Usually I try to not go under 20%, before I charge. And when I charge, I stop at around 80%. My battery on my Iphone is on 96% maximum performance capacity, and I bought it late 2017. I do think it helps the battery, not let it go all the way down to 0%, and charge it to 100% every time. But then I close it every night, after midnight. That has probably helped a lot. But I admit I dont know about all the battery theories. Some say do that, and another something else.