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/voltage-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.

[Steve Baumgartner]

Here's an anecdote that fits the topic:

A number of years ago we were shopping for a used car and found a 3-year old one that we liked at the dealer. But the battery was dead, so he had to swap it before we could test drive. The car was indeed what we wanted, so we bought it and I drove it to work without issue for about 5 years. Then my wife's car died of rust, so I got another and she took over my old one. Within a month, she went out one morning and it wouldn't crank. I put the charger on it, and it was fine for about another month then died again. Charged it again, and same story. We thoroughly tested the charging system on the car as well as the battery and found nothing wrong.

So, like the OP, I set out to see whether there was a parasitic drain killing the battery. This made sense, as my wife drives only about a mile to work. The alternator doesn't have enough time to recoup the energy spent cranking to start and lost to the parasite, so the battery would slowly sawtooth downhill until it was too weak to start the car. As recommended here, I put an ammeter between the battery and the negative lead and measured the current. Indeed, there was a greater drain (over 100ma) than should have been. But I pulled fuse after fuse and could not find the culprit. None of them eliminated the draw.

Finally, one time while connecting the meter I heard a faint "tick!" from under the dash. I got my wife to make and break the connection while I peered under there and found that the noise was coming from one of the relays. It was pulling in immediately any time there was battery power. I looked up the part number on the relay and found it was for the electric fuel pump. But when I looked up the locations of relays, I found that socket was for an overdrive (which my car doesn't have)! I consulted the car gurus, who were stumped but said "pull the relay out and see what fails". So I did, and the only thing that changed was that the parasitic drain went away!

It has now been more than 12 years and the car hasn't drained the battery again. It seems that someone (who knows, maybe even at the factory) had put a spurious relay into an unused socket, and the connections were mismatched such that what should have been the supply for the overdrive was instead activating the relay coil. That's probably what killed the battery on the dealer's lot and probably why the original owner traded in the car after 3 years of repeatedly dead battery!

[Mike Lassiter]

Most decent quality VOM's have a amp reading for DC that is ran directly thru them. Disconnect battery cable from post and use alligator clip ends to connect one lead to post and the other to the cable end. I think most limit current to 10 amps with a fuse inside meter. At least the Snap On meter I have does. You can record the min/max readings it gets while hooked up, but also note many will auto power off after a time so I doubt you could test like this and leave it all night or day and hope to get something. Mine also has a clamp around wire attachment that will do low current (about 40 amps I think) and higher current (400 on mine I think). I ready doubt this would be of much help to you because you would be looking at a current draw most likely that would be hard for this to accurately read.

[Steve Rozmiarek]

Most decent quality VOM's have a amp reading for DC that is ran directly thru them. Disconnect battery cable from post and use alligator clip ends to connect one lead to post and the other to the cable end. I think most limit current to 10 amps with a fuse inside meter. At least the Snap On meter I have does. You can record the min/max readings it gets while hooked up, but also note many will auto power off after a time so I doubt you could test like this and leave it all night or day and hope to get something. Mine also has a clamp around wire attachment that will do low current (about 40 amps I think) and higher current (400 on mine I think). I ready doubt this would be of much help to you because you would be looking at a current draw most likely that would be hard for this to accurately read.

I hadn't thought about how good my meter was for a while, it'll do this job with ease. If anyone is in the market, I recommend checking one out.

A funny story, I once used a couple Flukes as data loggers to prove that the local power company had a defective meter, after they had tested it three times and said it was ok. They ended up replacing the meter, and refunding for the entire time that meter was installed.

[Art Mann]

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.]

There are all kinds of Hall Effect sensors that will measure DC current. Just Google "Hall Effect Sensor" and you will see what I mean. If you want to pay enough money, you can measure even fairly small DC currents. I know this because many years ago, I designed and manufactured Hall Effect probes that were used to test small loads on new automobiles coming off the assembly line. Our probes were industrial hardened and cost several thousand dollars. In this context, cost is no object. The probe was clamped over the positive battery lead and measurements were taken as the test operator cycled various devices and lamps inside the vehicle. Prior to that, we used series shunts that required the battery be disconnected and the shunt inserted in the current path. The shunts were harder to use and less durable.

[Art Mann]

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

The 376 appears to be a nice meter but the owner's manual listed the resolution as 100 milliamps. That is not the same as accuracy. What that means is that the original poster's vehicle could be leaking enough current to discharge the battery in a few days an the meter would not even pick it up. On the other hand, it might read 200 mA when there is no current at all.

[Steve Rozmiarek]

The 376 appears to be a nice meter but the owner's manual listed the resolution as 100 milliamps. That is not the same as accuracy. What that means is that the original poster's vehicle could be leaking enough current to discharge the battery in a few days an the meter would not even pick it up. On the other hand, it might read 200 mA when there is no current at all.

Art, you are certainly a FAR more knowledgeable authority than I, you are right of course. In my experience using it though, the range they list is far better in practice. Especially in a trouble shooting situation like this, you are really just looking for a reading of any sort to give a clue. Personally I'd start at the battery and work my way up the wiring harness to find the issue. If I had to guess though, I'd suspect the battery is bad rather than something still drawing.

[Jim Koepke]

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.

That is a tested and true trouble shooting method of many a mechanic.

It can even be done without a meter setting back the tool budget to the tune of a few hundred dollars.

jtk