An underperforming battery will fail to properly run the starter motor on your vehicle. But to properly determine its internal components have gone bad, you can perform a quick and simple battery load test. This is one of the best and preferred methods to test battery state of charge and performance. And all you need is a battery load tester.
Preparing for the Test
Before performing the car battery load test, make sure your vehicle battery is fully charged. If your battery comes equipped with a charge indicator on the cover, the eye will show as green. Also, your car battery tester provides a way to check the state of charge as well. Read your tester instruction manual, if necessary. Connect the tester as indicated in your instructions manual for this reading and turn on your vehicle headlights. A battery with a good charge will show 12.4 to 12.6 volts on the meter, says James E. Duffy in "Modern Automotive Technology." Otherwise, charge the battery for this test.
Now, install the load tester on the battery. If you have an inductive type tester, connect the tester red clamp to the positive terminal and the black clamp to the negative terminal of the battery. Then clip the tester pickup terminal to the ground (black) battery cable. If you have a regular tester, connect it in series with the battery following the instructions on the instruction manual.
Next, read the label or markings on the battery and find the amp-hour or cold crank rating. If the label provides the amp-hour rating, multiply this number by three. Or, if the label provides the cold crank rating, divide this number by two. The product will be the amount of current draw you will be applying to the battery during this test.
Performing the Load Test
When ready, turn on your load tester and apply the appropriate current load to the battery. Hold the current for 15 seconds and then read the volts on the screen or dial of your tester. As soon as you have this voltage number, turn down the current load completely and turn off the tester.
Now, interpret your results and verify the condition of your battery. Take as reference a battery under normal room temperature of 21.1C (21C) with a load test result of 9.6 volts, which will indicate a good condition.
For every -12.2 degrees C under the reference temperature in the example above, subtract one decimal of voltage to arrive to the appropriate voltage for that temperature. For example, if you tested your battery under 15.6C (16C) of room temperature, a satisfactory voltage result would be 6.5V; under 10C (10C) of room temperature, a satisfactory voltage result would be 9.4V and so on.
If your reading falls below the satisfactory voltage result, your battery is under poor condition and its charge is not good enough to properly run the starter motor, blower motor and other components in need of high current in your car. The battery plates might have failed already. But before deciding you need a new battery, you might want to take it to your local auto electrical repair shop for confirmation.