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Battery Testing, Maintenance And Myths
Your vehicle's battery is not very demanding, and most often only thought about when it fails. But just a small amount of care and mantainence will help insure it doesn't let you down when you need it most.
 More of this Feature
Testing The Battery
• Testing The Battery (cont.)
Testing The Battery (cont.)
Buying A New Battery
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4. Remove The Surface Charge.

The surface charge, if not removed, will make a weak battery appear good or a good battery appear bad. Eliminate the surface charge by allowing the battery to sit for between four to twelve hours in a warm room.

5. Measure The State-Of-Charge.

To determine the battery's State-of-Charge with the battery's electrolyte temperature at 80° F (26.7° C), use the following table. The table assumes that a 1.265 specific gravity cell average and 12.65 VDC Open Circuit Voltage reading for a fully charged, wet, lead-acid battery.

If the electrolyte temperature is not 80° F (26.7° C), use the Temperature Compensation table to adjust the Open Circuit Voltage or Specific Gravity readings.

The Specific Gravity or Open Circuit Voltage readings for a battery at 100% State-of-Charge will vary by plate chemistry, so check the manufacturer's specifications for a fully charged battery.

Temperature Compensation Table

Open Circuit Voltage Approximate State-of-Charge at 80°F (26.7°C) Hydrometer Average Cell Specific Gravity Electrolyte Freeze Point
12.65 100% 1.265 -77°F(-67°C)
12.45 75% 1.225 -35°F(-37°C)
12.24 50% 1.190 -10°F(-23°C)
12.06 25% 1.155 15°F(-9°C)
11.89 or less DISCHARGED 1.120 or less 20°F(-7°C)

For non-sealed batteries, check the specific gravity in each cell with a hydrometer and average cells readings. For sealed batteries, measure the Open Circuit Voltage across the battery terminals with a digital voltmeter. This is the only way you can determine the State-of-Charge. Some batteries have a built-in "Magic Eye" hydrometer, which only measures the State-of-Charge in ONE of its six cells. If the built-in indicator is clear, light yellow, or red, then the battery has a low electrolyte level and if non-sealed, should be refilled and recharged before proceeding.

If sealed, the battery is bad and should be replaced. If the State-of-Charge is BELOW 75% using either the specific gravity or voltage test or the built-in hydrometer indicates "bad" (usually dark or white), then the battery needs to be recharged BEFORE proceeding. You should replace the battery, if one or more of the following conditions occur:

Next page» Testing The Battery (cont.) » Page 1, 2, 3, 4, 5, 6, 7, 8

Additional information provided courtesy of ALLDATA

© 2004 Vincent T. Ciulla

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