Good reason for back up battery

[cessnadriver850]

Nice weather and wanted to try out my new sv-d700 installed in a vintage 1955 plane with a-65 motor and generator. Had been testing the system in hangar and wanted to check out the compass calibration of the adhars. Fired up and powered up the display, which started to reset on and off and act all crazy. Thought flying not a good idea and returned to hangar and started looking for power problems. Found battery voltage below 10 volts. Evidently skyview does not like that too much. Good reason to buy the battery back up on a generator equiped aircraft. Lesson learned and a temp volt meter install until I get the EMS module.