W3C Battery Status API. The mechanism allows a web site to read the battery level of a device (smartphone, laptop, etc.). One of the positive use cases may be, for example, stopping the execution of intensive operations if the battery is running low.

Battery readouts provide the following information:

the current level of battery (format: 0.00-1.0, for empty and full, respectively)
time to a full discharge of battery (in seconds)
time to a full charge of battery, if connected to a charger (in seconds)

Those values are updated whenever a new value is supplied by the operating system

Frequency of changes in the reported readouts from Battery Status API potentially allowed the monitoring of users’ computer use habits; for example, potentially enabled analyzing of how frequently the user’s device is under heavy use. This could lead to behavioral analysis.
Additionally, identical installations of computer deployments in standard environments (e.g. at schools, work offices, etc.) are often are behind a NAT.

The information provided by the Battery Status API is not always changing fast. In other words, they are static for a period of time; it may give rise to a short-lived identifier.

This was surprising! It turned out that in some circumstances it was possible to approximate (recover) the actual battery capacity in raw format; in particular on Firefox under Linux system.

he study achieved an impact.

a W3C standard is updated to reflect the privacy analysis
Firefox browser shipped a fix
the work received some recognition .

Trackers use of battery information

Expected or not, battery readout is actually being used by tracking scripts, as reported in a recent study. Some tracking/analysis scripts (example here) are accessing and recovering this information.

Battery Status API
W3C Candidate Recommendation 07 July 2016
https://www.w3.org/TR/battery-status/

A group of researchers have demonstrated how to track users with nothing more than their remaining battery power, which could compromise privacy