I was searching the threads for some tips on constantly dead batteries and I saw this thread. If i put a DMM between the positive battery cable wire and the positive battery terminal, what should the reading in volts be? I am confused. Part me says it should be a little north of 12 volts since the DMM is part of the closed cirtcuit. The other part says it should read zero if there are no accessories on or nothing is pulling power. Which is correct and why?

Hank,
A voltmeter needs to be in parallel with the circuit being tested to give a meaningful reading. Hooking your meter up in series with the battery lead on a voltage scale would do nothing.

Most DVM's have a current scale though. Many have both a low amps scale, and a 10 amp scale (and a different place to put the red lead into the meter for the 10 amp scale.)
Ammeters need to be inserted in series. When connected as you described, an ammeter will directly indicate how much current your car is drawing. With nothing on, 100 milliamps or less is good (.01 amps)
If you open the door, the courtesy light will draw about 3 amps.
Be careful not to exceed your amp scale, it will damage the meter!

And don't forget to put the meter leads back in the volt/ohm socket after you're done. Otherwise, the first attempt you make to measure voltage afterward will blow the fuse in your meter or damage the circuit.

If you're looking for the source of an overnight draw on an old car, we used to (in 60's and 70's) disconnect a battery cable (doesn't matter which one), then connect a 12v test light to the cable end and to the battery terminal. If something is drawing juice, the light will be on. You can disconnect one fuse at a time until you locate the circuit that's causing the draw (the light will go out).

There are some accessories which will intermittently draw power, such as a clock. So, if the test light is on, then touch the battery end to the terminal for a few seconds and then try the test again.

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