Sometimes you just need more battery voltage. You may need to light more LED Christmas lights, or you may have an electronic device that requires more voltage than your battery can put out. One of the easiest ways to increase voltage is to use more batteries. Kirchoff’s Voltage Law, a fundamental law in electricity, states that the sum of the voltages in a closed electrical loop is equal to zero. This means that when you connect two batteries end-to-end (in series) the total battery voltage will equal the sum of the voltages of the individual batteries.
Obtain two working batteries with different voltage ratings. Use low voltage batteries that are under 10 volts, such as flashlight batteries. Use a 5-volt battery and a 3-volt battery for this example.
Measure the voltage of each battery: Connect the positive terminal of a DC voltmeter to the positive terminal of the first battery and the negative terminal of the DC voltmeter to the negative terminal of the first battery. Record the voltage measured. Repeat the procedure for the second battery. Add the two measured voltages together and write down the result.
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Connect the two batteries in series (end-to-end): Connect the negative terminal of the first battery with the positive terminal of the second battery. Connect the negative terminal of the second battery to ground.
Measure the voltage of two batteries in series: Connect the ground connection of the second battery to the negative terminal of the DC voltmeter and the positive terminal of the first battery to the positive terminal of the DC voltmeter. Record the voltage. Verify that the voltage measured is equal to the calculated voltage in Step 2 (the sum of the voltages of the two individual batteries). If not, check the connection.