[MPaulHolmes]

It's a stick woman, with a beautiful mane of red flowing hair.

Also, I want to walk through why the shunt works, just because I thought of it and felt really proud, and found out that everyone else already knew and it made me real sad way down deep.

Resistance of 2 gauge copper: 0.1593 Ohms / 1000 feet

V = I*R

So, since the setting on the harbor freight ghetto volt meter will be set to the 0.2v range, and since you don't want to be too close to 0.2v in that setting so nothing blows up (so use 0.1v as the maximum voltage that will be measured), and because 400amp is the most current that will ever come from the batteries, we must find the R in that situation:
0.1v = 400amp * R

So, R = 0.1/400 Ohms
R = 0.00025 Ohms

So, what length of cable gives 0.00025 Ohms?

(1000 ft / 0.1593 Ohms) * 0.00025 Ohms = 1.569 ft

Now, since there are going to be lots of chunks of copper cable used to connect the batteries, just make sure one of them is about 1.6 feet long, and attach the volt meter (used as ammeter) across one of the cable pieces that is 1.6 feet long.

If you want to use other cable diameters, just look up the resistance per 1000 feet for that size cable.

On the ghetto volt meter display,

0 will mean 0 amps
100 will mean MAX amps (what it's really reading is 0.1 volts, but it shows 100 because it shows the number of millivolts! haha!)

The potential problem with this is, if the cable get warm, it will seem as if I'm using more current than I actually am.

Near room temperature, the resistance of copper increases by 0.393% per degree C, which means the error of the ghetto ammeter increases by 0.393% per degree C. So, to get an error of 10% on the ammeter (too high), the cable must be

0.00393*T = 0.1
T = 25.44 degrees Celcius HOTTER than the environment.

So, if the cable is holding at around 114 degrees F (20 Deg C + 25.44 deg C), it will be an error of only 10% too much on the ammeter. Hopefully the cable doesn't get that hot.

Dang it! That's way cooler than 145! Oh dear! I made a booboo. New plan:
I must compensate??? Maybe, once I find out how hot the cable typically will be most of the time, I can make it so the 0 to 100 scale has an error of 0 in that situation. Then, when it's cooler, I'll be drawing more amps than I think, and when it's hotter, I'll be drawing less amps, but hopefully the error will be less.