How to Test for a Bad Cell in 12V Automotive Battery
All batteries have a predetermined life. The harsher the climate or environment, the shorter the life of a battery. Most automotive batteries must remain fully charged to extend the life of the battery. Over time, the individual cells of the battery may go bad. All 12-volt automotive batteries have six individual cells. Each cell has the capacity of delivering up to 2.2 volts. These cells are electrically connected to give the final output of 12 volts and greater. Testing for a bad cell in a 12-volt battery may require special testing equipment.
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Instructions
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Charge the battery to be tested on a good battery charger. Attempting to test a battery not fully charged will give inaccurate results.
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2
Remove the battery's surface charge by attaching a 12-volt car headlamp to the battery. Leave the lamp connected for approximately three minutes. Remove the lamp from the battery. The battery is now ready for testing.
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3
Switch the voltmeter to read DC (direct current) volts. Place the red lead into the "volts" connector and the black lead into the "com" connector. Touch the red lead to the positive (+) terminal on the battery. Touch the black lead to the negative (-) terminal on the battery.
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4
Read the meter. A good battery will read from 12.4 volts up to 12.7 volts. A voltage reading below 12.3 volts may indicate that sulfation has occurred to one of the cells. Proceed to the next step if the voltage is 12.3 volts or below. This next test can only be performed on a non-SLA (sealed lead acid) battery. You must have access to the individual cells. An SLA battery allows no access to the cell cavities.
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Remove the six access caps to the battery. These caps are located on the top and center of the 12-volt automotive battery. Insert the hydrometer into the cell. Draw out the battery fluid into the tester. The hydrometer should read a specific gravity of 1.225 or greater. If the reading is below 1.225, the battery cell is bad. Drain the battery fluid back into the same cell it was withdrawn from and perform the same test on each cell.
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Tips & Warnings
The surface charge to the battery is the uppermost voltage just after the charging process. It is best to remove this voltage by draining the battery through a type of load. A car headlamp is the easiest method for discharging the car battery for accurate testing levels.
Battery voltage readings 10.5 volts and below generally indicate a shorted cell. These are internal connections and cannot be repaired.
A battery load tester can be used for testing the condition of the battery but may not identify a bad cell.
