The T420s screen refresh rate is dropping to 50Hz when on battery despite setting "Never" change Refresh rate when on AC on Lenovo Power Manager.
This symptom has been resolved through the latest version of video drivers and power manager on the support site. Download and install the latest version for your specifc OS. Here is the link to the drivers for T420s
After installing, reboot system and test functionality - you should now be able to change refresh rate settings.
The following steps should now no longer be required and are included for historic reference:
A previous work around solution was to change some values in the inf files by follow steps provided below: