Hi

You asked, Why not fit a battery switch next to the battery? Just interrupt the earth.
Well, if you used the big ground (earth) cable, you would have to have a big O switch to carry the 300 amperes for starter current!
Now you could use one on the smaller body ground as it the one that most likely has the offending circuit for a small drain.

The best place is to check from the battery’s negative positive for everything going out of the battery and then divide the two grounds and use the method listed in “aleekat’s” posted link!

“How To Measure Key-Off Battery Current Drain”
The third paragraph in the post is my favorite to use.
It a way of looking at the complete car and then you can divide from there to which half.
Each half will read something and tell you how big the draw is to look for!

The third method is, again, the best , since voltage drops and things like that can get complicated, with or when, normal resistances are not know!

Those other methods like Using the same side (polarity), voltage check will spot excessive resistances in a certain “on” circuit.
That test shows resistances, with a minimal reading above all zeros and that means an exponential multiplier is in there being a gremlin! Especially in a charging circuit.
It’s good for finding a loss of power, that is not getting to someplace, like a dim headlight or slow motor.

This has been a nice thread tonight, with some great post from everyone!

Phil