Battery Voltage Test
Battery Voltage Test
STATE OF CHARGE
Testing the battery voltage with a voltmeter is a simple method for determining the state of charge
of any battery. The voltage of a battery does not necessarily indicate whether the battery can perform satisfactorily, but it does indicate to the technician more about the battery’s condition than a simple visual inspection. A battery that “looks good” may not be good. This test is commonly called an open circuit battery voltage test because it is conducted with an open circuit, no current flowing, and no load applied to the battery.
1. If the battery has just been charged or the vehicle has recently been driven, it is necessary to remove the surface charge from the battery before testing. A surface charge is a charge of higher-than-normal voltage that is just on the surface of the battery plates. The surface charge is quickly
removed when the battery is loaded and therefore does not accurately represent the true state of charge of the battery.
2. To remove the surface charge, turn the headlights on high beam (brights) for one minute, then turn the headlights off and wait two minutes.
3. With the engine and all electrical accessories off, and the doors shut (to turn off the interior lights), connect a volt-meter to the battery posts. Connect the red positive lead to the positive post and the black negative lead to the negative post.
4. Read the voltmeter and compare the results with the state of charge. The voltages shown are for a battery at or near room temperature (70°F to 80°F, or 21°C to 27°C).
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| Estimated State of Charge - 12V Battery |
What Is the Three-Minute Charge Test?
A three-minute charge test is used to check if a battery is sulfated, and is performed as follows:
• Connect a battery charger and a voltmeter to the battery terminals.
• Charge the battery at a rate of 40 amperes for three minutes.
• At the end of three minutes, read the voltmeter.
Results: If the voltage is above 15.5 volts, replace the battery. If the voltage is below 15.5 volts, the battery is not sulfated and should be charged and retested. This is not a valid test of many maintenance free batteries, such as the Delphi Freedom. Due to the high internal resistance, a discharged Delphi Freedom battery may not start to accept a charge for several hours. Always use another alternative battery test before discarding a battery based on the results of the three-minute charge test.
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