How to Measure 12 Volt Amps
The ampere (amp) is the standard unit of measure for electrical current. A standard 12-volt car battery may produce 600 amps or more, especially those designed for extremely cold weather. An ammeter designed for use on a car battery will have very thick cables that can handle this level of current.
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Instructions
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Obtain a car ammeter. These ammeters are readily available from auto supply stores and should be rated for at least 800 amps. Car ammeters typically use cables made with 10-gauge or thicker wire. This is necessary because an ammeter must carry the current in the circuit, unlike a voltmeter.
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Disconnect the battery cable from the positive terminal of the battery to install the ammeter temporarily for testing purposes. You would typically install the ammeter just before the alternator in the circuit if you wished to install a permanent meter.
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Attach the alligator clamp at the end of the ammeter's positive cable to the battery's positive terminal. The current flows from the positive terminal to the negative terminal, so the ammeter will typically be on the positive side of the circuit.
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Connect the alligator clamp on the ammeter's negative cable to the positive battery cable.
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Measure the amperage of your car battery. The dial on a car ammeter indicates the direction of current as well as the amount of current. The dial's position is in the center of the dial and indicates that there is no current flow. The values on the left side of the ammeter dial indicate negative current, meaning that the battery is being discharged to run electrical devices. The values on the right side of the ammeter dial indicate positive current, meaning that the battery is being charged by the alternator.
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