Same monitor refresh rates on A/C and battery

I am using a Dell Latitude E5520-4 laptop, and a few days ago I found out that the refresh rate of the laptop's monitor was changed to 40 Hz. I changed it back to 60Hz, but ever since everytime I plug out the charger, the monitor flashes for a few seconds, and it changes back to 40Hz. When I plug it back it flashes again, and changes back to 60Hz. At the state of charging it is 60Hz, while with battery it is always 40Hz. I have to change it back to 60Hz manually everytime I use it from battery.
Can anyone help me to make the computer doesn't change the refresh rate in either case? Just stay with 60Hz, and leave it there no matter if it is on charger or on battery.
Thank you in advance.

Try creating a custom resolution and check.