I got to thinking about a way to find which circuit is drawing current after the 30-40 mins by which time the car is meant to have gone fully to sleep (apart from about 50mA for any alarm and the like).

The "traditional" way uses a meter on amps/mA in series but the problem is where to put it and that you can't move it to a different circuit without restarting the 30-40 mins.

I figured a fuse is a small value resistor. So, put the meter on mV (millivolts) across the 2 terminals of the fuse.

I figured this was "too easy" and would by now have been mentioned on here if it worked but decided to try it anyway - and it works!

It's very easy and fast to do so it also means it's reasonable to ask another owner to see what theirs reads when you find a suspect mV reading (i.e. a suspect circuit).

And then... knowing what terminology I might use to describe this (fuse voltage drop, battery drain) I did a web search and of course someone's discovered this technique already, such as:
A new approach to testing parasitic draw | Search Autoparts

Oh well. It's still useful.

 

Is the technique not really a variant of the classic "voltage drop" measurement used to look for resistance in connections?

It works because the drop varies with current, so a circuit drawing higher current will show a higher voltage drop across those terminals. The terminals on the fuse act as a "standardised" circuit segment for each fused circuit since the terminal spacing is the same on all of the blade fuses.

 

It is, but I'd never heard of it in the context of looking for battery drain in a car. I don't recall it ever mentioned in all the threads here where people have hunted desperately for the bad circuit(s).

I "invented" it by wondering (not in connection with a car or a fault, just one of those wondering things) how low a resistance a fuse was (it had to be non-zero or it wouldn't ever blow) and then made the little mental leap to thinking it could be used this way.