The Battery Voltage data item is exclusively designed to report battery voltage from a NMEA 2000 battery voltage sensing device. Similarly, the Alternator Voltage data item is exclusively designed to report voltage from a NMEA 2000 engine gateway. The a/c/e/eS/gS-Series MFDs are not designed to report the voltage supplied to the display.
Raymarine neither manufactures nor recommends any specific manufacturer's NMEA 2000 NMEA 2000 battery monitor. When seeking NMEA 2000 devices to interface to one's system, it is recommended that NMEA 2000 certified devices be sought. It is recommended that a Google search including the words/acronyms be used to seek the device in question "nmea 2000 battery monitor".