The state-of-charge test checks the battery's chemical condition. One method uses a hydrometer to measure the specific gravity of the electrolyte. Another method uses a digital voltmeter to check the battery's open circuit voltage and, for a general indication of the battery's condition, check the indicator eye (if the battery has one) or check the headlamp brightness during starting.
Specific gravity means exact weight. The hydrometer compares the exact weight of electrolyte with that of water. Strong electrolyte in a charged battery is heavier than weak electrolyte in a discharged battery.
By weight, the electrolyte in a fully charged battery is about 36% acid and 64% water. The specific gravity of water is 1.000. The acid is 1.835 times heavier than water, so its specific gravity is 1.835. The electrolyte mixture of water and acid has a specific gravity of 1.270 is usually stated as "twelve and seventy."
By measuring the specific gravity of the electrolyte, you can tell if the battery is fully charged, requires charging, or must be replaced. It can tell you if the battery is charged enough for the capacity, or heavy-load test.
TEST PROCEDURE: The following steps outline a typical procedure for performing a state-of-charge test:
1 . Remove vent caps or covers from the battery cells.
2. Squeeze the hydrometer bulb and insert the pickup tube into the cell closest to the battery's positive (+) terminal.
3. Slowly release the bulb to draw in only enough electrolyte to cause the float to rise. Do not remove the tube from the cell.
4. Read the specific gravity indicated on the float. Be sure the float is drifting free, not in contact with the sides of top of the barrel. Bend down to read the hydrometer a eye level. Disregard the slight curvature of liquid on the float.
5. Read the temperature of the electrolyte.
6. Record your readings and repeat the procedure for the remaining cells.
TEMPERATURE CORRECTION: The specific gravity changes with temperature. Heat thins the liquid, and lowers the specific gravity. Cold thickens the liquid, and raises the specific gravity. Hydrometers are accurate at 80-F (26.7°C). If the electrolyte is at any other temperature, the hydrometer readings must be adjusted. Most hydrometers have a built-in thermometer and conversion chart. Refer to the temperature correction chart. For each 1 O°F (5.5°C) above 80°F (26.7°C), ADD 0.004 to your reading.
TEST RESULTS: Specific gravity readings tell a lot about battery condition.
1. A fully charged battery will have specific gravity readings around 1.265.
2. Specific gravity readings below 1.225 usually mean the battery is run down and must be charged.
3. Readings around 1.190 indicate that sulfation is about to begin. The battery must be charged.
4. Readings of 1.155 indicate severe discharge. Slow charging is required to restore active materials.
5. Readings of 1.120 or less indicate that the battery is completely discharged. It may require replacement, but slow charging may restore some batteries in this condition.
6. A difference of 50 points (0.050) or more between one or more cells indicates a defective battery. It should be replaced.
7. When the specific gravity of all cells is above 1.225 and the variation between cells is less than 50 points, the battery can be tested under load.
An accurate digital voltmeter is used to check the battery's open-circuit voltage:
1 . If the battery has just been charged, turn on the headlamps for one minute to remove any surface charge.
2. Turn headlamps off and connect the voltmeter across the battery terminals.
3. Read the voltmeter. A fully charged battery will have an open-circuit voltage of at least 12.6 volts. A dead battery will have an open-circuit voltage of less than 12.0 volts.
Toyota original-equipment batteries have an indicator eye for electrolyte level and specific gravity. If the eye shows red, the electrolyte level is low or the battery is severely discharged. If some blue is showing, the level is okay and the battery is at least 25% charged.
NOTE: The indicator eye should be used only as a general indication of electrolyte level and strength.
Was this article helpful?
You can now recondition your old batteries at home and bring them back to 100 percent of their working condition. This guide will enable you to revive All NiCd batteries regardless of brand and battery volt. It will give you the required information on how to re-energize and revive your NiCd batteries through the RVD process, charging method and charging guidelines.