Battery Open Circuit Voltage Test

An open circuit voltage no load test shows the state of charge of a battery and whether it is ready for a load test at 50 percent of the battery's cold crank rating. Refer to Starting and Charging/Battery/Battery Load Test. If a battery has open circuit voltage reading of 12.4 Volts or greater, and will not pass the load test, replace the battery because it is defective. To test open circuit voltage, perform the following operation.
1. Remove both battery cables, negative cable first. Battery top, cables and posts should be clean. If green dot is not visible in indicator, charge the battery. Refer to Starting and Charging/Battery/Battery Charging Procedures.
2. Connect a Volt/Ammeter/Load tester to the battery posts. Rotate the load control knob of the Carbon pile rheostat to apply a 300 amp load. Apply this load for 15 seconds to remove the surface charge from the battery, and return the control knob to off.

Testing Open Circuit Voltage:

3. Allow the battery to stabilize for 2 minutes, and then verify the open circuit voltage.

4. The voltage reading will approximate the state of charge of the battery. It will not reveal battery cranking capacity.
Battery Open Circuit Voltage
Open Circuit Voltage = Charge Percentage
11.7 volts or more = 0 %
12.0 volts = 25 %
12.2 volts = 50 %
12.4 volts = 75 %
12.6 volts or more = 100 %