Go to Settings, General, Battery, Battery Saver, Turn Battery Saver On, open that, and you can change the charge percentage at which Battery Saver switches on.
Posted via Android Central App
(Yes, I added a bit to my post just before you posted I imagine.)
Posted via Android Central App
You can disable. You need root, then g3 tweaksbox, then you can disable both the 15% and 5% notification, or just one.