Electricity follows the laws of physics. Standard equations have been discovered throughout history to calculate properties of circuits. Terminal voltage is no exception. In a discharging battery, you can determine terminal voltage by using the formula TPD = e  Ir. This can be expressed as "terminal potential difference (or voltage) = electromagnetic force minus current times resistance.
Things You'll Need
 Multimeter (digital or analog)
 Calculator

Touch the positive probe of a multimeter to the positive terminal of the battery to determine the electromagnetic force of the battery. Touch the battery's negative terminal using the negative probe. Electromagnetic force (EMF) pushes electrons though conductors (e.g., copper wire) and is measured in volts. Strong car batteries often have an EMF between 12.5 and 13.

Set your multimeter to the ohms setting and test the leads for resistance. Always test resistance with the power of any device turned off.

Determine the current using Ohm's law. Georg Ohm discovered that voltage could be calculated by multiplying amperage and resistance. Likewise, you can compute current by dividing voltage by resistance. If you know the current but not the resistance, divide the voltage by the current to calculate.

Multiply current by resistance. Subtract this product from the voltage reading. The difference is a battery's terminal voltage. If you want to know the terminal voltage of a charging battery, add the voltage reading to the product of current multiplied by resistance.
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