Surely the answer here is for the browser to not give such fine-grained information. For example always give the charge level to the nearest 10 percent and the battery life to the nearest 10 seconds.
The benefit is that the user doesn't need an additional setting for each potential web app to set the threshold for a low power mode, it's all done once at the OS level.
... I suspect many wouldn't understand the tracking implications, and even many who can understand it wouldn't immediately make the connection.
That said, this isn't the first such thing used to identify users. Other things like resolution, browser, extensions, etc, can be used to make a nearly unique fingerprint and this has been known for years. Search "browser fingerprint" and you'll find many pages on the subject.
It is is similar in any other case of low resources. There probably is not much of a difference for a website if user is low in power than if it is low in memory. There should be then just one flag/event really: low resources (catch-all for battery, memory, CPU time, storage etc.).