Most Internet users probably know they can be tracked and targeted based on what they search for and which websites they visit.
But it turns out that even the battery charge level on our phones and laptops may be giving away personal information.
That, at least, is according to a new paper on online tracking from two researchers at Princeton University, first reported by The Guardian.
The researchers crawled the top one million websites and found that a small number are now tracking how much charge is left on the device's battery.
The battery status information can then be paired with many other data points -- everything from the IP address to the list of fonts and browser extensions -- to help websites better "fingerprint" your activity online.
The more businesses know about you online, the more they can try to target ads and products to you.
"There is no piece of information about our devices that we can consider to be innocuous anymore. That era is gone," Arvind Narayanan, one of the researchers behind the report, told CNNMoney.
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So far, just a dozen sites are using "fingerprinting scripts" with this battery data included, according to data Narayanan pulled for CNNMoney. But the concern, he says, is that a large advertising exchange or analytics service could start doing it.
"That number can change in the blink of an eye," he said.
Security researchers have raised similar concerns before. It comes in response to the Battery Status API, which lets online applications track battery levels with the goal of making some features less battery intensive when the user's battery is low.
The new research report takes that a step further by suggesting some websites, however few, are actually tracking that data specifically to create a digital fingerprint of the device.
Narayanan can't say for sure what the battery data is being used for right now. His goal is simply to make the public "more educated" by shedding light on these and other data collection practices.
"Without this external oversight, companies don't really have the incentive to be very forthcoming about what data they are collecting," he says. "Somebody has to hold them accountable."
