How to judge a phone's battery life before you buy it
Battery life is the thing buyers ask about most and the thing spec sheets describe worst. The capacity figure printed on the box tells you how much charge the cell holds, and almost nothing about how long the phone will last, because the whole question is how fast that charge gets spent.
What actually drains the cell
The screen is usually the biggest consumer, and its appetite depends on size, panel type, brightness and refresh rate. A high refresh rate is genuinely nicer to look at and genuinely more expensive to run, which is why phones that offer it also offer a way to cap it or switch it dynamically. Brightness matters more than most people expect: the same phone used outdoors all day and indoors all day is effectively two different phones.
The processor is the next factor, and here the marketing is upside down. A newer, more powerful chip built on a finer manufacturing process often uses less energy for the same work than the cheaper one it replaced, because it finishes the work sooner and goes back to idle. Efficiency cores, sensible scheduling and a modem that is not constantly hunting for signal all matter as much as raw capacity.
Then there is software, which is where identical hardware produces different results. Aggressive background syncing, an assistant listening constantly, a launcher that never sleeps, and a dozen apps that all decided they need push notifications — these are the difference between a phone that idles overnight losing a trace of charge and one that loses a fifth of it doing nothing.
How to read a review
Look for screen-on time rather than a total endurance score. Screen-on time, measured at a stated brightness and refresh rate, is the closest thing to a comparable figure, because it describes work done rather than hours elapsed. A test that does not say how bright the screen was is not telling you anything you can use.
Then check whether the reviewer's usage resembles yours. A test built on video playback flatters a phone with an efficient panel and tells a heavy camera user nothing. Games, navigation with the screen on and hotspot use are the punishing cases; if the review covers them separately, that is the section to read. Also check whether the review ran on the shipping software, since a substantial update can move the result in either direction.
Charging is not the same as endurance
Fast charging is a real convenience and a poor substitute for endurance. It also interacts with the life of the cell: heat is what ages a battery, and sustained fast charging in a warm room is the fastest way to lose capacity over a couple of years. Phones that let you cap charging overnight, or hold at a lower ceiling most of the time, are protecting the thing you paid for.
Once the phone is in your hand, spend the first week measuring rather than guessing. Use the battery screen in settings to see which apps accumulated time in the background, not just which ones you opened. Note the charge level at the same hour on two ordinary days. If the phone is losing noticeably more than it should while idle, the cause is almost always one app with a wakelock, and finding it is an afternoon's work that pays for itself for the life of the device.
The cheapest gain of all is free: turn the screen down, turn the refresh rate to automatic, and stop letting apps you opened once keep running. Most of the difference between a phone that lasts a day and one that does not was never in the hardware.