Next is my DS lite, which I imported from the UK 9 years ago. To get a "worst case scenario" for the ancient battery, I let it run the demanding "Metroid Prime Hunters". Constant camera rotation wasn't possible via shoulder buttons, but with a bit of weight on the d-pad I kept the 3D engine busy:
As "best case scenario" for the battery I chose the simple 2D game "Ace Attorney 3" and let it idle in the courtroom... I was surprised when the results were almost the same as "Metroid Prime Hunters" in the same brightness settings, so the SoC seems to run always with full power.
Totally different were the results running a GBA game with one of the displays turned off... the energy efficiency in GBA-mode is great!
4.5 years ago I bought my red 3DS XL. The "worst case scenario" for the battery was "Resident Evil: Revelations". I tested it in 3 brightness settings with auto-stereoscopic 3D turned on and off. The circle pad can't be rubberbanded like a stick to keep a constant camera rotation, but a paper clip did the trick.
As "best case scenario" for the battery I chose the simple 2D game "Ace Attorney 3 HD" and let it idle in the courtroom.
The battery life in both games is a bit better when turning off 3D, but I had expected a much bigger difference in "Resident Evil: Revelations", since only half the resolution has to be rendered that way. The minor differences in "Ace Attorney 3 HD" weren't a surprise... even in 3D these are still simple 2D-graphics placed more in the foreground or the background.