Understanding the Difference Between +12 Volts & -12 Volts

Voltage

• Voltage is a pressure exerted by electrons pushed by a force such as a magnetic field in a generator or a chemical reaction in a battery. The electrons will flow only when there is a path for them to move across. If the force is present but a path is unavailable, this pressure remains.

  Voltage, or potential, is measured as a comparison to zero. The difference in potential between the terminals of a 12-volt battery is 12 volts when the battery is at full charge. Measuring between two battery terminals, the difference or potential is 12 volts because the electrons are being pushed from the positive terminal to the negative terminal. The negative terminal doesn't push, so it has a reference point of zero.

Voltage Reference Points

• If the reference point for a voltage measurement is not zero, the potential between the reference point and measured point will be the difference in potential between the points.

  For example, two 12-volt batteries are connected in series -- the positive terminal of battery A is connected to the negative terminal of battery B. The result is 24 volts measured between the negative terminal of battery A and the positive terminal of battery B.

  If the voltage is measured between the positive and negative terminals of either battery A or battery B, the voltage measurement remains 12 volts. The reference point for each battery is the negative terminal. The reference point for both batteries connected together is the negative terminal of battery A for a total difference in potential of 24 volts.

Differences of Potential

• Continue with our example of two connected batteries. Place the common lead of a multimeter at the connection point and the positive multimeter lead to the positive terminal of battery B and the voltage reading is +12 volts.

  Move the positive multimeter lead to the negative lead of battery A and the multimeter now will read -12 volts. This demonstrates difference in potential. The reference point already is at 12 volts as measured on battery A from the negative to the positive terminal.

Negative and Positive Voltages

• If the connection point between the two batteries is used as ground, then connections between the positive terminal of battery B and ground provide 12 volts above ground.

  Connections between ground -- which actually is at a potential of 12 volts with respect to the negative terminal of battery A -- and the negative terminal of battery A provide 12 volts of potential below ground or -12 volts.

  The difference between +12V and -12V is the reference point. +12V is 12 volts above the reference point, and -12V represents 12 volts below the reference point.