On my S204 2008 C220 with Comand NTG4, key to position 1, hold down steering wheel buttons make/answer phone call, end phone call (both on right) and OK (on left) all together for about 5 seconds, new menu appears in dash, first item highlighted is Vehicle details, press OK and battery voltage and amps useage appears in dash, start engine and display changes to alternator output voltage and amps useage (-ve) or going into battery (+ve). Display continues until either you press back button on steeering wheel or turn engine off.