The battery test, seen here in action, makes phones set to equal brightness scroll through the same set of websites until each runs out of juice. (Geoffrey A. Fowler/Washington Post)

Phone makers promise “all-day battery life.” Sure, and you haven’t stolen any of the kids’ Halloween candy.

If you recently bought a new flagship phone, chances are its battery life is actually worse than an older model.

(Washington Post graphic)

For the last few weeks, I’ve been performing the same battery test over and over again on 13 phones. With a few notable exceptions, this year’s top models underperformed last year’s. The new iPhone XS died 21 minutes earlier than last year’s iPhone X. Google’s Pixel 3 lasted nearly an hour and a half less than its Pixel 2.

Phone makers tout all sorts of tricks to boost battery life, including more-efficient processors, low-power modes and artificial intelligence to manage app drain. Yet my results, and tests by other reviewers I spoke with, reveal an open secret in the industry: The lithium-ion batteries in smartphones are hitting an inflection point where they simply can’t keep up.

“Batteries improve at a very slow pace, about 5 percent per year,” says Nadim Maluf, the CEO of a Silicon Valley firm called Qnovo that helps optimize batteries. “But phone power consumption is growing up faster than 5 percent.”

Blame it on the demands of high-resolution screens, more complicated apps and, most of all, our seeming inability to put the darn phone down. Lithium-ion batteries, for all their rechargeable wonder, also have some physical limitations, including capacity that declines over time – and the risk of explosion if they’re damaged or improperly disposed.

And the phone power situation is likely about to get worse. New ultrafast wireless technology called 5G, coming to the U.S. neighborhoods soon, will make even greater demands on our beleaguered batteries.

My test has limitations. Your experience will depend on how you use your phone, and there are steps you can take to make your phone life stretch.

We’re not without hope. Two phones that performed well in my tests, Samsung’s Note9 and Apple’s iPhone XR, offer ideas about how to design phones to last longer – at least until a totally new battery tech comes along.

My results made me do a double take, so I called up a squad of other tech journalists also obsessed with testing at CNET, Tom’s Guide and Consumer Reports. “Our overall average battery life is coming down,” says Mark Spoonauer, the editor in chief of Tom’s Guide, who also found the iPhone XS battery died sooner than the iPhone X. Many of the phones with the longest battery life, he adds, are a year old.

But not all other reviewers have noticed the same declines – and the differences in our results help shed some light on what’s going on.

Larger phones often last longer, but it’s not as simple as the size of the battery inside. Remember the Blackberry? Those had much smaller batteries than today’s smartphones, but could go days without being charged.

There’s no perfect battery test. Mine, which I borrowed from an industry group called the Embedded Microprocessor Benchmark Consortium, particularly stresses the screen.

I use a light meter to set all the phones at the same brightness and then force their web browsers to reload and scroll through a series of sites I serve through a local WiFi network. I rerun the tests as many times as possible, and then average the results.

CNET, which like me found conspicuous dips in battery life between the iPhone 8 and iPhone X (and Samsung’s Galaxy S8 and S9), tests screens at 50 percent brightness playing a looping video with Airplane Mode turned on.

What we both discovered: Phones with fancy screens that are especially high-resolution or use tech such as OLED perform worse. (That tech can require more power to push out light.) So if you want your phone to last longer, turn down the screen’s brightness. Or stop looking at your phone so many times each day, if you can break our nationwide spell of phone addiction.

Tom’s Guide throws another factor into the mix: The cellular connection. It makes phones run through a series of websites streamed over LTE. Unlike me, it also saw a big battery life hit to the Pixel 3 XL versus the Pixel 2 XL.

Another lesson: If you want the battery to last longer, use WiFi when possible – or even Airplane Mode when you don’t need to be reachable. Both Apple and Android phones also offer low-power modes (not reflected in our testing) that reduce some draining data functions without taking you offline.

The counterexample is Consumer Reports, which found the new iPhone XS lasted 25 percent longer than last year’s iPhone X. Its test uses a finger robot – yes, you read that right – to make phones cycle through lots of different functions and apps, including pauses in use where the screen turns off.

Consumer Reports is likely better testing the phone’s processor, an area where a number of companies – but particularly Apple – have made efficiency gains.

So overall, are battery lives decreasing or increasing? “You can’t make a straight trend,” says Consumer Reports director of electronics testing Maria Rerecich.

I wish companies had more standardized ways to talk about battery life. Since the earliest days of the iPhone, Apple has described battery life through specific measures, including “talk time” and “Internet use.” Recently it’s also taken on some more squishy language: The iPhone XS “lasts up to 30 minutes longer than iPhone X,” it says, a measure based on data about how long people go before plugging back in.

So what about the two 2018 phones that did better in my tests?

Samsung’s Note9 succeeds by stuffing in more battery. It contains a battery capacity of 4,000 mAh, up from the already-huge 3,300 mAh in the Note8. (The iPhone XS battery is only 2,659 mAh, and actually slightly downgraded from the X.)

Lots of phones have followed the bigger battery trend. iFixit, a repair community that performs teardown analysis of phone components, says battery capacities have almost doubled in the last five years.

How much further can the size game go? Huawei just introduced a phone called Mate 20 Pro, not sold in the U.S., with a 4,200 mAh battery. Larger, denser batteries can be more dangerous (remember Samsung’s exploding Note7?), not to mention heavier. The Note9, which also has a giant screen and a stylus, weighs 7.1 oz – more than twice a deck of cards.

Apple’s iPhone XR, the new phone I recommend to most people, has a different approach. It scales back on the screen tech – lower resolution, less bright and lower-quality color – in ways that benefit battery life tremendously: The XR lasted 3 hours longer than the top iPhone XS, even though the its screen is actually a smidge larger. (Bonus: It also costs $250 less.)

“Consumers have to start getting ready for compromise,” says Maluf, the CEO of the battery optimization company.

Perhaps the market will fragment further, making phones more like buying cars. That market was eventually upended by fuel-economy models; instead of the gas-guzzling Cadillac, you could choose the Honda. Apple’s iPhone XR is the Civic of smartphones.

Our near-future choices are likely either: Get an economy phone – or plug in more often. Faster and more convenient charging is the strategy for some makers. Lots of phones now support wireless charging, though still few cafes, offices and airport lounges offer it.

And then there’s the plug itself. Apple, which has shipped the same 5 watt charging brick for years, could take a lesson from Google, which sells its Pixel phones with an 18 watt charger and claims you can get 7 hours of use from just 15 minutes of charging. The one thing that’s almost as bad as running out of juice is being tethered to an outlet.