The Correct Mixture of Sulfuric Acid and Water in a Lead-Acid Battery

In a lead-acid battery, the typical mixture of sulfuric acid (H2SO4) and water is approximately 30 percent sulfuric acid and 70 percent water by weight. This concentration can vary slightly depending on the specific requirements of the battery, but maintaining this ratio is essential for optimal performance.

Mixing Procedure

When mixing sulfuric acid and water, it is crucial to add acid to water, not the other way around. Adding water to concentrated sulfuric acid can cause exothermic reactions leading to splattering and potentially dangerous situations. Always follow safety precautions and add acid slowly to water while constantly stirring.

Electrolyte Specific Gravity and Application

The mixture of sulfuric acid in water electrolyte used in a lead-acid battery depends on the application and the battery's intended lifespan. For example:

High Current for Short Periods - Engine starting batteries or UPS backup until a genset comes online typically have a specific gravity (SG) of 1.265 or 36 acid.

Low Current for Long Periods - Central office telephone or switchgear backup may require a lower SG of about 1.215 or 30 acid.

Motive Power Applications - Electric forklifts need both high current and long run times, so these types may contain an SG in the range of 1.290 (38) to 1.315 (40).

In general, the higher the specific gravity, the shorter the battery's life. Some standby power batteries have been known to last up to 35 to 40 years due to their low acid concentrations and thick plate design.

Conversely, during World War II, US ship torpedoes used DC electric motors for propulsion. Their batteries contained very high SG acid to maximize speed to the target before deployment. The specific gravity of about 37 or 38 weight H2SO4 (corresponding to a SG of 1.28) is optimal for performance as it maximizes conductivity and reduces internal resistance, improving battery performance.