My Samsung Galaxy S6 with Android 6.0.1 always dims the screen when hitting 5% remaining battery. This is very bad in bright sunlight because the screen gets so dark that it's impossible to use the phone and pure guesswork to brighten it again. This “feature“ seems to override both auto and manual brightness setting. It also happens irrespective of power saving mode on/off. How can I stop it from doing this?
I believe you are looking for Power Saver Mode. This is under the Settings > Battery. There is usually an option to turn on Power Saver Mode altogether or adjust what it does.
Samsung Galaxy Phones do dim their phones once it reaches the low level of battery. You may try this: http://www.samsung.com/ie/support/skp/faq/1056771