Hello All,

I recently read (and need to re-read) a very informative post on a motorcycle forum about voltage drop testing. Thought I'd pass it along to those here.

I've never learned about this and I'm sure this knowledge could come be handy.

http://www.motorcycleforum.com/showthread.php?t=151506

Excellent post. There was one on a jeep webpage of the same title, at vernco.com with a very clear analogy bringing the truth in this a little closer to us all. I'd post a link but it seems to be infected currently.
--
Art Benstein near Baltimore

You one of those guys that does it till it hertz?

I understand the theory of voltage drop testing, but what confuses me is the bit about including the device in the voltage test. In other words, if testing the positive leg of the circuit, why not just check the voltage from the battery + terminal to the positive feed going to the device. When testing the negative leg, check from the ground of the device to the - battery terminal.

Wouldn't including the device in the circuit test cause a drop in voltage because of the load the device is putting on the circuit? Or am I confusing volts and amps?

re: "...why not just check the voltage from the battery + terminal to the positive feed going to the device..."

Because voltage measured from the source (battery) to the connection to "a device" [as you put it] won't reveal the difficulty of higher current flowing through that leg of the circuit. You could get a measure of full voltage (i.e., battery voltage) measured through an almost infinitely long wire the thickness of a hair (only a slight exaggeration), simply because the voltage measurement only takes a slight amount of current. But as current increases (as in powering a "device"), slight resistances (at connections, broken strands of wire, etc.) become greater impediments to the flow.
In other words, serious points of resistance won't show up as a reduction of voltage when measured as you suggest; but they will nevertheless reduce the flow of current as if the voltage were lower.

On the other hand, when you're trying to force larger currents through such points of resistance, they will reduce the effective voltage. You can measure these between such points as actual voltages, or voltage drops.

You can think of this as a sort of "law of conservation of voltage"*: the sum of voltage drops along a circuit is equal to the difference between the original voltage source (e.g., the battery) and the final voltage seen at the "device", or
V(at source) - V(at device) = Ʃ(from 1 to N) V(voltage drops)
[ * sort of like a law of conservation of energy]

Hi Ken!

I have been reading your posts over.
I think I understand your first post completly and agree with it totally!

In reading over this post most of it makes sense to my simple mind as well!

Maybe you can help me figure out the "conservation law" you have going on here.
I cannot say I have read anything on this energy law. I think I like its inferences of something held back or just lost.
It is cute how you put out there at the end.

From what I understand about DC circuits you can calculate the voltage drop by knowing the resistance of the load or device. The inverse of knowing amperage draw will give you an expected voltage drop.
In the case of motors or items that heat up changes happens as work is done, wattage surge draw, is also a voltage drop or rise as the dynamic changes.


I hope I make sense here!
I am trying to write to more than just us.

Now to my other understanding or the lack of it, as the case may be.
I use another technique that has helped me. I have no name for it. Yours inspires me to steal it, for me only! (:-)

This technique uses the fact that when a voltmeter is put in parallel of one side, or either side, of a circuit the reading should be a zero.
I picked this up from reading an article in my subscription to Auto Restorer Magazine.

In order for a voltmeter to read voltage of a circuit or part of it, it has to be across the sides of the circuit. To a show the complete source voltage or to show a voltage drop.

No matter the type reading, a voltage drop is one side of the load or of several loads paralleled are referenced back to the opposite side of the source.

When I check the sides and the cables or wiring that is good I expect zeros! Anything other than a zero is conferred an amount of resistance of voltage or voltage difference can be bad wiring or corrosion.

A very minor reading above zeros equate to a significant amount of resistance that may not or cannot be read with an ohmmeter for the reason sighted as wire strands!

I use 0.020 as my maximun highest reading for a good side of a circuit. This covers up the instruments on impedance or error of theory that I understand "sneaks" in.

A higher reading shows up as the missing link, that is overlooked by most, until it affects operation.

My understanding of Kirchhoff's laws of circuits everything must add up to source input voltage.
I believe he is right on despite what lab classes I took in, for control circuit electronics, proved that the math was closer than the instruments we have back when!

Now with all this said, it seems, I should call this difference in a zero voltage reading is a resistance. Maybe your term of "energy conservation" is that link "held up" in the wire itself or impedance of flow shown, as a voltage reading?

Kirchhoff's law points to Something in science and the above zero readings means Something. Thank heavens for digital meters with very high impedances.

I think a voltage drop is caused whenever energy is used.
A corroded and crusty wire is rather hard to detect for heat coverted from electricity.
Yet it is hiding in there under any normal voltage drop study.
Thus, it's considered part of the load with the wiring section as a unknown! The value fills in Kirchhoff's theory for me.

My readings as small as they may seem, can be used as an indicator of your "conservation"law!
In this case, a "conversation" opportunity. (:-)

Like I said, I do not profess to know all about how electricity can be used. But a digital voltmeter, is a wonderful tool for me.
In the old days, I used to use an analog meter. Now it's kept for seeing needle swing a large motor capacitor, charge and discharge, of which I might still replace, out of questionable reliability.
A digital bar depends on speed sampling. Where for me, a needle has that smooth even movement without changing ranges.

It's old habits that die hardest!

Jabbering might be a another one. (:-)

Phil

Hi Lord Volvo,

You've got a point there. Yes why not disconnect the device and just test the cable going to the device?

Unfortunately in such test there is no load on the cable, only the load of the voltmeter (which has very small load). The effect of voltage drop in a big cable with almost no electrical load is negligible. As such you might have very little voltage drop at the end of cable.

As to my earlier posting, a faulty item (those that fails with an open circuit) would produce no load on the cable. And voltage testing would reveal negligible drop in voltage. This would give the false impression that the cable is OK (whereas the cable is also faulty along with the item).

Conversely, a faulty item with a partial electrical short within, would add more load on the cable and further drops the voltage. This might give the false impression of really faulty cable. And not all electrical shorts produce a sum of zero ohms. Some are in-between e.g in coil windings.

Amarin.

Here's a fun real world Volvo example of this. Five or so years back I picked up a rough 88 240. I had it for a week, and hadn't gotten around to any sort of zero-mile restoration work on it when it no-started (no crank) in my driveway. My first suspect was the positive battery cable because the end of it was just beyond super nasty. I wanted to see the voltage drop between the battery terminal and the lug on the starter. I had my wife come out to crank the car while I had the multimeter leads on--one on the battery, one on the starter terminal. Well wouldn't you know it, she hit the key and the car started right up! The extra voltage flowing through the multimeter was enough to render a non-functional battery cable functional. Surprised the heck out of me, what with my head and both hands wedged under the hood.

--John
'68 220 (vintage boat tower)
'92 245 (now x2, one red, one black)
'92 945t (>M46)
'98 XC (highway mile eater)

Hope you don't take a personal offense, but your explanation is really hard to accept. I'd rather chalk it up to coincidence, or to changes that occurred from the sequence of events of your efforts in this case.

The reason for my reluctance to accept that "...The extra voltage flowing through the multimeter was enough to render a non-functional battery cable functional...." is because we would have to accept that the meter's probe's cables could conduct enough current to make a difference for the starter motor -- those meter cable's insulation would have dripped off in a molten mess, with the wire's becoming red hot, if they were carrying the several hundred amps needed for your starter. Electricity flows through the easiest path -- if the meter's path had less resistance than the main battery cable, it would have been cooked.

I think it's more likely that you inadvertently did something, such as prying or moving the battery clamp, or even merely jamming the probe into the cable's clamp, which was just enough to jostle the layers of corrosion just enough to create a less resistant path that allowed your starter to work. Or,..., just having tried to crank it once, that might have been enough to make an electrical path good, at least temporarily. Bottom line: I think that the car would have started regardless of whether you hooked up the meter, except perhaps for what you may have done when you applied your probe.

That makes sense. Thanks for the info!

Hi Art,

The article doesn't cover it all. Especially to owners of older vehicles like us.

Here are the excerpts:
"So what is the most COMMON use of voltage drop testing? No-crank situations. Either the starter itself, or the starter solenoid is typically NOT getting enough voltage because of a voltage drop SOMEWHERE in the starting circuit. Common causes? Loose battery connections, bad grounds, worn out starter switches, tired batteries."

The author MISSED faulty items/parts. This is also COMMON cause of no-crank. Ohm's Law unfortunately doesn't apply here. The vehicle may have cranked yesterday but not today. And all the good voltages are there.

Regards,
Amarin.