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Using a voltmeter to check battery terminal
connections: An automotive starter can draw up to several hundred amps when starting a car or truck engine. Poor connections at battery terminals can often cause a loss of voltage (and power) to the starter. You can use a voltmeter to check the quality of the terminal connection and to check the battery and charging voltage. If you do not already have a voltmeter, they are normally available at Radio Shack, some auto or building supply stores, or mail order companies. The easiest to use is an auto-ranging type, that allows you to set only the function (such as D.C. Volts), and the meter selects the proper voltage range. What you measure: If you have a poor connection at a battery post, there will be a voltage difference between the post of the battery, and the terminal when something on the car is drawing current. The more current, the larger the voltage difference. A connection may look fairly good, yet still have a voltage drop. To check the positive terminal, set your voltmeter to DC Volts. With the car off, place the positive lead of the of the voltmeter on the positive battery post, and the negative lead on the cable terminal that clamps around the positive post. If your voltmeter is not an autorange type, set it to about a 2 volt range. Turn on the headlights, if the connection is good, the voltage will be zero, or a few millivolts ( 1 mV = 0.001 Volt). If you read in the tens or hundreds of millivolts (100 mV= 0.1 volt), a cleaning of the terminal may be in order.You can repeat this test for the negative terminal, except reverse the leads from the voltmeter. Side mount terminals will be a bit more tricky, but if you can get access to the bolt and the terminal on the cable, the same process should work. Battery post cleaning tools are available at auto parts stores. The author has found, at the recommendation of a former auto company engineer, Permatex Dielectric Tune Up Grease does a good job of reducing corrosion after a good cleaning, and allows for a good connection. Spread a very thin coat on the terminals. Battery voltage and charging voltage While you are at it, if you are having trouble starting your car, you can measure your battery voltage (remember to set the range up on manual meters to around 20 volts). The - terminal on the meter goes to the negative battery terminal, and + to positive. A fully charged battery will measure around 12.6 - 13.2 volts with the car off. With the car running, you can measure the battery voltage at idle with lights, and accessories off. Most cars will charge from around 13.2 to 15 volts. If it is lower, check your connections, belt tension on the alternator, and the alternator. If the voltage is high, you may have a defective regulator (often inside the alternator - so you have to replace the alternator). Alternators can fail gradually, with a bad diode, for example. In this case the alternator may still work, however not be able to keep up with accessories such as lights and the heater / air conditioner. When this happens, the car battery may slowly run down. |
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