Interpreting Multimeter Voltage Reading
 

1991 MB 300-SEL

Working a battery drain issue. Found a maybe somewhat different way to testing for current drain in a car magazine article. Demonstrated on this youtube video as well:

Fixing Battery Drain In Your Car - YouTube

Using the infamous Craftsman multimeter that reads duty cycle. This meter has auto-ranging. Meter set to DCV. With the meter hooked up, I see the following reading:

037.8

If it read 3.78 I would think 3.78 volts. The 037.8 reading, especially the decimal place has me stumped.

I do realize that I can test for amp draw at each fuse with fuse out and a lead set to each end of the fuse holders with the meter on the appropriate amp scale. What I'm doing here is more of an academic exercise. The voltage reading has me stumped.

Thanks for your time.

If you are doing like the you-tube video and are using a 1 ohm 10 watt resistor then what you are seeing is 37.8 mili volts or .0378 volts. Using the formular I=E/R or amps = voltage/ohms. Then amps = .0378 or 37.8 mili-amps. That seems like a normal reading for a 420.
My 84 500SEL will go dead if not started for 3 or 4 weeks and it reads about .03 milamp if I remember correctly.

Paul

I'm on a slow connection so u tube won't work. Can you describe the test setup?

With auto ranging, 037.8 might be Milivolts = 0.0378 V . The best way to test for an amp draw is to use a clamp on amp meter. Clamp Meters within Electrical Meters / Testers - MCM Electronics Category I use an older Temna brand ( this is the house brand of MCM )

Make sure the meter can read DC , the less expensive ones don't read DC,figure on spending around $100 for something decent.

The big hitters for battery drains are trunk / glove box lights and shorted diodes in the alternator. Testing draw per circuit lets you find the specific circuit causing the draw.

Paul:

Thanks.

Yes - that would make sense. The battery is dropping of fairly quickly. How many days 'til it's dead? I can't say as I normally have a Quick Disconnect attached.

I see the Open Circuit Voltage dropping quickly if I leave it attached. This is not the case of a bad battery or alternator. I strongly suspect the after-market Clifford alarm, but the test I'm using would suggest that I do not have a serious drain?

Onward.

Pickup up a 1 ohm 10 watt resistor from a place like radio shack. Looks like this:


http://www.radioshack.com/product/index.jsp?productId=2062290


Connect some electrical line(I used 18 gauge wire) to each wire protruding from the ends of the resistor. Some people solder; I wrapped the wires together and then wrapped in electrical tape. Used about a 1 ft. length of wire on each side. youtube test leads are much longer than mine. I was concerned that excess length would increase resistance and skew readings. On the end of each wire, connect an alligator clip large enough to connect to batteries negative post. Walmart sells a set of these(red and black) for $2 and change.

Remove batteries negative cable. Connect one alligator clamp from your test lead to the negative cable and the other to the negative post. In my case, this sets off the alarm. I have the horns disconnected, but see the lights flashing, indicating this process has activated the alarm. The cars lights flashing stops after awhile. I wait 10-15 min for things to go to sleep, then connect my meter with small alligator clips on the meters probe sticks to the wires coming out of the resistor - one meter lead on one side of the resistor; the other meter lead on the other side of the resistor. Set meter to millivolts; then turn on and take reading.


Partial photo of test lead in this article:


http://www.motor.com/magazine/pdfs/042010_02.pdf

OK, you are making a shunt and measuring voltage drop across it. With such a low voltage drop across a 1 ohm resistor you will be seeing milivolts.

The setup will work, it is just a bit more cumbersome than an amp clamp as it requires breaking the wiring..

Using a resistor to determine the power drain is not a good idea.
If you measure 3,5 volt over the resistor it means you measure the power consumption at 9 volt not at 12 volt. Especially with electronics that is not good, it is possible the electronics won't work at 9 volt at all or al least not as they should.

An amp meter is much more accurate.

Rob

Rob and 97, I agree with both of you. A clamp on meter is much easier to use but not better in this case. In our shop, we had both a clamp on amp meter and an in series meter. Both had their place. We also had a little box that did exactly what the OP is doing. It also had its place. The two meters cost about $750 each. The little box (that I made) cost less than $5. For intermittent drains, the little box did just fine, and it only needed a DC digital meter which you can not get for less than $20. That would leave the expensive, easy to use meters for checking alternators, starters, etc. In fact, for small drains of 100 miliamps or less, the $25 setup was more accurate.
I have to disagree about the 9 volts damaging the electronics in an automotive application. If it were a problem, then any car/truck with a weak battery would be history the first time the key was turned to start. I agree that some things may not work correctly but that is not what we are working on in this case. The problem with using the 1 ohm resistor in this case is the resistor getting hot and burning your fingers. That is why I put mine in a box and added an inline fuse for the times it was being used when it shouldn't.

Paul

Most multimeters have a amp setting, even cheap ones.
The amp circuit is often not protected, so you will fry your meter if the amps are to high.

I never said that a low voltage damages electronics, only that i may give a different power consumption at a lower voltage. A car alarm may not work at all if the voltage is to low.
If you want to know the power drain at 12 volt you have to measure it at 12 volt, not at 9 volt.

Rob

How is it than I'm measuring with 9 volts vs. 12? Could someone throw up the math? Not doubting anything that's been said, just looking for clarification.

Thanks.

I stand corrected. Thanks. You are correct, you will get unreliable information with low voltage. But the question still remains, Is an expensive clamp on meter worth the cost when it may only be used 1 or 2 times over the life of a car when a $2 resistor and cheap digital meter will give the same information under the same conditions?
When I made that post I knew I would regret it.

Paul

Will pursue a traditional amp draw test which tests in series; then go back to the test I started out with - resistor in the middle of a lead and compare the two. Millivolts read using the resistor lead supposedly line up one for one with milliamps.

.040 mV equates to .040mA, or so it's been said by others with an electronics background on another forum.

Still curious about the math.....9 volts?

It is rather simple.
If you measure 3 volt over the resistor it means that the voltage drops 3 volt over the resistor, leaving 9 volt for the rest.
The same as putting two 6volt lamps in series on a 12 volt battery.

Rob

An expensive meter is only useful if you can use it professionally.

Even simple 20$ multimeters have an amp setting, so there is no need for resistors to check the power drain. Make sure you buy one with a 10amp setting and always start with the 10amp and go lower from that (otherwise you blow up the meter).

I have a couple of cheap meters that I keep in my car and motorbike, but I nearly always use my 30 year old Fluke because it is much easier to use.
For 200$ you can buy a very good meter that is practically indestructible and last a lifetime.

Rob

I appreciate the math, but...

Not measuring 3 volts. Measuring about 38mV. If 3 volts were measured, you have approx. a 3A draw.

12 - .038 = 11.962

I was a bit confused, in your first post you wrote 3,7 volt.
38mA can't drain your battery very quickly, a good battery should last several weeks.

If it is really only 38mA and your battery drains it is probably at its end.

Rob

I wrote 037.8......the 3.7 ref was an "if" situation. Review my opening post.

The battery is fresh as were a few that came before it. Left connected it will drain in a handful of days...maybe a week. I've been living with a Quick Disconnect for a good long time because the local MB dealer and independent shops want nothing to do with it. Oddly enough, all of the wide-eyed 19 yr. olds that work in radio/alarm installation shops tell me they can remove the Clifford alarm without issue. This alarm was installed before any of them were born. I doubt they would understand it's construction and more importantly how it interfaces with MB electrical/electronic systems in a 23 yr. old car which is likely why the MB dealer and the indy shops turn and run.

There's a draw somewhere. I'm very suspicious of the alarm or after-market radio, but suspicion is bad science. I'll get to the bottom of this sooner or later.

Thanks for everyone's participation.

I will throw in my .02 for what it is worth. I am an electronic technician and EE.

The value of the resistor shunt method is that it is something the average person can understand and do, usually without causing much damage, and it gives accurate results.

Yes, most meters can measure DC current through their probes, but it is my experience that almost no one can do this without blowing the fuse on the meter. The meter needs to have its probes connected correctly and then the meter probes need to be put IN SERIES with the circuit, which is different than how voltage is measured. Easy if you understand but most people not "skilled in the art" do not get this detail. So the result is they put the probe in the current measuring hole, then immediately measure a voltage and blow the fuse. Then they wonder why the meter doesn't work. Even an experienced person can forget where the probes are plugged in and do this. I work with a bunch of professional wiremen and half the meter fuses are blown all the time.

A DC clamp-on meter is a wonderful thing. As was said you have to be careful buying one since the vast majority of clamp-on's measure AC current only, including the ones on eBay advertised as "AC DC CURRENT METER". You have to look at the details. A good cheap one is the Mastech MS2108A which you can get for about $40. It will not read small currents as accurately as the resistor method but will measure them well enough. I used one this weekend to track down a short in some trailer wiring. I just sniffed the current along until it stopped. Like magic. Took 5 minutes and I didn't have to touch a wire.

I never blew a fuse of a multimeter.

I did blew up a complete multimeter once (a rather nice and brand new one) when I discovered that Olivetti used 220V AC fans in their power supply instead of 12V DC like the rest of the world. The display ended up upside down on the case. Replaced it with a Fluke, less features but nearly indestructible.

The clamp on meter I use is not very accurate with small currents.

Rob

You can't expect 19 year olds to understand eight tracks.
The electrics of a 21 year old car is simple enough to make removing a car alarm not that complicated.

If the battery is 55A and drains +/- 40A in week the power drain is (40/7)/24 = 0,23A/H.
If you measure 38mA (0,038A) and your battery is good it can only be an intermitted power drain. These are sometimes hard to find.

You can disable the radio and alarm to check if one of these causes the drain.
Other suspects are the light switches in the "always power on" circuits, like interior, doors, trunk and glove compartment.

If not, a multimeter with a MAX hold option can help to find the problem. If you know how much current is used during the drain it is easier to detect the component.

Rob

Ohm's Law is applied.
Let's back up a little.
The 1 OHM resistor will restrict the current that flows form the source back to the same source, your Battery. In the course it causes a Voltage drop across the resistor. That's what the DVM or whatever Volt meter indicates.
According to OHM's Law if a Voltage of 1V is applied across a 1 OHM resistor the resulting current is 1A
If there is only 38mV drop across one ohm resistor then 38mV/1Ohm=38mA

The 1 Ohm resistor makes it easy to calculate, however I don't like the Current Drain measurement using a 1OHM and measure the voltage drop across either.

Here is an example why not:
assuming there is a higher current flow than 38mA, lets say 3.8A
according to OHM's law the voltage drop would be 100 times greater at 3.8V.
We have only a 12 Battery so 12-3.8=8.2V is left for the rest of the car since we already dropped 3.8V across the resistor. Aside the fact that this is already over 14W of power, most of the gear will not work properly if at all.

As stated before, the 1OHM will probably work if the current flow is very little.
A 55AH battery can supply a current of 1Amp for about 55 Hours.
if the Current is double (2A) of course the battery is drained faster at about 27 hours.
and so on.

If measuring the idle-load (your car does nothing, only keeps a minimal equipment alive) battery current first of all one should give it an educated guess from last time the battery was drained and the time it took. If this would be around 1AMP or so, you should use a meter that is fused at 2Amp, many do this only the 10AMP is usually not fused. Set it up for current (DC) measurement, disconnect a battery terminal, which one doesn't matter, in the worst case your Meter shows a negative current. If the Positive is disconnect attach the positive lead to the positive battery post and the negative lead to the battery clamp. Now you can read the current directly off the meter.
The advantage, it is a current measurement where the voltage drop is very minimal and fairly stable with the consumer operational.
I believe Mercedes states that idle currents of ~200-350mA are within specifications. This would be about 14 days on a 66AH battery.
Don't quote me on the current, it may be much less.
So, your reading might be right it also could be wrong, current measurement as above.
If it is within this range and your battery is drained too quickly, then either your battery is too small in terms of capacity or it simply met the end of it's life.

Boy that was along one.....

Word of caution, **DO NOT START THE ENGINE** while the current meter is attached. A current surge would probably burn the fuse, to say the least.
The starter can draw 300Amps or more, plus your test leads will not cope with this type of current. e.g. your car would only make one click....

8-Tracks were done in the U. S. by the late 70s; early 80s. My 23 yr. old 300-SEL came equipped with a CD player. Purchased used a handful of yrs. after it was released.

Tenma is a decent house line of electronic equipment

Clamp Meters within Electrical Meters / Testers - MCM Electronics Category

I've had an alternator on a Ford that would occasionally drain the battery when the engine was off so don't rule out a failing diode.

There are actually youtube ect tutorials on meter use. Nothing wrong in not knowing but if you can read, you can do.

Yeah, but they usually have a range from 40A upwards!
Maybe good to test the current flow while starting the engine.

Tenma Compact True RMS Clamp Meter | 72-7226 (727226) | Tenma

40A is the limit for that specific DC Amp range, it will work fine for finding low draws. We are less interested in a specific reading and more interested in if the draw goes away when we lift a fuse.