Battery load testers were once too expensive for the do-it-yourself mechanic. Today, a battery load tester is affordable enough for even the casual shade tree mechanic to have in his garage. My favorite is a handheld digital battery loads tester made by KAL Industries, the KAL model 4780, 130 it loads Amp Digital Battery Load Tester that costs less than $50 online.
You can use a Digital Multimeter to check Automobile storage batteries, charging systems, and starting systems, but it is not the right tool for the job. Every serious shade tree mechanic needs to have a stand alone battery load tester on his or her tool kit.
General Guidelines for Using Any Battery Load Tester To Test A Battery
- Make sure that the ignition switch is in the "OFF" position and that all-electrical load has been removed from the battery. If there are electrical loads on the battery, you will have to disconnect the battery's negative ground cable. CAUTION: Do not disconnect a battery without first plugging a battery keeper in to the cigarette lighter socket to hold the computer codes.
- Do not attempt to load test a battery while the battery is connected up to a battery charger.
- Make sure that the battery posts and terminals are clean and free of corrosion before connecting the load tester. Any corrosion present will cause a high resistant connection and produce a false reading.
- If you are working with a battery that has side terminals, you need to screw in "Charging Posts" in order to connect the load tester.
- All load testers become hot during use so allow them to cool down for 2 to 3 minutes between tests. Never hold the load test button down for more than 10 to 15 seconds. Holding the load test button down for a longer period will cause irreparable harm to the load tester.
Voltage Correction For Temperature
When testing a battery where the ambient temperature is between 40°F and 70°F add 0.1 volt for every 10°F below 70F. When testing a battery where the ambient temperature is between 70°F and 100°F, subtract 0.1 volts for every 10°F above 70°F.
Conducting A Battery Load Test
- Connect the Red "+" cable to the Positive "+" battery post and the Black "-" cable to the negative "-" battery post. The LCD readout will display the battery voltage. The battery must be at a 75 percent charge before a load test can be conducted. At 70°F, a 75 percent charge is equal to at least 12.45 volts for a 12-volt battery and 6.23 volts for a 6-volt battery. If the temperature is above or below 70°F, adjust the voltage as explained in the above section on voltage –Vs- temperature correction.
- If the state of charge is below 75%, recharge the battery before continuing with the load test. If you have not read my article on How To Use A Battery Charger, you should read it now.
- Once the battery is at 75% charge or above, reconnect the load tester to the battery, and hold the "Load Switch" down for 10 seconds. Note the voltage reading on the LCD and release the "Load Switch."
- Disconnect the Red lead then the Black lead.
Interpreting The Test Results
- A battery in good condition should read between 10.00 and 12.00 volts.
- A weak battery will read between 6.10 and 10.00 volts. If the test indicates a weak battery, recharge the battery and conduct the test again. If the results are the same, there are one or more bad cells in the battery and the battery needs to be replaced. If the battery checks good the second time, the battery was not adequately charged before the first test.
- A bad battery reads below 6.10 volts. The battery may have been totally discharged before the first test but odds are that the battery is bad and you should replace it.
Using The Load Tester To Test The Starting System
- Connect the load tester to the battery as before but do not depress the "Load Test" switch.
- Disable the ignition system so the car does not start.
- Use a remote start button or have a helper crank the engine for 15 seconds.
- Note the voltage readout on the tester's LCD. Assuming a fully charged battery, the voltage should not drop below 9.0 volts. If the voltage drops below 9.0 volts during the test there are electrical or mechanical problems with the starter system
- Recharge the battery and repeat the test to confirm the original results.
Conducting A Charging Test
- Start the engine and allow the engine to reach operating temperature.
- Turn the ignition switch to "OFF" to stop the engine.
- Turn the ignition switch to "Accessory" position. Turn the headlights on and the fan on to the high position for one minute. You need to start with a partially discharged batter for this test.
- Turn the ignition switch to "OFF" and connect the load tester to the battery.
- Start the engine and run at a fast idle (1200 to 1500 RPM.) A charging system in good working order will maintain a voltage between 13.5 and 15.5 volts for a 12-volt system and between 6.7 and 7.4 volts for a 6-volt system. F the voltages are above or below those readings, there is a problem with the charging system.
Conducting An Alternator Test
- Connect the red and black battery clamps as before. Plug the small test lead into the jack on the tester and attach the alligator clip to the positive terminal on the alternator. In the case of GM alternators, attach the lead to the N1 terminal without disconnecting the plug.
- Start the engine and allow to idle.
- If you are testing a Gm alternator, watch the red LED and do not touch the load switch. If the red LED flashes and then goes out or if it never comes on, the diodes are good. If the red LED comes on and stays on the diodes could be bad and the alternator need to be replaced.
- If you are not testing a GM alternator, Rev the engine to 1200 to 1500 RPMs.
- Press and hold the "Load witch" for 5 seconds. If the red LED flashes and then goes out or never comes on the diodes are good. If the red LED continues to flash or remains on, the alternator or its integral regulator are bad.
Most battery load testers come with detailed instructions on how to conduct all these tests.