Aside from any change the Windows update may have made, screen brightness is an issue with many laptops when you run off the battery. The manufacturers want to squeeze every last second out of battery up time so that the specs look good, and push things like screen brightness to the usability limit. You might be able to make some adjustments in the power saving settings.
Adjust brightness automatically in HP pavilion dv5 laptop
Some PCs adjust screen brightness automatically based on how much light there is in the room where you're using the PC. To see if your PC has this setting, follow these steps.
1. Swipe in from the right edge of the screen, tap Settings, and then tap Change PC settings.
(If you're using a mouse, point to the lower-right corner of the screen, move the mouse pointer up, click Settings, and then click Change PC settings.)
2. Tap or click PC & devices, then tap or click Display.
3. Make sure the Adjust my screen brightness automatically slider is turned on.
(If you don't want the screen brightness to change automatically, tap or click the Adjust my screen brightness automatically slider to turn it off.)