Specific Gravity Battery Test

The lead acid battery used in today's automobile is made of plates, lead, lead oxide, all in a solution of electrolyte. The electrolyte solution consists of 65% water and 35% sulfuric acid. The specific gravity or weight of this solution increases as the battery charges and decreases as the battery discharges. As the battery discharges the sulfur moves away from the solution and toward the plates. The opposite is true as the battery is charged, the sulfur returns to the electrolyte solution.

The specific gravity of the electrolyte depends on this 65% to 35% ratio for the chemical reaction necessary create the electrons in the battery. This ratio is effected by the amount of sulfuric acid and the temperature of the solution.

As the temperature drops the electrolyte contracts increasing the specific gravity and changing the reading. As the temperature increases the electrolyte expands deviating from its optimal ratio and effecting the specific gravity reading.

The same is true for the level of the electrolyte solution. As the level gets lower the specific gravity is higher than optimal. When the technician adds water to the cell the specific gravity is lowered bringing the solution closer to its optimal ratio.  A battery cells specific gravity is a great way of measuring a batteries state of charge. This is because during a discharge the specific gravity decreases linear with ampere-hours discharged. The specific gravity also increases as the battery is recharged.

Use a hydrometer to measure the specific gravity of the electrolyte solution in each cell. It's a tool used to measure the density or weight of a liquid compared to the density of an equal amount of water. A lead acid battery cell is fully charged with a specific gravity of 1.265 at 80° F. For temperature adjustments: get a specific gravity reading and adjust to temperature by adding .004 for every ten degrees above 80° F and subtracting .004 for every10 degrees below 80° F.