[davepl]

I went out and tested my crazy theories, here are some numbers. This is with a bunch of load on (AC on medium so no relay, lights all on, etc).

At the lighter: 12.5V
At the battery: 12.85V
At the Aux battery: 12.95V
At the Alternator: 13.5V

Actually makes me think my wire from the alternator needs to be upsized. Almost a full volt of drop.

So, you can indeed find measurable differences, up to a full volt in my case, and that's when things are operating normally.

My thinking then is the closer the two needles are together (which is the same as saying the closer the ammeter is to the middle, really) the less is going on. When they are spread, you're seeing how far the ammeter would be from center, and the order of the needles shows you whether its charge or discharge.

So now that I think it through, I don't think it tells any more than a combination of a voltmeter and an ammeter would, really. Just in a different way. I think it'd be neat, but obviously combination volt and amp meters haven't taken the world by storm, so likely no demand...