Battery Status API: Why Browsers Deprecated Battery Level Tracking

The Innocent API That Became a Tracking Threat

When the W3C introduced the HTML5 Battery Status API (navigator.getBattery()), the goal was innocent: allow web applications (like video editors or games) to throttle resource usage and save power when a laptop or mobile device was running low on battery.

However, security researchers soon discovered that high-precision battery readouts created a severe privacy vulnerability.

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How the Battery API Created a Short-Term Fingerprint

The API exposed four key properties:

1. battery.level (e.g. 0.542385...)
2. battery.charging (true/false)
3. battery.chargingTime
4. battery.dischargingTime (seconds remaining)

When combined, these four values produced millions of distinct combinations. Because battery discharge is continuous and predictable, tracking scripts could follow a user moving between Incognito tabs or across different browsers on the same device within a 30-minute window.

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Browser Deprecation and Privacy Lessons

Recognizing this threat, browser vendors took decisive action:

• Mozilla Firefox: Completely removed the Battery Status API in Firefox 52.
• Apple WebKit (Safari): Never implemented the API due to fingerprinting concerns.
• Google Chromium: Restricted precision and deprecated unprompted access.

This case study demonstrates why modern privacy diagnostics, such as Device Info and Browser Fingerprinting, audit all browser hardware APIs.