Enter any two of current, resistance, and voltage to calculate the missing value using Ohm’s law. If you use a battery’s internal resistance, the calculated voltage is the voltage drop across that resistance (ΔV = I·R), not the battery’s open-circuit voltage.
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Voltage Drop (Ohm’s Law) Formula
The following formula is used to calculate the voltage drop across a resistance (for example, the drop across a battery’s internal resistance when current is flowing).
\Delta V = I_b \cdot R_b
Variables:
- Where ΔV is the voltage drop (volts)
- Ib is the current (amps)
- Rb is the resistance (ohms)
To calculate the voltage drop, multiply the current by the resistance. If you also know a battery’s open-circuit voltage (often written as Voc), an approximate terminal voltage while discharging is Vterminal ≈ Voc − I·Rinternal (and while charging, Vterminal ≈ Voc + I·Rinternal).
How to Calculate Voltage Drop?
The following example problem outlines the steps and information needed in order to calculate the voltage drop.
Example Problem:
- First, determine the current. In this example, the current is measured to be 10 A.
- Next, determine the resistance. For this problem, the resistance is 0.045 Ω (45 mΩ).
- Finally, calculate the voltage drop using the formula above:
ΔV = Ib * Rb
Inserting the values from above into the equation yields:
ΔV = 10 * 0.045 = 0.45 (volts)
FAQ
What factors can affect battery voltage?
Battery voltage depends on the battery chemistry and state of charge, and it also changes with temperature, age, and load (how much current is being drawn). Under load, cold temperatures generally increase internal resistance and cause a larger voltage drop, while warmer temperatures typically reduce internal resistance (though excessive heat can damage the battery and shorten its life).
How do you measure battery resistance?
Battery internal resistance is typically estimated using a load test: measure the open-circuit voltage (Voc), apply a known load and measure the loaded voltage (Vload) and current (I), then approximate Rinternal ≈ (Voc − Vload) / I. Dedicated battery resistance testers often use short pulses or AC methods and can be more reliable than a simple multimeter alone.
Why is it important to calculate battery voltage?
Knowing battery and pack voltage helps you confirm system compatibility (for example, how many cells in series are needed) and estimate performance under load. Estimating the voltage drop (I·R) is especially useful for checking whether a device will see enough terminal voltage when drawing current. For state-of-charge estimates, voltage is most meaningful after the battery has rested (no load) and should be interpreted with the correct battery chemistry in mind.
