Measuring electrical loss?

[MushCreek]

This is automotive, 12 VDC power. I want to check the drain on my car's battery. I think it's in the dome light circuit, but not sure. Once I removed the fuse, no more battery drain. What I want to do is check the power drain as I make various repairs so that I can isolate the problem without killing my battery. What equipment do I need to measure current drain? It's not a big, lights-on type of drain, but it will eventually drain the battery.

 

[bluecomputer]

What size is the fuse? If its less then 10 A just connect a multimeter across the fuse in while the meter current mode.

 

[MikeF2316]

Generally you disconnect one battery cable and put a current meter between the cable and battery post. Then pull fuses one by one while watching the meter. There are a couple of unfused things that can cause trouble, the main one being the alternator.

That works great on older cars. Newer cars with networks can take a while to get to the rest current after a battery disconnect / reconnect - I've heard of more than 30 minutes! So the trick is to install a battery disconnect switch, put your ammeter across the switch. When the switch is closed (on) all the current flows through it. When open (off) all the current will flow through the ammeter.

They say under 30 mA is ok, but I like to see less than 10 mA.

 

[rlitman]

What size is the fuse? If its less then 10 A just connect a multimeter across the fuse in while the meter current mode.

Fuses act as a shunt. You can get approximate shunt resistance values for fuses online. From there, leave the fuse in, and measure the millivolts across the pins at the top of the fuse. That will be proportional to current, with the ratio being the resistance (Ohm's law and all that goes with it).

 

[firebirdparts]

You need an ammeter. Big ammeters are not something you'd see in a hardware store, but what you need is one that can go up to 5 or 10 amps only. Those are common. The multi-meter that they give away free at Harbor Freight, for instance, will do 10 amps on a car.

Rlitman's advice, above, won't work because you won't be able to find a volt meter sensitive enough to do it that way. So don't get sidetracked.

 

[wyliesdiesels]

The easiest way to measure current is with a clamp meter.

Not all clamp meters measure DC current so need to make sure the one you get measures DC current.

 

[Bert_]

The easiest way to measure current is with a clamp meter.

Not all clamp meters measure DC current so need to make sure the one you get measures DC current.

If we're talking a fraction of an amp then a clamp type meter might not be the best choice. I don't think they are very accurate at low current.

 

[rlitman]

...Rlitman's advice, above, won't work because you won't be able to find a volt meter sensitive enough to do it that way. So don't get sidetracked.

I guess you've never tried it. Just about every DMM I've owned does millivolts just fine. But if you don't want to believe me, try reading this:
https://www.searchautoparts.com/motorage/electrical/parasitic-battery-drain?page=0,2

If we're talking a fraction of an amp then a clamp type meter might not be the best choice. I don't think they are very accurate at low current.

You can get a milliamp clamp meter with a small but elongated clamp for just this purpose. Many of these handle DC. This might make sense if you do this a lot, but it's a big expense for a single job.

 

[BillK]

How about year, make and model ??

Does it have lighted mirrors in the visors ? Had the switch in one of those go bad in my Tahoe and took me a little while to figure out what it was.