an apparatus for measuring battery voltage is provided that can detect battery voltage with high accuracy , and also can be miniaturized with cost and noise reduced . the apparatus comprises : a capacitor which is charged to the voltage of each module , switches which connect and disconnect this capacitor and the module , a voltage follower which outputs the voltage across the capacitor , and switches which connect and disconnect the voltage follower and the capacitor . for the aforementioned switches , photo mosfets are used . furthermore , to prevent discharge from the capacitor , a voltage follower is installed . therefore , by using not a complicated circuit but inexpensive and simple elements , a low cost and miniaturized apparatus for measuring battery voltage is realized .