Power Reserve lets you see the time on your Apple Watch while preserving your battery life. While your Apple Watch is in Power Reserve, press the side button to show the current time. In Power Reserve, your Apple Watch and iPhone won't communicate, and you can't access other watch features.

What should reserve battery level be?

The default percentage for Reserve Battery Level is 9%. If you have a laptop with higher battery storage capacity, then 9% means that you can use it longer compared to other laptops which have lesser battery capacity. E.g., 9% of 5000 mAh is greater than 9% of 3000 mAh capacity.

How do I charge my laptop battery at 80?

If your system has Power Management Options, then select the drop-down for Battery Health Manager and select Maximize my battery health. This setting maximizes the battery health by lowering the maximum battery charge level to 80%. Next, press the F10 key to save the changes and exit.