I think it comes on at 10%, which was around 16 miles of estimated range the first time it popped up on mine.
It closes Android Auto, and opens the the built in Nav and gave me the choice of local chargers. I've taken the car down to 4 miles but that was still 8% when I started charging.
I find the estimate range figure very misleading.
I ran mine as low as I dared a few weeks ago. As far as I remember, the low battery level warning came on as the power reduced into the final red bar on the gauge which is 12½%. I ran it down to 4% which I think showed 16 miles remaining.