My phone says my battery is to low for the phone to be able to update itself. I have around 34% left. How much is needed for it to update? 50%? I'm trying to update to Windows 10 Insider Preview for phones from Windows Phone 8.1.
I advise you to plug your phone to a charger and then begin with update. During update to W10 battery percentage can go wild (my Lumia 520 went from 90% to 10% during update, and after update it had around 40% battery) or it can get stuck on last(or any other) step for a long time.
Play it safe - plug it in and do not worry about the battery.
P.S. When I was updating my Ativ S from WP8.0 to WP8.1 it wouldn't update if I had less than 50%,