Why is my screen battery usage so high?

[AC Question]

My 18 Months old Samsung Galaxy S6 is using the battery very fast now.
On a normal day the screen will use 35-45% of the battery capacity and my phone generally lasts for half a day with normal usage (a bit of gaming, facebook, email and web browsing)
I would estimate it now lasts about half the time of when it was new. I would expect the battery to degrade over time, but I would not expect the screen percentage to increase over time, which it has.
I always have "auto" brightness enabled and most of the time the brightness slider i at max (except at night) and it has always been like that.

 

[Rukbat]

That's 35% to 45% of the amount of battery used, not of the total battery capacity. (The only way they're equal is if you run the battery down to 0%, and if you do that you need a new battery - lithium batteries are not deep-discharge batteries.)

Since the percentage you're seeing is percentage of total use, not total capacity, if you use less, the percentage for the screen goes up. Running an Amoled screen at maximum brightness eats a lot of battery - so if you're always doing that, and usually draining the battery down lower than 40% (that should be the "turn the phone off now" point), it's not going to last more than about 6 months.

(We could solve that problem by using lead-acid batteries, but you don't want 5 pounds of battery in your phone - lithium is the lightest metal, so they chose weight, not capacity, as the criterion.)