Hydrometer Test
HYDROMETER TESTThe hydrometer test reveals the battery state-of-charge by measuring the specific gravity of the electrolyte. This test cannot be performed on maintenance-free batteries with non-removable cell caps. If the battery has non-removable cell caps, refer to Diagnosis and Testing for alternate methods of determining the battery state-of-charge.
Specific gravity is a comparison of the density of the battery electrolyte to the density of pure water. Pure water has a specific gravity of 1.000, and sulfuric acid has a specific gravity of 1.835. Sulfuric acid makes up approximately 35% of the battery electrolyte by weight, or 24% by volume. In a fully-charged battery the electrolyte will have a temperature-corrected specific gravity of 1.260 to 1.290. However, a specific gravity of 1.235 or above is satisfactory for the battery to be load tested and/or returned to service.
Before testing, visually inspect the battery for any damage (a cracked case or cover, loose posts, etc.) that would cause the battery to be faulty Then remove the battery cell caps and check the electrolyte level. Add distilled water if the electrolyte level is below the top of the battery plates. Refer to Battery System Cleaning for the proper battery inspection procedures.
Hydrometer - Typical:
See the instructions provided by the manufacturer of the hydrometer for recommendations on the correct use of the hydrometer that you are using. Remove only enough electrolyte from the battery cell so that the float is OFF the bottom of the hydrometer barrel with pressure on the bulb released. To read the hydrometer correctly, hold it with the top surface of the electrolyte at eye level.
CAUTION: Exercise care when inserting the tip of the hydrometer into a battery cell to avoid damaging the plate separators. Damaged plate separators can cause early battery failure.
Hydrometer floats are generally calibrated to indicate the specific gravity correctly only at 26.7 °C. When testing the specific gravity at any other temperature, a correction factor is required. The correction factor is approximately a specific gravity value of 0.004, which may also be identified as four points of specific gravity For each 5.5 °C above 26.7 °C, add four points. For each 5.5 °C below 26.7 °C, subtract four points. Always correct the specific gravity for temperature variation.
EXAMPLE: A battery is tested at -12.2 °C and has a specific gravity of 1.240. Determine the actual specific gravity as follows:
1. Determine the number of degrees above or below 26.7 °C: 26.7 °C + -12.2 ° C = 14.5 °C below the 26.7 °C specification
2. Divide the result from Step 1 by 5.5 °C: 14.5 °C + 5.5 °C = 2.64
3. Multiply the result from Step 2 by the temperature correction factor (0.004): 2.64 X 0.004 = 0.01
4. The temperature at testing was below 26.7 °C; therefore, the temperature correction factor is subtracted: 1.240 - 0.01 = 1.23
5. The corrected specific gravity of the battery cell in this example is 1.23.
Test the specific gravity of the electrolyte in each battery cell. If the specific gravity of all cells is above 1.235, but the variation between cells is more than fifty points (0.050), the battery should be replaced. If the specific gravity of one or more cells is less than 1.235, charge the battery at a rate of approximately five amperes. Continue charging the battery until three consecutive specific gravity tests, taken at one hour intervals, are constant. If the cell specific gravity variation is more than fifty points (0.050) at the end of the charge period, replace the battery.
When the specific gravity of all cells is above 1.235, and the cell variation is less than fifty points (0.050), the battery may be load tested to determine its cranking capacity. Refer to Standard Procedures for the proper battery load test procedures.