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I'm coming from a computer science background, and will be writing some software to automate battery testing. I have background in math, but little to none in physics or electronics. Please consider this in your answers.

That said, how would I go about testing the internal resistance of a small lithium polymer cell by hand?

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Measure open-circuit voltage \$V_{oc}\$, then load the battery with resistor \$R_{load}\$ and measure voltage \$V_{loaded}\$.

$$ R_{in} = R_{load} \cdot \frac{V_{oc}-V_{loaded}}{V_{loaded}} $$

This is the internal resistance at DC and at your specified load. Internal resistance varies with load and temperature and battery charge and age, etc. And you'll need more advanced equipment to measure at higher frequencies.

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    \$\begingroup\$ Internal resistance varies with temperature yes, but I have never heard of it varying with load. Do you have any info of the subject? \$\endgroup\$
    – Gunnish
    Jan 10, 2013 at 16:08
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    \$\begingroup\$ Everything I read indicates that batteries' internal resistance changes with load. Maybe when you apply a load to the battery, it performs some work thus heating up and thus affecting the IR? \$\endgroup\$
    – Igor Zevaka
    Mar 12, 2013 at 2:48
  • \$\begingroup\$ Internal resistance is just an approximation. There's not actually a resistor in there. If you take the small-signal response of voltage vs. current, that slope is a resistance. It's not a perfect model. Battery chemistry is complex. \$\endgroup\$
    – markrages
    Mar 12, 2013 at 3:50

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