simplicity 950 minivator batteries?

I have a Simplicity 950 Minivator that lost its display. I checked the batteries....one is at 5.9 vdc, and the other reads dead. Will the display still work if the power supply is plugged in? Or do the batteries have to be within range for the display to work?

Also...can the batteries be charges with a standard 12v charger directly? Or do the batteries need replaced?