As reported by engadget,
Adaptive Charging is one of the most useful features Google rolled out this month for Pixel phones. It makes sure the device’s battery gets filled slower than usual if you leave it plugged in overnight, which is something that can help maintain the battery’s original capacity. As 9to5Google has spotted on a support page, though, there’s something important you need to know if you plan to put it to good use: The feature will only kick in if you charge your phone and set your alarm within a specific time frame.
The phone will only use Adaptive Charging if you plug it in after 9PM. Further, you’ll need to set an alarm to go off anytime between 5 and 10AM. According to the support page, the feature “uses your alarm’s settings to completely charge your phone right before you wake up,” so your Pixel will charge like normal if you don’t meet those requirements. If your schedule’s in line with those hours, and you want to take advantage of what the feature can do, you can switch it on under Battery in Settings on the Pixel 4/XL, Pixel 4a, Pixel 4a (5G) and Pixel 5.