I think I have something drawing on the battery when the car is idle. I have a good voltmeter and an academic understanding of the principles, but I don't know how one measures amperage without "being in the circuit." More specifically, I don't know how to measure, or at what point to measure, whatever current is flowing out of the battery even when everything is off. Guidance?

Unhook the + from the battery.
Put the meter in "Amps" mode (this usually requires you to move the Red lead to a different plug on the meter).
Put the meter between the battery + & the + cable that you unhooked.

It will measure current if any is "leaking".
Do NOT try to turn much on in this configuration.
Most meters will blow a fuse at 10 Amps.
10 Amps is not really much - the starter can pull hundreds of amps ;-)

I used to have one of those battery disconnect things on the battery.
I would turn it to OFF if I planned on letting the car sit for more than a day or two.

Stu

Thanks for the response. I, too, just put in a battery disconnect to solve this until I can figure out where the leak is. I have a music amp in the trunk. Power light is always on. Trigger wire going to an unswitched circuit?

What may be happening is that the voltmeter is falling as you reach idle. You did not mention volts. If it is falling from 13 to 12, then it is normal. The alternator is pushing a small amount of current through a charged battery at 13 volts when running and then gets tired of doing that at low revs - revealing your true battery charge. It should be the same number if you turn off the key and leave the ignition on.

On the other hand, if it is dropping down to 10 it may be that there is truly a load drawing the battery down, or it may be that the battery is not able to put out that much despite the charging efforts of the alternator. If it is the battery, then the voltage across the terminals will not change if you shut off the engine and disconnect the wires.

If it is a load, then the voltage between the battery terminals will be higher when the engine is shut down and the terminals connected, than it is when you hook up the wires.

My alternator is new with new regulator and still won't really kick in till well over 1500- more like 2500. Then it runs fine.

My Fluke Process Meter can see DC current with an inductive clamp. Most other inductive clamps only do AC.

BTW, current draw is fairly normal. The maximum alawable is about .04 Amps. Anything more will kill the battery in a few days.

As described above you can check the current draw with a meter in series. Pull 1 fuse at a time until you find the circuit. Then figure out what on that circuit is the problem.

Radio, clock, ECU are things that normally draw when the car is off.