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Tech Tip #4 – Load Testing / Starter Draw
If the battery’s State-of-Charge is at 75% or higher or has a “good” built-in hydrometer indication, then you can load test a car battery by one of the following methods:
- With a battery load tester, apply a load equal to one-half of the CCA rating of the battery for 15 seconds and observe the lowest voltage reading (Recommended method).
- Disable the ignition and turn the engine over for 15 seconds with the starter motor and observe the lowest voltage reading.
DURING the load test, the voltage on a good battery should NOT drop below the following table’s indicated voltage for the electrolyte at the temperatures shown:
To perform this test using the Power Probe 3, first disable the ignition, next select Negative Peak Mode by pressing the oval button once, this will hold the lowest reading on the display during your cranking load test.
Now connect the probe tip to the positive battery terminal and begin cranking the engine, immediately press the reset button while continuing to crank for 15 seconds. Note the Minimum voltage displayed.
If you are a Hook user, you can also use normal Hook Mode or select the Voltmeter Min/Max screen which will capture the highest and lowest voltage during this test.
Typically during the load test, the cranking voltage on a good battery should NOT drop below 9.6V, but that varies depending on temperature. Use the Battery Load Test Voltage Chart shown.
If the battery passes the load test, you should recharge it as soon as possible to prevent lead sulfation and to restore it to maximum performance. If not, remove the load, wait ten minutes, and measure the State-of-Charge. If the battery bounces back to less than 75% State-of-Charge (1.225 specific gravity or 12.45 VDC), then recharge the battery and load test again. If the battery fails the load test a second time or bounces back to less than 75% State-of-Charge, replace the battery because it lacks the necessary CCA capacity.
Performing a Starter current draw test can tell you a lot about the health of the starter motor and the circuit supplying power to the starter. To much current draw could indicate a starter that is binding or has shorted windings. Specifications vary considerably, depending on engine size, type (gas or diesel), compression ratio, etc. A good general rule is about 1 Amp per Cubic Inch for gas vehicles and about 2 Amps per Cubic Inch for diesel vehicles. Using this formula, a 350 Cubic Inch gas engine should have a starter current draw no more than about 350 Amps. The following auto and light truck current draw information (for 12 volt systems) is a general guideline. It is normal for some starting systems to draw more amperage. To test current draw in this range will require the use of an inductive pickup or amp clamp plugged into a DMM or the Hook. Once the amp clamp has been calibrated, clamp it over one of the main battery cables, disable the ignition, and crank the engine while observing the current draw on your meter. A current reading that is higher than normal usually indicates a starter motor starting to short internally and likely need’s replacing.