I think the mistake may have actually been writing ammeter instead of voltmeter. Ammeter is not safe as general advice as a typical one won’t be anywhere near large enough capacity for working with a car battery and should come with a lot of disclaimers about how to hook it up. Measuring voltage drop would work but only for fairly high loads.

> Measuring voltage drop would work but only for fairly high loads.

Yah, that doesn't really make sense for small parasitic loads.

> Ammeter is not safe as general advice as a typical one won’t be anywhere near large enough capacity for working with a car battery

It should be fine with the car not "on", but the general level of danger of doing anything near a car battery requires caution.

> It should be fine with the car not "on"

No. Not if the meter is in "Ammeter" mode (or is purely an ammeter only). An ammeter presents a very low resistance to the circuit you are attaching it to, low enough that for almost all circuits, it is effectively a "short".

If one connects an ammeter across a low resistance voltage source (i.e., across the car battery) the ammeter will appear as a short circuit to the battery, and one or more bad things will happen. The 'least bad' will be blowing a rather expensive fuse in the ammeter that protects it from this kind of accidental use. Several of the "most bad" will involve hot molten metal and/or extreme heat.

> If one connects an ammeter across a low resistance voltage source (i.e., across the car battery) the ammeter will appear as a short circuit to the battery

Yes. The thread you're replying on starts with me saying not to do that, and suggesting an alternative is "Ammeter between a battery's terminal and the car's electrical system." instead.

I was replying to "anywhere near large enough capacity" in the next post. The typical 10A range is massive overkill for typical parasitic loads in a car, and even things like the fuel pump deciding to kick on and run in most cars.

Using an ammeter as you describe would work fine, but the topics this thread including mine should come with a footnote[1] at least. Especially when this thread shows a simple mistake can cause a lot of confusion about what to do. My thought process with my comment about capacity was that muscle memory makes it likely you'll eventually start the car if you're touching the ignition switch/button during troubleshooting, although on further thought and reading the other comments I'd do the following: pull the starter fuse, then use a 10A ammeter function of a common multimeter, connected inline between the negative harness and the negative terminal and not on the positive side.[1]

[1]This is just what I would do. Only do this based on a reputable guide if you don't have experience working around car batteries.

> Yah, that doesn't really make sense for small parasitic loads.

If your multimeter has a sub-mV range -- which a decent meter should -- you can measure a voltage drop across the battery ground lead. I've successfully done that to diagnose a battery draw in my car.

Exactly, you are measuring a change, so its the precision of the meter not the absolute V. Maybe easier to measure on a nearly dead battery?

I'm not talking about measuring the terminal voltage of the battery. What I mean is using the ground lead as a current shunt by connecting one meter lead to the negative battery terminal and the other to where the ground lead connects to the vehicle chassis. The voltage measured across that shunt is proportional to the amount of current being drawn from the battery.

It's the same principle as measuring the voltage across a fuse, except it ends up measuring the draw for the whole car, not just one circuit.

heh true, edited post to maybe save someone some sparks.