I managed to get it work with a Powershell script. I'm pretty sure this is a badly managed Dell-Microsoft issue. Too bad they don't bother to resolve it.
Hi, Hardware: Vostro 15 3510 laptop with Intel/nVidia graphics, i7, 16 GB memory, 512 GB ssd. OS: Windows 10 21H1 with latest updates. Newest drivers, including graphic drivers from Dell download center. Intel Graphics Command Center from Dell, not from Intel or MS Store. Problem: screen won't dim when charger cable unplugged. Setting in BIOS checked (brightness on battery: 50%). Found no Power Settings in Intel Graphics Command Center, except Adaptive Brightnes (but it's not working). Tried reinstalling graphics drivers, Intel Command Center, reinstalled latest BIOS. Nothing. Screen dim when on battery power doesn't work. Any ideas?
Screen auto dimming not working