How to Use a Battery Hydrometer

by Justin Chacos

Your car’s battery is the starting point for all travels. It dates back to 1915 when a Cadillac engineer named Charles Kettering designed the first electric starter motor to eliminate the need for dangerous manual cranking, Kettering then designed the battery, voltage regulator and other starting system components.

Obtain the Battery Electrolyte Specific Gravity

The battery hydrometer tells you the state of battery charge by giving the specific gravity of the electrolyte inside the cells. Identify the cells starting from the positive side of the battery to the negative with a simple one-through-six system. Only open and test one cell at a time to prevent yourself from drawing electrolyte from one cell then depositing it in another. Draw enough electrolyte into the hydrometer to barely enter the bulbous section of the tool. Read and record the number on the integral float that is level with the top of the fluid. Test each cell twice to verify the readings.

Interpret the Results

The specific gravity of each cell should be within .050 point of each other. If one or more cells differ from the others by more than .050 point, the low cell is likely bad and the battery needs replacement. A battery is fully charged at 1.265 points specific gravity averaged across the six cells and discharged at 1.230 points. However, the electrolyte temperature affects the reading. For every 10 degrees above 80 degrees Fahrenheit, add 0.004 point to the reading. Conversely, subtract 0.004 point for every 10 degrees below 80. Most hydrometers have a built-in thermometer to measure the electrolyte temperature as it is usually different from ambient temperature.

About the Author

Justin Chacos is a professional mechanic with experience on all vehicle types, from cars to boats to airplanes. He has been writing since 2006 and has been published in multiple maintenance manuals and journals. He holds a Master of Science from the University of Arkansas.

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