Indeed, all are valid arguments.
My engine is self suistaining - if the battery explodes (granted I have a bigger problem than worrying about the actual voltage at the plotter), the engine will keep going. I have a Garmin handheld that will instantly switch to internal battery power, and a handheld VHF in the seat.
A lot of this is as I said above - I can calibrate any one of a number of sensors, but not the built - in one!
Having said that Jezza2012 has solved the problem nicely in the bits'n'pieces section and soon I will have a voltmeter across the battery -ve and swtched side of the main isolator. Main problem now is the finding the console space to mount it!