I would not expect miners to voluntarily stop mining just because the user agent reports low battery. Any miners that care about mobile users could check the user agent string and throttle mining.
If the battery API ever becomes a thing, the first thing I will do is to hack the browser to perpetually report an almost-dying battery. Similarly, if there ever comes a standard for network link quality, I'll make my browser report I'm on a 56k modem.
The reason is simple: whatever cruft the site decides to only serve to high-end users is the cruft I'm very much not interested in seeing.
And because I'm definitely not alone in this thinking, those APIs will be failures for anything other than getting additional bits for tracking and profiling people.
It'd be fine. You would do that, I would do that and most people here would do that. The vast majority of users would not, it's just like adblockers: only a minority uses them.
The reason is simple: whatever cruft the site decides to only serve to high-end users is the cruft I'm very much not interested in seeing.
And because I'm definitely not alone in this thinking, those APIs will be failures for anything other than getting additional bits for tracking and profiling people.