Answer: Run the test above — it reports your battery level, charging state, and discharge timing to gauge battery health right in the browser.
Live charge readout, charging state and a 60-second drain test
The Battery Status API (where your browser supports it) reports charge level, charging state, and estimated charge/discharge times, which this page displays live. Watching the drain rate during a timed idle period gives a rough indication of whether runtime has degraded versus the device's original spec. No app installation is required.
Common causes are high screen brightness, poor cellular signal forcing the radio to work harder, background apps and syncing, and cold weather, which temporarily reduces lithium-ion capacity. A battery that drained quickly even in airplane mode with the screen dimmed suggests age-related wear. Battery health below about 80% of design capacity is the usual threshold for replacement.
Most lithium-ion phone batteries are rated for 500–1000 full charge cycles before dropping to roughly 80% of original capacity, which for typical users means two to three years. Heat and habitually holding the charge at 100% accelerate wear. Keeping the battery between roughly 20% and 80% when practical slows degradation.
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.