This is a known problem with iPhone 6s, here is a link to help replace your battery. https://www.apple.com/support/iphone6s-unexpectedshutdown/
While capturing video of a multiple alarm fire my iPhone 6s+ Went from 30%+ battery remaining, to showing me a 10% battery remaining box, to shutting off, to rebooting itself. When I travelled away from the scene I notes my key fob battery was also depleted. While driving away from the scene, and after being plugged into my car charger, I saw the red battery icon (full screen) but very shortly thereafter it turned back on and showed 30% battery again. Any idea what is at work here?
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Question: Power seemingly depleted, then restarted...