Live charge readout, charging state and a 60-second drain test
The page uses the browser's navigator.getBattery() Battery Status API, which reports the real charge level, charging state and time estimates straight from the device's power management. The ring animates with every change, and the page subscribes to levelchange, chargingchange and chargingtimechange events so the readout updates live — plug or unplug the charger and watch it react within a second.
The drain test gives you a rough real-world number: it snapshots the level, waits 60 seconds with the screen on, and reports the percentage drop. Because the API quantizes level to whole percent (and some devices to 1–5% steps), a 60-second window is short — treat it as a smoke test. A healthy phone idles at a bright screen and loses roughly 0–1% per minute; consistent drops of several percent per minute at idle point to a worn battery, a runaway app, or a swollen cell — get the last one checked immediately.
Why does it say "not supported" in Firefox or Safari? Firefox and Safari removed the Battery Status API because exact battery levels turned out to be a fingerprinting vector — two visitors with 87.234% battery could be told apart. Chrome ships it on Android and, on desktop, only reports battery when the OS exposes it. Nothing is broken on your device; it's a browser policy.
Is my battery healthy? The browser can't read cycle count or design capacity — only the OS can (iOS: Settings → Battery → Battery Health; Android: dialer codes or the manufacturer's diagnostics). This test's value is behavioral: unexpected percentage jumps (e.g. 40% → 12% in seconds) or shutdowns while the readout still shows charge are classic signs of a degraded cell that the health percentage alone won't always reveal.