Re: "Everybody knows that you should measure battery voltage under load, but exactly what load"

The best load conditions to measure are those that apply to your loads. Different for a speedlight vs a remote control vs a battery powered LED strip vs a remote temp/humidity sensor.

Also I am doubtful about the usefulness of constant current load testing these days. The most common loads for NiMH batteries these days are "constant power" loads: boost regulators. As their input voltage falls, they draw more current to supply the same voltage and current to their own load.

I put "constant power" in quotes because the efficiency of boost regulators usually goes down with decreasing input voltage, so they are usually "increasing power" loads. Constant current testing would overpredict.

I have a project on my list to make a NiMH capacity tester using as the load a boost regulator loaded with a range of resistors. But at my current rate of progress through the list, I'll be dead first. :-) :-/