EVERYONE Needs a Multi-Meter

Step 4: Corrosion?

See the first photo. Your car battery may be in good condition, but the starter still barely cranks the engine. The problem could be corrosion at one or both of the battery terminals. Sometimes you will see white or blue powder around the terminals, but often the corrosion is not visible. The photo shows an old-style battery terminal.  If your car (or motorcycle or riding lawnmower) has this type of terminal, see the text boxes for where to place the voltmeter probes. Set the voltmeter to a DC setting slightly in excess of 12 volts. Have a helper turn the key to crank the engine.  If the battery connections are good, the voltmeter reading should remain at zero (0).  If the battery connections are corroded, they will provide a high resistance, perhaps even an open circuit, and current will try to go through the voltmeter as an alternate route. The voltmeter reads the difference in voltage between one probe and the other. That means the voltmeter will show a reading equal to the voltage of the battery.  Clean or replace the battery terminal by taking it apart, dissolving the corrosion with baking soda in water, and scraping the parts of the connection. If you use your multi-meter just once for this, you will have saved enough money to pay for it.

See the text boxes in the second photo. The newer side post battery terminals are now more common than the old-style shown in the first photo. Much of the terminal is covered with insulation. One meter probe touches the terminal's bare metal bolt. The other probe can touch a straight pin pushed through the cable's insulation. 

(The first photo is from Bing Images.)

Comments

[ac-dc]

It is not true that corrosion could be a problem if it is not visible. For there to be enough corrosion to cause a problem it is progressed to the point it is very plainly visible and there is no need to check it with a multimeter as it is obviously in need of cleaning.

[askjerry]

Sorry ac-dc but I am a Senior CET with 30+ years of experience... and I have seen equipment fail because of a micron or so of corrosion build-up. The metal looked shiny, everything seemed normal... hitting the contacts with some emery cloth and a bit of denatured alcohol fixed the problem. (Or sometimes a rubber pencil eraser... keep one in your toolbox!)

Depending on the current going through a circuit... it may only take a tiny film to provide enough resistance to cause a problem... in some circuits the addition of as little as 1K ohm is enough to cause a failure.

Jerry
http://en.wikipedia.org/wiki/Electronics_Technicians_Association

[Phil B]

Last week I had a great example of the point you are making (i.e. corrosion not visible to the eye can cause a circuit to malfunction). My '99 Oldsmobile Alero has an electronic instrument cluster that sometimes refused to come to life recently when I turned the ignition key. On some GM cars that symptom is the beginning of problems that will worsen until the car will not start. I fully expected I might be forced to spend for a new car. In my case, a set of unsoldered metal on metal circuit connections had oxidized enough over 12 years to keep the instrument cluster from operating. I cleaned the terminals and bridged them with solder. You can view the resulting Instructable here.

[johnny3h]

Howdy askjerry.  Thanks for the comeback to ac-dc as his comment could send a novice on a lengthy time-wasting chase "around Robin Hood's barn" by ignoring connections that "look" good.

I have been down that road myself, and what you describe is definitely a commonly occurring problem.

[sschoemann]

OH almost forgot... they are getting almost impossible to find these days but I also carry a couple old fashioned typewriter erasers (ink eraser in a wooden pencil type stick) for cleaning contacts, they are abrasive enough to clean the metal contacts but not so that they remove metal, and since they are sharpened in a pencil sharpener they are pointed enough to use on some pretty small packages.

[Phil B]

Thank you for your comments. And, the typewriter eraser you describe is dielectric so you can use it on "hot" terminals. Your comment about lead battery contacts being phased out reminds us that if something works the federal government is always ready to ban it for our good until they make a mess out of everything.

[sschoemann]

Jerry You speak gospel! Prior to becoming a full fledged EE I to was a CET (ISCET). I discovered a fantasitc product worth it's weight in gold on any socketed (plug in) type device that I have used on car batteries since I always have it on hand despite it's cost... DEOXIT. I keep the pen dispenser in my kit and the bottle on my bench (I'm a Ham radio operator, not one of those idiot design engineers who forgot what it's like to work on equipment in the field). It works and thats all that it takes in many instances... I cant count the number of times I've pulled a prom, eeprom, or any number of chips or micro procs, cards or so on that were flaking out, applied the deoxit to the contacts, reinserted them and all wierdness was completely eliminated for evermore. Not only does resistance cause problems but a minute amount of oxidation can act like a diode and even though it will conduct, it will "rectify" a 'stray' radio signal or two adding enough noise to really screw things up at the most Murphy's Law moment.

[ac-dc]

The devil is in the details, I was writing within the context of the prior post. On that topic (battery and lights), you will not have sufficient corrosion to cause any problems if you cannot see it. The contact area is sufficiently large, the clamping force too, that even an invisible film is not enough resistance to be an issue with lights consuming only a single digit # of amps.

Of significance is also the metals used for the battery and clamp and the nature of the corrosion causing discoloration that would be visible. Further, if the film is so slight that it can't even be seen, the abrasive action of taking the clamped connector off and putting it back on alone would have cleaned it sufficiently for a light load like lights.

You wrote "1K Ohm" as being a small resistance, but between a battery terminal and the clamped connector where you cannot even see any corrosion, you will not find even three orders of magnitude lower resistance... less than 1 ohm.

As for dissimilar circuits and different connectors, I do agree in those circumstances a contact can be fouled by corrosion or other contaminants and cause a problem without it being visible, particularly logic circuits where you end up with a capacitive junction.

[sschoemann]

Actually I've measured considerably more resistance on fully functional battery clamp/post connections the worst I found was a full meg ohm. The residue from sulfuric acid is very resistive, often with the only conductive path being a single arc or two. Living in Wisconsin, the dead of winter proves these situations time and again, where a simple dirty post and clamp are the difference between the battery charging and going dead and freezing. This is going to become much more apparent as time goes on as autos become more Rohos compliant, as the lead battery clamp is now mandated for extinction and will be replaced with plated or tinned brass or copper clamps which will not hold up to the corrosion.

[ac-dc]

Car batteries are exempt from ROHS, the whole inside of them is LEAD plates. Other car electronics are also exempt. Note how ROHS was already implemented in the EU, changing global design materials where they are affected, but they are still lead.

With enough (a visible level) corrosion, you could measure 1 Mohm, but it would not be a "fully functional battery clamp/post" anymore. The key to the battery post and connector is that when you tighten it down, the areas where mechanical stress occur are the primary current path. If there are air gaps elsewhere, you'd have corrosion sooner in that area but should measure where the connector friction scrubs away the surface layer on the contact point.

[sschoemann]

Actually there has not been an automotive battery made with true lead plates in decades. They are currently manufactured with a lead grate, and less and less lead is used every year as larger and more powerful alternators are taking over the demand for continious electrical power, and the battery is no longer acting as a reserve capacitor while the vehicle is running. And the EPA is targeting the car automotive battery due to the health and environmental concerns involved in the recycling of them... Give the government an inch and they WILL take your testicles...

[static]

Wow nice misleading rant. No mention that the newer plate construction is a major reason that batteries now have more cranking capacity. in the same package. Auto starting batteries are a comprise, and where never, ever intended to handle any large part of a car's electrical load for anything but brief periods. Alternators have gotten larger because of an increase of the continuous electric load not because of less lead in the battery. Just as the hazards of manufacturing LA batteries have been been mitigated so have the hazards of recycling them been mitigated. You are creating a bogeyman about recycling LA batteries to lodge a complaint against the EPA. The stamped metal battery terminal clamps where being used on some imports before  RoHS.  Judging by the longevity of the brass battery clamps on old tractors I have had to use, brass would be an acceptable substitute for the lead clamps. The only advantage with the softer lead is the ease of restoring a clean contact surface.

[skrubol]

1k ohm? Do you mean 1m ohm?
With car batteries it is pretty rare for corrosion to cause problems without being plainly visible. There is a reason the terminals are typically made out of lead and not a better conductor like copper.

[askjerry]

Mo, I meant 1K ohm.

Depending on the circuit you have... 1K ohm can make a huge difference. Let's look at an LED running on 12 volts.

An LED uses 2.2 volts and takes 20mA... or 0.020 amps. (typical)
So you take 12v and subtract 2.2 volts... that leaves 9.8 volts that needs to be dissipated. Okay... 9.8 volts divided by 0.02 amps is 490 ohms. Now... add 1K of resistance from corrosion... 1,490 ohms total.

9.8volts / 1,490 = 0.0065 amps... or 6.5 mA. That is a WHOLE lot less current than the LED expects... so it barely glows if at all. Now imagine a microprocessor that is connected to an input via an optocoupler... and the LED above is the one for the optocoupler... see the issue?

Jerry

[skrubol]

1k ohm is huge for resistance (no such thing as a K ohm btw, lowercase is the proper terminology for kilo.) A car battery with a 1 k ohm terminal resistance would basically read near 0 volts from just the indicator lights in the car. Even 1 ohm would not let the starter budge.
I was referring to 1 milliohm, not 1 Megaohm.

[Phil B]

I am sure I remember reading that not all corrosion is visible. Before responding I did a search and found someone had problems with corrosion he could not see without removing the battery connectors. I have found the newer side post terminals can be quite corroded, but the corrosion remains covered and not visible from a quick glance under the hood.

[ac-dc]

I cannot speak for what anyone else meant, but when I think or write about visible corrosion I do mean visible with/after the connector (is) disconnected.

[NaturalCrafter]

You are right. I have watched my husband working on a small circuit board that would not work he took out the battery and used very fine sand paper on the contacts, suddenly the thing just stared working right again. This always amazes me. I have to say my first experience helping to fix something was my grandmothers toaster would not work. My dad took out his meter, tested good, then tried another plug and looked inside and scratched his head. The two copper leaf contacts were only very slightly brown like a dull penny. I offered maybe an eraser on the contacts. So he chuckled and let me try it. Then when we plugged it back in and it lit up, he was amazed...My dad was an electrical engineer...only time I ever got over on him as he was sure it was a broken internal element...lol

[ac-dc]

Apples and oranges, brass and copper especially degrade much more resistively and faster than lead, which is why lead is chosen, BUT we were speaking about visible corrosion too - brown like a dull penny is considered visible corrosion on copper.

[Phil B]

If you are familiar with electrolytic capacitors, those are made by allowing an oxide to form on one side of the aluminum foil. It serves as an insulator and has the advantage of being thinner than paper or mylar film. It is a powerful reminder that oxides look like conductive metal, but actually insulate. Your father's experience with the toaster shows we look for what we expect to find, and sometimes that blinds us to what is.

[79spitfire]

This is true, corrosion can "hide" in the contact areas between the terminal and post. What your doing is called a "voltage drop test". It's a quick and easy way to locate where a bad connection is raising havoc. There are several good videos showing how to use a voltage drop to solve real world problems, including cars that won't start with battery terminals that look clean!

[Phil B]

Thank you. I assume the videos are at YouTube. Is that correct?

[79spitfire]

Yes, and they scattered about the web as well. Several auto manufactures are teaching their factory techs how to use voltage drops to more effectively repair cars and especially computer circuits in cars.

[Phil B]

I first became aware of the voltage drop test for auto battery connections testing from an old Motor's Auto Manual. That particular manual was published in 1969. Thanks.

[ElectroFrank]

I once replaced my car battery that seemed to have expired, with very poor response from the starter motor.  But the new battery made no difference.  

After carefully tracing volt drops, I discovered substantial corrosion on the engine block where the battery ground cable terminal bolted to it.  

Just cleaned and greased, and no more problem.   Thinking about it, two different metals, plus a bit of moisture and a current, is a beautiful recipe for corrosion. 

I always grease or oil nuts and bolts and electrical connectors when reassembling things, particularly on my car, both for preventing corrosion and ease of extraction the next time.

[Phil B]

Your information is very good. I knew a man back in the 1970s who had spent over $100 to have a bad ground discovered and fixed. To complicate things, grounding terminals are often hidden under layers of other things. Thank you for the important reminder.

[r.effuse]

If the voltage difference is more than 2 volts between the cable and the battery post, you have a corrosion problem. Auto technicians treat 0.5 volts as acceptable for commercial work. This is true for any 12v circuit in the car and is a great way to test for tricky problems that only act up under load or, say, in bad weather.

[Phil B]

Thank you for the additional information and thank you for looking at this Instructable.