Connect the positive terminal of the voltmeter to the positive terminal of the first battery. Connect the negative terminal of the voltmeter to the negative terminal of the first battery. Write down the voltage on the voltmeter's display. Do not use the battery if the voltmeter measures less than 11.8 volts. Repeat the procedure for the second battery. Continue until you find two batteries that meet the minimum 11.8 volt specification.
Connect the positive terminal of the first battery to the negative terminal of the second battery. Connect the positive terminal of the voltmeter to the positive terminal of the second battery. Connect the negative terminal of the voltmeter to the negative terminal of the first battery. Connect the positive terminal of the voltmeter to the positive terminal of the second battery. Connect the negative terminal of the voltmeter to the negative terminal of the first battery.
Write down the voltage on the voltmeter display. Call this voltage "the measured battery voltage of two 12 volt batteries connected in series." Add the recorded voltage of the first battery to the recorded voltage of the second battery. Write down this result and call the result "the calculated value of two 12 volt batteries connected in series." Recheck your connection if the measured voltage value is not the same or very close to the calculated voltage value.