How to calculate battery internal resistance

Written by joseph west
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How to calculate battery internal resistance
Batteries have numerous imperfections that must be accounted for in some circuits. (battery image by Sergey Juchkov from Fotolia.com)

Most people think of a battery as a perfect voltage source. A new nine-volt battery supplies nine volts, regardless of the device it is powering. In reality, all batteries have internal resistance, which means that the battery's voltage changes according to how much current it must deliver. At low current, the internal resistance of the battery is negligible. But in circuits that draw higher current from the battery, the decrease in voltage may be a problem. A battery's ability to supply current can be determined by measuring its internal resistance and comparing this resistance to the overall resistance of the external circuit.

Skill level:
Easy

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Things you need

  • Voltmeter
  • Resistor

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Instructions

  1. 1

    Choose an appropriate value for the resistor to test the battery. Measurements will be more accurate if the value of the test resistor is not vastly different from the value of the battery's internal resistance. A test resistor that is 10 or 20 times larger than the internal resistance is usually a good choice. Find an approximate value for the internal resistance of your battery by searching the web. For example, a typical AA battery might have an internal resistance of 1 ohm. If you cannot find any information for your battery, take measurements with different test resistors to ensure you get consistent results.

  2. 2

    Measure the ideal voltage of the battery by touching the red probe of the voltmeter to the positive terminal and touching the black probe of the voltmeter to the negative terminal. Call this measurement V-bat. As an example, an AA battery might have a V-bat of 1.5 Volts.

  3. 3

    Connect the leads of the test resistor to the terminals of the battery. Wrap the leads around the terminals or use little metal clips to keep them attached.

  4. 4

    Measure the voltage across the resistor by touching the red probe to the side of the resistor connected to the positive terminal and touching the black probe to the side of the resistor connected to the negative terminal. Call this V-test. The example battery has a V-test of 1.4 Volts.

  5. 5

    Subtract V-test from V-bat to find the voltage across the internal resistance, and call this V-res. In the example, V-res = 1.5 - 1.4 = 0.1 Volts.

  6. 6

    Divide V-test by the value of the test resistor to calculate the current flowing through the test resistor. The example uses a test resistor of 20 ohms, so the current flowing through the test resistor = 1.4 / 20 = 0.07 amps.

  7. 7

    Calculate V-res dividing it by the current flowing through the test resistor. The test resistor and the internal resistor have the same current. The result of this division is the internal resistance of the battery. In the example, V-res = 0.1 V, so the internal resistance = 0.1 / 0.07 = 1.4 ohms.

Tips and warnings

  • Large batteries, such as car batteries, can cause serious electrical shocks. Exercise caution.

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