Re: Voltage tester idea
David, it will work but I'd suggest two other options.
One being a plug-in voltmeter, the display is on the "lighter" plug itself. Digital or LEd bars, both are available.
The other is to buy a pair of "pygmy" hippo clips from Rat Shack, crimp a run of duplex wire and a fuse onto them, and bring out the leads direct from the battey for a "cleaner" reading.
In either case the voltage will be a good reference for you, relative to the charge state of your batteries, but subject to all the usual ways to misread the information. Starting with the accuracy of the meter, no matter what kind you have, and whether it has been calibrated. I finally stopped wondering which meter was wrong and by how much, and built a little "10.00" voltager reference supply. That's all it puts out, and exactly what it puts out, accurate to a few thousandths of a volt. So when I need to know if my meter is on drugs, or just a little slow, I check to see if it says "10.00" when it is connected to the calibration source.
As the Wicked Queen said, "Meter meter on the wall, who's the foulest of them all?"