What instrument tests for slow battery drain on cars?

[Stephen Tashiro]

What's the name of an instrument that can measure whether there is a slow drain on the battery as a car sits overnight?

(I know there are methods to check for short circuits with a voltmeter and by pulling fuses. But what is used to test for slow "leaks"? )

[Chuck Wintle]

i used an ammeter once in series with the battery, it worked to show a steady drain of current.

[Steve Rozmiarek]

I'd use my clamp type amp meter. Clamp it around the positive, or negative lead off the battery, and you'll know instantly. If your battery is going dead, it could also be the battery. They are pretty easy to check out too, there is a tool to test their capacity, or you could disconnect the battery and check voltage after it sat overnight. A loss in voltage means the battery is junk usually.

[Jim O'Dell]

On the newer cars with all the computers, be sure to give it 30 minutes or so for the modules to "go to sleep" to check numbers. (amp meter hooked up during this wait). Also make sure no lights are on and no phone chargers plugged in. Jim.

[Mike Lassiter]

there will be current draw for radio station memory and clock and such but it should be very small. This will not drain a battery "overnight" or even for weeks with a good battery. Diodes fail in alternators and create a short circuit that can. Doesn't occur often but it does happen. Personally I would suggest having the battery tested. A dead cell can short the battery internally and discharge it over a few hours enough that it likely couldn't start the engine. Also consider that a "bad" battery taxes the alternator more and can cause it to fail. Most automobiles have much smaller batteries now and much higher output alternators than back in "older days". With that being said the battery fails more abruptly now because it is smaller and doesn't have as much reserve capacity so it gets discharged quicker.
You can have the battery tested at most auto parts stores free of charge. I know locally Autozone does this in the parking lot with the new testers they can detect far more than just cranking amps in just a few seconds testing time.

[Steve Baumgartner]

On the newer cars with all the computers, be sure to give it 30 minutes or so for the modules to "go to sleep" to check numbers. (amp meter hooked up during this wait). Also make sure no lights are on and no phone chargers plugged in. Jim.

Actually, even on older cars there are things such as clocks and radio memories that draw some current when the car is off and never "go to sleep", so don't expect to see zero. But it takes a significant part of an amp to drain a battery in less than week unless the battery is failing. The typical drain varies from car to car, but if you see 50ma or less, it is most likely within normal limits. Also, if you try Steve R's suggestion, make sure your clamp-type meter can measure DC - most of them only sense AC via the clamp and need cables to measure DC.

[Lee Schierer]

I'd use my clamp type amp meter.

I don't think clamp on amp meters will work on a direct current circuit.

[Jim Koepke]

What's the name of an instrument that can measure whether there is a slow drain on the battery as a car sits overnight?

(I know there are methods to check for short circuits with a voltmeter and by pulling fuses. But what is used to test for slow "leaks"? )

My tendency is toward hesitancy in answering an inquiry of this nature.

The common name for such an instrument is an amp meter. What you really want to know is the intensity of the current flow when the vehicle is sitting with the engine off.

There are other factors at play in this situation.

Maybe a description of the problem that being experienced that brought on the original question.

How old is the battery?

Have you checked the system voltage with the engine running? If it is at battery voltage or less, it may be your alternator isn't working. (This is why knowing a bit more is helpful. My guess is your vehicle is not an old one with a generator.)

Someone knowing the year, make and model of the car might be able to provide other insights.

Many years ago a neighbor was always having trouble with his car battery. Looking out the window one night I saw his problem. Light was coming from around his trunk lid. The switch for the trunk light wasn't being actuated when the lid was closed.

Battery charging problems can be caused by many things from a loose cable to dirt on top of the battery.

Back in the old days we would remove a battery cable and then in a dark place touch the cable to the battery post. If there was a spark, something was drawing current that shouldn't be. This was before the day of clocks in radios that needed electricity to hold the station programming. Sometimes we would hold different light bulbs on the terminal and touch the cable to the light. The amount of illumination was used to guesstimate how much current was being used.

If you have a good volt meter and a couple of coins it is interesting to place the coins on top of the battery near but not touching the terminals. Then place one of the meter's leads on each coin. If you see any voltage, it means the dirt on top of the battery is draining current from the battery. This might not work as well with side mount terminals.

My experience with old VWs taught me a lot about current loss and voltage drop from loose connections and thin wires. There was even an after market relay available for the old 6 volt models to help with the starter circuit.

Some of those vehicles had some strange problems. One weird one was pulling in the driveway and shutting off the engine only to have it keep running until one took their foot off the brake or turned off the headlights. Cleaning the crud off the inside of taillight sockets was one job I didn't like.

jtk

[Kevin Godshall]

When I had a vampire in my vehicle sucking the life out of my battery (parasitic battery drain), I hooked up a voltmeter to the battery, and then begin systemically pulling fuses, and seeing if the volts changed. Turned out, my rear wiper motor was the culprit. Left the fuse out, until I got a bad wire fixed.

Not the easiest method, but it worked.

[Steve Peterson]

I don't think clamp on amp meters will work on a direct current circuit.

According to my physics classes, any current flowing through a wire will generate a magnetic field that is detected by the clamp on amp meter.

Most clamp on amp meters appear to only be set up for measuring AC current. Maybe they are designed for construction workers that only have a use for measuring AC currents. There are a few that will also measure DC current, but you have to look carefully at the specs.

Steve

[Stephen Tashiro]

Maybe a description of the problem that being experienced that brought on the original question.

I'm just curious how to detect current drain that might be intermittent. "The battery goes dead overnight" is a complaint I've often heard.

After more web searching, I see devices like these can be used: http://www.microdaq.com/data-logger/...-current/1.php

[Art Mann]

According to my physics classes, any current flowing through a wire will generate a magnetic field that is detected by the clamp on amp meter.

Most clamp on amp meters appear to only be set up for measuring AC current. Maybe they are designed for construction workers that only have a use for measuring AC currents. There are a few that will also measure DC current, but you have to look carefully at the specs.

Steve

It is possible to buy a volt-ohm-ammeter at some place like Harbor Freight with good enough accuracy and resolution for less than $10. A DC current probe that will read accurately in the sub 1 amp range will probably cost at least $450 and they require a millivolt meter to sense the output. Low current DC current probes are more complex than you are thinking.

[Steve Baumgartner]

It is possible to buy a volt-ohm-ammeter at some place like Harbor Freight with good enough accuracy and resolution for less than $10. A DC current probe that will read accurately in the sub 1 amp range will probably cost at least $450 and they require a millivolt meter to sense the output. Low current DC current probes are more complex than you are thinking.

Yes - the generally available inexpensive clamp-on ammeters work via magnetic induction, which is only effective for AC. More exotic sensors based on Hall effect can measure DC via the magnetic field, but I've never seen a cheap one!

[Mike Henderson]

According to my physics classes, any current flowing through a wire will generate a magnetic field that is detected by the clamp on amp meter.

Most clamp on amp meters appear to only be set up for measuring AC current. Maybe they are designed for construction workers that only have a use for measuring AC currents. There are a few that will also measure DC current, but you have to look carefully at the specs.

Steve

Any current carrying wire will have a magnetic field around it. However, for that magnetic field to induce a current in another conductor, the field must be varying. A wire carrying AC will have a varying magnetic field, and that varying magnetic field will induce a current in an amp meter probe that is clamped around the wire. A wire carrying DC (constant direct current) will not have a varying magnetic field, and because of that, it will not induce a current in the amp meter probe.

The amp meter probe is a type of transformer, and DC will not pass through a transformer.

I suppose there are alternate ways of detecting a magnetic field so there may be non-invasive techniques for measuring amps in DC circuits.

Mike

[Oops, I see others have already said the same thing, with more detail.]

[Steve Rozmiarek]

I don't think clamp on amp meters will work on a direct current circuit.

My Fluke 376 does, so does the my other fluke, but it's far more expensive then the 376.