Visual inspection alone is not sufficient to determine the overall health of the battery. Both open-circuit voltage and specific gravity readings can give a good indication of the battery's charge level, age, and health. Routine voltage and gravity checks will not only show the state of charge but also help spot signs of improper care, such as undercharging and over-watering, and possibly even locate a bad or weak battery. The following steps outline how to properly perform routine voltage and specific gravity testing on batteries.

I. Specific Gravity Test(Flooded batteries only)

  1. Do not add water at this time.
  2. Fill and drain the hydrometer 2 to 4 times before pulling out a sample.
  3. There should be enough sample electrolyte in the hydrometer to completely support the float.
  4. Take a reading, record it, and return the electrolyte back to the cell.
  5. To check another cell, repeat the 3 steps above.
  6. Check all cells in the battery.
  7. Replace the vent caps and wipe off any electrolyte that might have been spilled.
  8. Correct the readings to 80o F: Add .004 to readings for every 10o above 80o F /
    Subtract .004 for every 10o below 80o F.
  9. Compare the readings.
  10. Check the state of charge using Table 1.

The readings should be at or above the factory specification of 1.277 +/- .007. If any specific gravity readings register low, then follow the steps below.

  1. Check and record voltage level(s).
  2. Put battery(s) on a complete charge.
  3. Take specific gravity readings again.

If any specific gravity readings still register low then follow the steps below.

  1. Check voltage level(s).
  2. Perform equalization charge. Refer to the Equalizing section for the proper procedure.
  3. Take specific gravity readings again.

If any specific gravity reading still registers lower than the factory specification of 1.277+/- .007 then one or more of the following conditions may exist:

  1. The battery is old and approaching the end of its life.
  2. The battery was left in a state of discharge too long.
  3. Electrolyte was lost due to spillage or overflow.
  4. A weak or bad cell is developing.
  5. Battery was watered excessively previous to testing.

Batteries in conditions 1 - 4 should be taken to a specialist for further evaluation or retired from service.

II. Open-Circuit Voltage Test

For accurate voltage readings, batteries must remain idle (no charging, no discharging) for at least 6 hrs, preferably 24 hrs.

  1. Disconnect all loads from the batteries.
  2. Measure the voltage using a DC voltmeter.
  3. Check the state of charge with Table 1.4. Charge the battery if it registers 0% to 70% charged.

If battery registers below the Table 1 values, the following conditions may exist:

  1. The battery was left in a state of discharge too long.
  2. The battery has a bad cell.Batteries in these conditions should be taken to a specialist for further evaluation or retired from service.


TABLE 1. State of charge as related to specific gravity and open circuit voltage