Open circuit battery voltage is the voltage measured on a battery when it is disconnected from an electrical circuit and not under any load. Although you can test nearly any battery this way, the most common batteries that are tested for open circuit voltage are automobile or marine-use batteries. You need to fully charge the battery that you are going to test so it provides a precise reading. In addition, the only way to achieve the type of accuracy you need for open circuit battery voltage testing is by using a digital multimeter.
Remove any battery cables or wire clamps from the battery, if applicable.
Set your digital multimeter to read DC voltage, or if that option is not available, set it to read 20 volts.
Touch the red probe to the positive post of the battery, and then touch the black probe to the negative post.
Read the figures on the multimeter. A fully charged battery will put out 12.66 volts exactly. If the battery is not fully charged, the voltage will still be in the 12 volt range, but slightly less than 12.66. If the battery is faulty, it will put out less than 12 volts and you will need to replace it.