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How to Measure the Internal Resistance of a Cell

Nearly 120 reportable air incidents that involved batteries occurred in the decade to May 2011, according to data released by the Federal Aviation Administration. In many instances, shipping companies such as UPS or Federal Express reported that accidental short circuits caused batteries to overheat or, in extreme instances, catch fire. Generating the current needed to start a fire does not require a high voltage; all that is needed is simply a direct connection across the terminals of a cell that has a low internal resistance. Measuring internal resistance helps characterize a cell, particularly when determining the maximum current it can deliver.

Things You'll Need

Cell to be tested
Calibrated variable resistor
Digital multimeter
Switch
Wire
Calculator
Graph paper

Instructions

- 1
  Check that the switch is in the open position. Using wire, connect one end of the variable resistor to its own sliding contact. Make a second connection from the sliding contact to one of the switch terminals. Connect the other end of the variable resistor to the negative terminal of the cell to be tested.
- 2
  Connect the positive lead of the digital multimeter to the positive terminal of the cell and the negative meter lead to the unused switch terminal. Check that all the connections are secure. You now have a series circuit that you can complete by closing the switch.
- 3
  Set the variable resistor to a value that will limit the current from the cell to less than 1 amp. For example, if you are using a 9-volt cell, the resistance should not be below 9 ohms. Record the resistance value.
- 4
  Switch on the multimeter. Set it to a range that will measure a direct current, or DC, of at least 1 amp.
- 5
  Close the switch. Read the current in amps shown on the multimeter and open the switch. Record the current reading alongside the resistance value.
- 6
  Increase the resistance of the variable resistor by approximately 20 percent. For example, if the initial resistance was 9 ohms, set the variable resistor to around 11 ohms. Record the new resistance value.
- 7
  Close the switch and take another current reading. Open the switch and record the new current reading alongside the new resistance value.
- 8
  Repeat the process until you have six to eight sets of measurements, increasing the resistance by approximately 20 percent each time. Record your results.
- 9
  Calculate the reciprocal of each current reading using a calculator. For example, if the current reading is 0.5 amp, the reciprocal would be 1 divided by 0.5, which equals 2. Record the reciprocals alongside the corresponding resistance values.
- 10
  Plot a graph of the resistance values against the reciprocal current values. Use the X-axis for the reciprocal current values and the Y-axis for the resistance values. Draw a best-fit straight line through the points that you have plotted.
- 11
  Observe the resistance value at which the line intersects the Y-axis. This should be a negative figure. Multiply the intersect value by minus 1. The result is the internal resistance of the cell in ohms.

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