Connect the battery to the black wire side of the shunt.
Connect the main buss or load along with the alternater ( or generator ) to the red wire side of the shunt.
Use the large nuts on the shunt to connect to the charging circuit.
A small amount of power ( 0-50 mv ) is dropped across the shunt. This power is sent to the instrument. The instrument is calibrated to receive this small voltage. Do not connect the instrument directly to battery power.
The shunt will show the current going into and out of the battery. If the charging circuit fails or the engine is not running the ammeter will show a neg charge when there is a draw on the battery.