are rated for a specific number of charge cycles, or times they can be filled up. (These cycles are cumulative, so two charges from 50% to 100% count as one cycle.) So the more you use your phone, the more you have to recharge the battery, and the more it degrades.

In an informal poll of 32 Wirecutter staffers who use iPhones, the lowest battery capacity reported after two years of ownership was 85%. Of the survey respondents, just one person said they meticulously kept their phone's battery level in a certain range. The vast majority (29 respondents, or 91%) indicated that they simply charged it when the battery level was low, or overnight, while two people reported that they charged their phones more or less constantly when a charger was available.

Although our poll results show a general decline in iPhone battery health over time, as you might expect, they also suggest that there's little direct correlation between battery age, charging habits, and battery health. For example, one iPhone 7 owner reported that after 42 months, their phone still had 87% battery health despite their decision not to micromanage its charge levels. Another respondent said their iPhone 7 had just 64% battery health despite being six months younger than that other phone and being on the same charging routine.

A loss of 15% of your battery capacity over two years is noticeable, sure, but it leaves enough juice—especially with the larger batteries in newer iPhone models—that most people can still get through the day without plugging in. For heavier phone users who wear down their batteries quicker, or those who have older phones with smaller batteries and more marginal battery life to start with, the good news is that batteries can be replaced fairly cheaply. Apple charges $50 or $70, including labor, depending on your iPhone model. Best Buy will replace a Samsung Galaxy battery for $50. uBreakiFix, Google's walk-in repair partner for Pixel devices, charges about $80 to $110 to replace batteries. Or you can do it yourself, by following the guides on iFixit.

Ultimately, it's a matter of convenience

In the long run, you need to choose what's right for you: babying your battery to extend its life, or charging it at your convenience so that your phone is more likely to be juiced up when you need it.

"It would be better for a phone battery to be allowed to gradually lose its charge, then recharge when needed, perhaps to 80%, before stopping again," said Purdy. "Of course, some people don't want to risk having their phone keep only a partial charge before they head out. Or have a part-time job watching their battery percentage."

de Vries echoed that thought. "If you charge the battery only half way, okay, it will last longer, but it will be empty sooner than if the battery has been fully charged. So it is a trade off between the total life of the battery and the amount of times that you have to recharge it." He told us that even though he is intimately familiar with optimal battery hygiene, he doesn't always practice it. "I'm lazy," he said. "Back in the old days, I'd look every quarter or half hour to see if my cell phone, my laptop was charged already. And then I would stop, for instance at 90%, 95%. But sometimes you forget."

Smartphone Charging Continuously Smartphone Charging Wrong Way

What the science says

What the manufacturers say

Even if constant charging affects battery life, will you ever notice?

Ultimately, it's a matter of convenience

Further reading

The Best USB Car Charger

The Best USB Phone Charger

The Best Portable Solar Battery Charger

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