Check Battery Status Online

Because the percentage is counted, not measured. The battery’s gauge starts from what it believes a full charge holds and subtracts what flows out. When the cells have aged and hold less than the gauge believes, the count still says half at the moment the cells are empty: the voltage collapses, and the gauge snaps the number down to match — or the laptop simply switches off. On Windows the battery report records it, and this page lists every drop of ten points or more that happened faster than a battery can discharge. One full cycle — charge to 100 %, run on battery until the laptop switches itself off, charge to 100 % again — lets the gauge relearn the real capacity, so the percentage becomes honest; if the capacity it then shows is under about 60 % of the original, the battery needs replacing.

On Windows, yes — from the battery report the system keeps. Press Win + R, run powercfg /batteryreport /output "%USERPROFILE%\battery-report.html" and open the file on this page: it shows the full-charge capacity against the design capacity, the cycle count when the firmware reports one, and how the capacity changed week by week. The browser itself exposes none of this — only the level, the plug state and time estimates — so on other systems the figure is on the system’s own screens: System Settings → Battery → Battery Health on a Mac, Settings → Battery → Battery Health on an iPhone, the Diagnostics app on a Chromebook.

A typical Li-ion battery is rated for ~500 full charge cycles to 80% capacity (phones) or ~1000 cycles (modern laptops). Most manufacturers consider the battery worn out once it drops below 80% of its original design capacity — that is also the threshold for warranty replacement on iPhone, MacBook and Samsung devices.

Because plugged in is all the browser is told. The field is named “charging”, but on Windows it is set from the power cable and on Android from the plug, not from the current going into the battery — a laptop that stops at 80 % to spare its battery reports “charging” for as long as it stays on the charger. So the page says “charging” only after it has seen the level rise, and “level not rising” once fifteen minutes have passed without a change. If that is not a charge limit you set, the charger is giving less than the device is using.

Because the system does not report one. On Windows the call browsers use has no time-to-full figure at all, so it stays unknown for as long as the laptop charges; on Android it depends on whether the phone exposes its charging current. The browser interface marks an unknown time as infinity, which this page shows as a dash rather than as a number. Time until empty is different: Windows estimates it after a minute or so on battery, and where the system gives nothing the page works out its own from the measured drain rate.

Because it is an extrapolation from what the battery is doing right now, and what you are doing changes every few seconds. The estimate takes the current rate of discharge and divides the charge left by it, so a minute of video pushes it down and a minute of a still page pushes it back up. That is not a fault in the reading — it is what happens when a rate that varies is projected forward as if it were constant. Watch the discharge graph instead of the estimate: a line with a steady slope over ten minutes tells you far more than the number does, and a slope that steepens without you doing anything is the interesting signal.

It is measured from the moments the level changes. The percentage arrives in whole steps, and the time between two steps is an exact measurement of how fast the battery is draining. Nothing is shown until the level has changed twice — on a laptop losing 15 % an hour that takes about eight minutes — because a figure taken from the first half-minute only shows where in a step the page happened to open. The rate follows the last six steps, so it describes what the device is doing now, not an hour ago.

Because the battery status interface it reads was removed from most browsers on privacy grounds — the combination of level, charging state and discharge rate was distinctive enough to help identify a returning visitor. Chrome, Edge and Opera on Android and desktop still provide it; Firefox and Safari removed it, and on iOS every browser is Safari underneath, so an iPhone shows nothing regardless of which app you open. There is no way around that from a web page. The wear check does not depend on it: on a Windows laptop the battery report can be opened here in any browser.