Your amp meter goes in series with your circuit, not parallel.
If you place it in series with the wires going from the rectifier to the batteries, it will be in the right place to measure current in that circuit.
The meter is for one direction only If you put it in back to front the needle will go backwards to the stop, so put it in the other. The + goes to the most positive wire, (mill side +) and the - side goes to the positive terminal of your battery.
The mill- goes to the battery -
A shunt is for meters without an internal shunt.
All meters at heart are just ma meters. If we put a shunt (known resistance) in the circuit, of R ohms. it will drop E volts at I current.
The needle is simply glued (pressed) onto the moving coil, suspended in a magnetic field. As current moves through the field, it makes a magnetic field of it's own... this repels the main perm field, and it turns. It is also connected to a "clock" spring, opposing this force.... so the more current in the meter coil, the more magnetic field, the more up the scale it can go against the spring.
It does this with millivolts. A shunt may be rated as 50mv for 75 amps etc.... we then need a meter that will have full deflection for 50mv... and we now have a 75A meter etc.
Most smaller current meters have the shunt built internally, (up to 30 amps), after that it is wise to use a shunt style, as the heat in an internal shunt can lead to breakdown of the meter (particularly the connection point in and out).
..............oztules
Edit.. Woof beat me too it I see.... pressed the button sooner.
