Battery Status API Privacy Protections in Web Browsers

The Battery Status API was originally designed to allow web applications to adapt their resource consumption based on a device's remaining charge, but severe privacy concerns have led modern web browsers to aggressively restrict or entirely remove developer access to it. This article explores the specific privacy and fingerprinting risks introduced by the API, the mitigations implemented by modern browser engines, and the current landscape of browser support and developer restrictions.

The Privacy Risks of the Battery Status API

The primary driver behind restricting developer access to the Battery Status API (navigator.getBattery()) is browser fingerprinting and cross-site tracking. Researchers discovered that the API exposed four high-precision metrics: the battery level percentage, charging status, time until fully charged, and time until fully discharged.

When combined, these values yielded millions of potential combinations, creating a highly stable and unique temporary identifier for a device. Trackers exploited this data to:

  • Bypass Private Browsing Modes: Trackers could link a user's identity between a standard browsing session and a private/incognito window by matching identical battery drain rates and charge estimates.
  • Enable Cross-Site Tracking: Third-party scripts embedded across multiple websites could track users across different domains without relying on cookies or local storage.
  • Correlate Multiple Browsers: Because battery state is hardware-dependent, trackers could identify the same physical device even if the user switched between different browsers.

Modern Browser Protections and Mitigations

In response to these tracking vectors, major browser vendors implemented several layers of protection:

1. Complete Removal and Non-Implementation

The most effective privacy protection has been outright deprecation:

  • Mozilla Firefox: Removed support for the Battery Status API in Firefox 52 (2017) to eliminate the fingerprinting vector.
  • Apple WebKit (Safari): Refused to implement the standard, citing severe privacy and fingerprinting risks that outweighed the functional benefits to web applications.

2. Resolution Reduction and Value Rounding

Browsers based on the Chromium engine (such as Google Chrome and Microsoft Edge) that still maintain partial support have drastically reduced the fidelity of the exposed data:

  • Quantizing Battery Levels: Percentages are rounded to coarse intervals rather than high-precision floating-point decimals.
  • Removal of Time Estimates: Discharge and charging time predictions are either removed, set to fixed values, or rounded to substantial time brackets (such as full hours) to prevent micro-fluctuation tracking.

3. Secure Context and Feature Policy Enforcement

Where the API is accessible, modern standards restrict its execution environment:

  • HTTPS Requirement: The API is strictly limited to secure contexts (https://), preventing man-in-the-middle injection of tracking scripts that attempt to read hardware state.
  • Permissions-Policy Delegation: Access can be blocked in cross-origin iframe elements using the Permissions-Policy: battery header, preventing unauthorized third-party trackers and advertisers from querying device state without the hosting site's explicit permission.