Instructables

EVERYONE Needs a Multi-Meter

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Step 4: Corrosion?

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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.)
 
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ac-dc3 years ago
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.
r.effuse3 years ago
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.