LiPo INTERNAL RESISTANCE

Internal resistance is a very useful measurement of the health and quality of a lipo battery pack. Manufacturer-stated "maximum C discharge" ratings do not seem to have any uniform standard, and each battery's discharge rate at a given voltage will degrade over time anyway. Internal resistance is a preferable way of benchmarking a new battery pack as well as tracking its loss of performance over time, through normal use and unintentional abuse.
It must first be said that internal resistance measurement is highly sensitive to a number of variables such as the testing equipment and setup, pack capacity and cell count, temperature, and test load. Using a particular IR testing setup on a particular set of batteries to check their degradation over time is fairly accurate however be cautious about expecting accuracy between IR measurements of different battery sizes and with other users who may have different setups.
The absolute internal resistance for a given battery pack at a certain temperature with a certain test rig is a very reliable comparison when retested later. The per-cell normalized IR is a good but not perfect way to compare between packs of different cell counts. The 1C normalized IR is a very rough measure between packs of different capacities, but there is not a linear relation between C discharge rate and actual IR, so use this with caution.
All that is needed is the battery packs to be tested, a wattmeter, a calculator, and a test load. 12v halogen bulbs on a suitable base work well. I used a 100w (at 12v) test load to provide good resolution in the voltage drop measurement. It also approximates a low-to-medium draw RC powerplant, so I will know if a pack has a noticeable voltage drop at 100w, it is not fit for flying use!
Instead of using a calculator you may use the spreadsheet linked here to make all the computations for you. Download or save to your Google Drive.
My personal spreadsheet log is here if you wish to have a look:
For the test load it is only essential that there is some constant and safe means of resistance. 12v halogen bulbs singly or in parallel are a good option for testing 3S packs. These may also be used for 1S and 2S packs, though they will not deliver the rated wattage. The wattage is convenient to know but is absolutely not a crucial parameter. For a 4S pack a 12v bulb is probably OK, though a two bulbs in series would be safer. For a higher load connect four bulbs two series by two parallel. Use this arrangement for up to 6S. If you are using a higher cell count than that then you can probably afford a nice factory IR checker ;)