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With the onslaught of aftermarket electrical devices for older muscle cars and hot rods, there’s a renewed need for higher alternator output, leaving many car owners wondering how to test an alternator and charging system. The common issue is low voltage at idle where the voltmeter reads 11.5 or 12 volts instead of closer to 13.5 to 14.5 volts. While a higher output alternator is often the cure, it’s best to first check the charging system circuit to ensure that the connection between the alternator and the battery is not creating unwanted resistance.
This simple procedure is called a voltage drop test, an important part of servicing your battery charging system. The first step is with the engine running, the headlights on, and the heater blower fan running at high output. Next, use a digital multimeter that’s set to read voltage to measure the alternator output at the output post at the rear of the alternator. As an example, let’s say this alternator is putting out 14.0 volts.
The voltage recorded right off the alternator is 14.66 volts, which is actually a touch high because we had to crank the starter to get the cold engine running.
Now switch to the battery and measure the voltage reading directly at the battery terminals – and not just on the connectors. In nearly all cases, the reading at the battery will be lower. A typical rule of thumb for this circuit is to record no more than a 0.50 volt drop, with a 0.25-volt drop as ideal. So if the charge wire circuit is performing properly, the digital multimeter would read 13.5 volts at the battery.
Let’s say the reading at the battery is actually 13.2 volts. This would represent a 0.80-volt drop between the alternator and the battery – nearly triple the ideal voltage drop. This means there’s excessive resistance in the circuit. It will prevent the alternator from fully charging the battery, thereby reducing the system voltage to the rest of the vehicle. That’s important because nearly all automotive electrical devices are designed to operate at 13.5 volts or higher.
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Now comparing the voltage on the battery (on the terminals not on the connector), we can see that it has dropped from 14.66 to 14.11 volts. This is a slightly higher drop of 0.55 volts compared to the 0.50 spec and could be improved with a larger charge cable between the alternator and the battery.
In this example, we’re reading a lower voltage that indicates resistance in the circuit, even though we’re only reading voltage. If there’s excessive resistance in the charging circuit, this can be repaired by adding a larger charge gauge wire between the alternator and the battery.
Another good voltage test is to evaluate the ground side. Again, with the engine running and a load on the charging system as described above, place one multimeter probe on the alternator body and the other on the battery ground terminal. If the meter reads more than 0.25-volt, this means there’s excessive resistance on the ground side of the system. Usually this is due to insufficient or poor ground cabling between the engine and the battery.
This method is a quick and easy way to check any charging system, regardless of output. It can often point to deficiencies in the charging system that can be easily repaired. Ultimately, all the vehicle’s electrical devices will run more efficiently with a proper charging system circuit. Plus, improving your charging system circuit might also just put a smile on your face.
We can also check the ground side of the charging system by connecting the voltmeter to the alternator case and the ground terminal on the battery. As you can see, the meter reports 21 millivolts – which is only 0.021-volt which is well within the ideal spec of 0.25-volt. A high voltage reading here would indicate excessive resistance in the ground circuit.
