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The hydrometer is an instrument used to test the state of charge of a battery. The state of charge of a conventional lead-acid battery can be determined by testing the specific gravity of the electrolyte. Do not, however, make the hydrometer test immediately after water has been added to the electrolyte. Measurement should be made before the water is added in order to obtain on a representative sample. If the level is too low to obtain, a sample water should be added as needed.

After the battery has been used long enough to thoroughly mix the water with the electrolyte, the sample may be taken with a hydrometer. To read hydrometer, squeeze bulb to draw enough electrolyte from cell to raise float. The float should not touch barrel of the hydrometer. Release pressure on a bulb and observe specific gravity reading on a float. Check thermometer and compute temperature-corrected specific gravity of the electrolyte.

In heavy load test which is also called ‘high rate discharge’ or ‘capacity test’, it is a good test of the battery’s ability to perform under load. In this test, a good battery will produce current equal to 50 percent of its cold cranking rating for 15 seconds and still provide minimum voltage to start the engine.

To make the heavy load test:

  1. Charge battery, if necessary, until all cells are atheist 1.22S specific gravity. Remove vent caps, if so equipped, and install a thermometer in an electrolyte.
  2. Connect a battery load tester or a battery starter tester, directly to a battery posts: positive to positive: negative to negative.
  3. Turn carbon pile knob to apply a load equal to 50 percent of cold cranking rating of a battery being tested.
  4. Read voltmeter while a load is being applied for 15 seconds. Turn carbon pile knob to off position.
  5. Read thermometer, record temperature, and check temperature and voltage against a chart.
  6. Replace the battery if a minimum voltage is below specification.
  7. If reading is the same or greater than voltage, it shows on chart clean and fully charges. The battery should remain in service.
Load Test Chart
Minimum Voltage Temperature
Fo Co
9.6 70 and above 16
9.5 60 10
9.4 50 4
9.3 40 -1
9.1 30 -7
8.9 20 -12
8.7 10 -12
8.5 0

The chart above shows readings obtained after 15 sec reading by a battery load tester which has carbon pile.

In the open circuit voltage test, if the battery fails the heavy load test, check its state of charge by making a stabilizer open circuit. The voltage test allows at least 10 minutes after the load test for the battery to stabilize then measure and record open circuit voltage.

There are many types of test equipment available for making the open circuit voltage test. In general, a voltmeter and multi-meter are used by connecting its positive lead to the positive post of the battery and its negative lead to the negative post of the battery.

Here are steps to do that:

Before you can properly test any battery, it must be at full charge. You can verify the state of the charge in two ways: the simplest is done with digital millimeter. The other test involves the use of a hydrometer.

Using the digital millimeter to verify the state of charge: a fully charged 12 volts battery will read at least 12.6volts (2.1 volts per cell) on the millimeter when connected together.

Open Circuit Voltage Test

Open circuit volts Percent of charge
11.7 volts or less 0%
12.0 25%
12.2 50%
12.4 75%
12.6 or more 100%

Battery, Cells, Car, Electric, Charge

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