You make a good point regarding actual use versus a lab test, I could probably make my own watts-up type device but buying the device would be much easier, something to put on my "to get one day" list because it'd mean fiddling with the wiring on the ebike to accomodate the device.
I'm curious because I now have 3 identical ebike batteries, all over 2 years old, and want to find out the capacity of the not so older two.
I got a 3rd ebike last week for a steal, a 2 year old Urban Mover UM36X in black, looks and runs really well, better than my ageing 4.5 year old UM36 that has seen much better days.
I think I'm ok for battery capacity usage (even carrying a 2nd) until next spring but after that I'm going to seriously look into re-celling the oldest battery, especially as the price of a set of cells is 1/7 to 1/4 the price of a whole new battery pack.