How to Test an Automotive Battery with a Digital Voltmeter
The DMM (Digital Multimeter) is one of the most versatile test instruments in any auto mechanics tool kit. This one instrument combines the functions of a voltmeter, ammeter and ohmmeter, allowing you to measure circuit voltage, current and resistance quickly and accurately. Older styled auto batteries allowed the use of a battery hydrometer to measure the specific gravity of the electrolyte in each cell to ascertain the cells condition and the overall condition of the battery, but today's sealed battery construction makes this technique impossible. Today, we use a digital voltmeter to evaluate the overall condition of the car battery. Does this Spark an idea?
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Things You'll Need
- DMM (Digital Multimeter)
- Test leads
- Safety glasses
- Auto mechanic's gloves
- Wire brush
Instructions
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1
Locate the battery and clean any corrosion from the tops of the battery posts and battery cable terminals using the wire brush. Modern batteries are of sealed construction, but small amounts of electrolyte can escape past the posts and cause the corrosion to form on them. This corrosion contains sulfuric acid, which can burn your skin and eat holes in your clothes, so wear mechanic's gloves and work with caution. Always wear eye protection.
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2
Drain the surface charge from the battery by turning the headlights on for a couple of minutes. Surface charges are charges that build up on the surface of the battery's anodes, the battery's positive plates, which will produce a high, false reading. Surface charges will be found either on batteries that have been charged, by the vehicle's charging system or by a battery charger, within the last 48-hours.
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3
Select the DC (Direct Current) function on your DMM and set the Range Switch to an appropriate voltage range. For example, on the Sperry DM-4100A DMM that would be the 20-Volt range.
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Conduct a "No Load" test. With the meter probes making firm contact with the centers of both the "Positive" and the "Negative" battery posts, take a "No Load" voltage reading. "No Load" means there aren't any current drains on the battery. Note: This really isn't possible with the modern automobile because there are always small loads on the battery caused by the on-board computers. However, these loads are so insignificant they can be disregarded during this test.
Under "No Load: conditions:
• 12.6-Volts and above equals 100-percent charge.
• 12.45-Volts equals a 75-percent charge.
• 12.24-Volts equals a 50-percent charge.
• 12.06-Volts equals a 25-percent charge.
• 11.89-Volts equals a 0.0-percent charge, a "dead" battery. -
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Conduct a load test. With the probes still firmly pressed into the centers of the "Positive" and "Negative" battery posts, have an assistant crank the engine over while you observe the voltage readings. Under full cranking load, the battery voltage should never drop below 10.0-Volts. If the voltage drops below 10.0-Volts during the load test, the battery is either badly in need of a charge, low on water or has a high internal resistance due to the sulfated plates
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Charge the battery and repeat the load test. If the voltage still drops below 10.0-Volts under full load, it's time to replace the battery.
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