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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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  1. If there is a .050 (sometimes expressed as 50 "points") or more difference in the specific gravity reading between the highest and lowest cell, you have a weak or dead cell(s). Using the battery manufacturer's recommended procedure, applying an Equalizing charge may correct this condition.
  2. If the battery will not recharge to a 75% or more State-of-Charge level or if the built-in hydrometer still does not indicate "good" (usually green or blue, which indicates a 65% State-of-Charge or better).
  3. If a digital voltmeter indicates 0 volts, there is an open cell.
  4. If the digital voltmeter indicates 10.45 to 10.65 volts, there probably is a shorted cell. A shorted cell is caused by plates touching, sediment ("mud") build-up or "treeing" between the plates.

6. Load Test The Battery.

If the battery's State-of-Charge is at 75% or higher or has a "good" built-in hydrometer indication, then you can load test a car battery by one of the following methods:

  1. With a battery load tester, apply a load equal to one half of the CCA rating of the battery for 15 seconds. (Recommended method).
  2. With a battery load tester, apply a load equal to one half the vehicle's CCA specification for 15 seconds.
  3. Disable the ignition and turn the engine over for 15 seconds with the starter motor.

DURING the load test, the voltage on a good battery will NOT drop below the following table's indicated voltage for the electrolyte at the temperatures shown:


Electrolyte Temperature °F Electrolyte Temperature °C Minimum Voltage Under LOAD
100° 37.8° 9.9
90° 32.2° 9.8
80° 26.7° 9.7
70° 21.1° 9.6
60° 15.6° 9.5
50° 10.0° 9.4
40° 4.4° 9.3
30° -1.1° 9.1
20° -6.7° 8.9
10° -12.2° 8.7
-17.8° 8.5

If the battery is fully charged or has a "good" built-in hydrometer indication, then you can test the capacity of a deep cycle battery by applying a known load and measuring the time it take to discharge the battery until measures 10.5 volts. Normally a discharge rate that will discharge a battery in 20 hours can is used.

For example, if you have an 80 ampere-hour rated battery, then an average load of four amps would discharge the battery in approximately 20 hours. Some new batteries can take up to 50 charge/discharge "preconditioning" cycles before they reach their rated capacity. Depending on your application, fully charged batteries with 80% or less of their original rated capacity available are considered to be bad.

7. Bounce Back Test The Battery.

If the battery has passed the load test, please go to Tip #8, below. If not, remove the load, wait ten minutes, and measure the State-of-Charge. If the battery bounces back to less than 75% State-of-Charge (1.225 specific gravity or 12.45 VDC), then recharge the battery and load test again. If the battery fails the load test a second time or bounces back to less than 75% State-of-Charge, then replace the battery because it lacks the necessary CCA capacity.

8. Recharge The Battery.

If the battery passes the load test, you should recharge it as soon as possible to prevent lead sulfation and to restore it to peak performance.

Next page» Buying A New Battery » Page 1, 2, 3, 4, 5, 6, 7, 8

Additional information provided courtesy of ALLDATA

© 2004 Vincent T. Ciulla

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