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Testing Batteries

Testing Batteries
First, be sure to follow these important safety tips:
Put on protective eye wear, rubber gloves and work clothes and remove all jewelry.
Don't allow smoking, open flames or sparks near the battery.

How do I test my battery?
Here are some of the tests available to determine your battery's status:

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Headlight Test In the dark, pull your car up to a building wall or a garage door. Turn off the engine and leave on the headlights. If they're bright, then your battery is probably fine. If the lights are dim, but get brighter when the engine is started, this tells you the battery could be bad and requires further testing. If the lights are dim and stay dim when the engine is started, this tells you that your charging system requires further testing.
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Open Circuit Voltage (OCV) Test An OCV test may be performed with a voltmeter.
  1. To determine if the battery is experiencing a problem, turn off all electrical loads and the charging source. 

  2. For an accurate reading, allow the battery to sit with no electrical loads applied for at least one hour. 

  3. Connect a voltmeter to the positive and negative terminals and measure the terminal post voltage with no loads or chargers connected to the battery. 

  4. To determine the battery's state of charge, compare the OCV reading on the voltmeter to the Open Circuit Voltage Chart below.
Open Circuit Voltage VS. State of Charge
12.66V    100%    
12.45V    75%    
12.24V    50%    
12.06V    25%    
11.89V    0%    

We do not recommend that you substitute the OCV test for a specific gravity test, which is more accurate. The OCV test, however, is the only way to determine the state of charge of a sealed no-maintenance battery.

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Specific Gravity Test This test is performed with a hydrometer, which is the most accurate hand held tool for determining the state of charge of a lead acid battery.
  1. Draw electrolyte into the hydrometer a few times so that the float reaches the same temperature as the electrolyte. This will increase the accuracy of your readings. 

  2. Hold the hydrometer vertically so that the float is free and does not touch the inner walls of the barrel. 

  3. Hold the hydrometer so that the liquid is level in the barrel and at eye level. 

  4. When you draw the electrolyte, make sure that the hydrometer is full. 

  5. Check each individual battery cell. If the specific gravity varies more than .050 or "50 points" among the cells while the battery is at a 75% state of charge or above, then the battery is bad and should be replaced. The cells that have a specific gravity of 50 points less than the highest cell are bad cells. A hydrometer reading of 1.265 or greater at 80°F indicates a full charge for Interstate batteries. To determine the battery's state of charge, compare the hydrometer reading to the Specific Gravity Chart below.  
    Specific Gravity VS. State of Charge
    1.265    100%    
    1.225    75%    
    1.190    50%    
    1.155    25%    
    1.120    0%    
  7. To get the most accurate hydrometer reading, you should adjust your hydrometer reading according to the temperature. If the electrolyte temperature is ABOVE 80°F , ADD .004 (called "four points") to the hydrometer reading for each 10 degrees above 80°F. If the electrolyte temperature is BELOW 80°F, SUBTRACT four points from the hydrometer reading for each 10 degrees below 80°F.
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Load Test This test is a 15-second discharge of the battery at a 1/2 cold cranking amp level. A more accurate testing method than a voltmeter or a hydrometer, the load test is often required to determine whether a battery is good or bad. It is used by professional technicians.