Yep... that's the issue. I'm attempting to use the readings on the JPI
and the Davtron to ascertain the condition AT THE BATTERY. Because of
these differences (probably due to different grounds; maybe due to
voltage drop from main bus to avionics bus), I need some way to
calibrate JPI readings to battery voltage; best easy way for me is to
measure the main bus, at the cigar lighter.

I got the plug in voltmeter and calibrated it using the HP power supply
and a real nice digital shop multimeter. To my surprise, the plug in
unit was VERY accurate... right on below 10 volts, and maybe 0.1 low at
12 volts (some lag, though... a second or two to stabilise).
Sufficiently close for my purposes.

Too busy (Christmas) and too cold (Michigan) to check it out right now.
Further reports to follow.

Rich


mah wrote:
Joining this in mid-thread as an interested onlooker.

Was working a similar problem on my van recently. I'd see a difference
of 1.5 volts depending on where I tapped into the circuit, the ground
location, and load in the system.

That's the one item, load, not mentioned in the previous posts. When
I'd fire up my gps, laptop, and cell phone charger the voltage on that
circuit would become 12.2 volts while it read 13.5 at the battery.

Be thinking of what load you are drawing as the voltage will drop as it
pushes more amps down the wire.

MAH