Another option to read the current, is to use a resistor. I have access to lots of resistors, so it was easy for me. I twisted 5, 5 ohm resistors together, to make a 1 ohm resistor. Then undo one terminal, touch the resistor to the terminal and the battery, and read the voltage across the resistor. In this case, the reading will also be in amps, since Volt/1 Ohm = amps. Mine was reading about 0.16A. To me, that sounds about right, with all the electronics in a car drawing minimal current in the off state.
A fully charged battery should read 12.7 V. With tthe car running, the alternator charges, you should read more than 13.5 V (more like 14 -14.5V)
Another thing that can cause some of your problems, is dirty connections. Insure that your battery to cable connections is clean. Get a batter terminal cleaner (wire brush type). Remember to turn the brush in one direction only. Then, check the ground connection to the engine (and other places). Mine was right below the power steering pump. If someone says "its only ground", they do not understand that currnet needs to have a full circle pathway.
If you have a 2 ohm resistance dirty connection, and try to draw 1 amp, there is a 2V drop right there, "robbing" voltage from where you want it - and of course as the resistance goes up form there, it only gets worse.
I see you are in Georgia, so rusty connections should not be an issue - unless you are near the ocean. Do you know the history of the car - was it near the ocean, or from the rust belt?
