Internal resistance testing is a relatively simple way of measuring the health of your LiPo batteries. Internal resistance is the electrical resistance that occurs inside the battery cells themselves as the battery creates electricity. The higher the internal resistance the less current can flow from the battery to your ESC and motor.
Older, abused, puffed, crashed, and lower quality batteries have a higher internal resistance and therefore cannot maintain voltage to your ESC and motor when you apply power. Because the battery itself creates current, you cannot simply hook up an ohmmeter and measure resistance. Instead you must measure the voltage, then apply a known electrical load (halogen light bulbs in this demonstration), re-measure the voltage and also measure the amperage. The internal resistance is the voltage drop divided by the amperage.
Measuring this occasionally will give you a good idea of the "health" of your battery packs. If you have a battery that doesn't give the "juice" or "oomph" like it used to, this is a good way to actually quantify the loss in power and determine whether the battery should be reassigned to bench work or the garbage.
The video will demonstrate how to actually perform the test. Once set up it takes about 30 seconds per battery and you can do all the testing and another 30 seconds to do the calculations. I like to use a 100 watt load, but this is up to your discretion.
Below is a link to a shared spreadsheet you may download to your computer and fill with your own values (some sample values are entered by default). This spreadsheet will do all the calculations for you, including compensating for different numbers of cells, different battery pack capacities, different C-discharge test variables. Enter your specs and measurements on the left from "DATE" to "AMPS" and it will calculate the rest. This will give you pretty close to an apples-to-apples comparison of internal resistance for all your batteries.
This should not be considered a highly precise measurement for purposes of publishing or comparing specifications. But it is a great way to keep tabs on the status of your batteries.
Please contact me if I can be any help.