The sulfuric acid electrolyte in lead-acid batteries serves as the ionic conductor and active material, participating directly in the electrochemical reactions during charge and discharge. The electrolyte system consists of a solution of sulfuric acid and water, typically ranging from 29% to 39% acid by weight in fully charged batteries, corresponding to specific gravities between 1.22 and 1.28 at 25°C. The concentration varies with state of charge, decreasing during discharge as sulfuric acid is consumed to form lead sulfate on both electrodes.

Acid stratification occurs due to density differences in the electrolyte, where heavier concentrated acid settles at the bottom of the cell while diluted acid rises to the top. This phenomenon develops during cycling, particularly in flooded lead-acid batteries, as the charging process generates higher-density acid at the plates, which then sinks. Stratification reduces battery capacity and accelerates plate corrosion at the bottom due to localized high acid concentration. Mitigation techniques include controlled overcharging to induce gassing and mixing, periodic equalization charges, or mechanical agitation in industrial applications.

The conductivity of sulfuric acid electrolyte follows a nonlinear relationship with concentration and temperature. Maximum conductivity occurs at approximately 30% weight concentration (1.22 specific gravity) at 25°C. Conductivity decreases at higher concentrations due to increased viscosity and reduced ion mobility. Temperature has a significant effect, with conductivity increasing by roughly 1-1.5% per °C within the operational range of -40°C to 60°C. The freezing point of the electrolyte varies with acid concentration, reaching a minimum of -70°C near 30% concentration but rising sharply at lower concentrations, making discharged batteries more susceptible to freezing damage.

Electrolyte maintenance in flooded lead-acid batteries requires regular inspection and correction of fluid levels using distilled or deionized water to compensate for evaporation and electrolysis losses during charging. Acid should never be added to cells except after complete spillage, as this disrupts the concentration balance. Specific gravity measurements using calibrated hydrometers or refractometers provide state-of-charge indication when corrected for temperature. A fully charged battery typically shows 1.265 specific gravity at 25°C, decreasing linearly to about 1.10 at full discharge. Temperature compensation for specific gravity measurements follows a standard coefficient of 0.004 per 5.5°C deviation from 25°C.

Temperature effects on performance are substantial. Capacity increases with temperature at approximately 0.6-1% per °C above 25°C, while decreasing similarly below this reference point. However, high temperatures above 40°C accelerate grid corrosion and water loss, reducing service life. Low temperatures increase electrolyte resistance and reduce reaction kinetics, limiting power delivery. The Arrhenius relationship describes the temperature dependence of electrochemical reactions, with typical activation energies for lead-acid systems ranging between 35-50 kJ/mol for discharge processes.

During charging, the electrolyte undergoes concentration changes as sulfuric acid reforms at the electrodes. The recombination efficiency in flooded designs is typically 95-98%, with the remainder lost to gassing. Overcharging causes excessive water decomposition into hydrogen and oxygen, leading to water loss and requiring replenishment. Modern charging algorithms adjust voltage based on temperature to minimize these effects, with temperature compensation coefficients of -3 to -5 mV/°C per cell being common.

Electrolyte contamination significantly impacts performance. Metallic impurities such as iron, copper, or chlorine increase self-discharge rates through parasitic reactions. Organic contaminants can promote gassing or form insulating layers on plates. Maintenance practices include keeping battery surfaces clean and preventing introduction of foreign materials during watering.

The acid absorption process in plates follows complex diffusion kinetics. During high-rate discharges, concentration polarization occurs as acid diffusion cannot keep pace with consumption at the electrode surface, causing voltage drop. This effect becomes pronounced above C/3 discharge rates and limits practical power delivery in starter battery applications.

Electrolyte volume design considers multiple factors including sufficient acid quantity for the desired capacity, reserve for losses over life, and space for gas recombination. Industrial batteries typically allocate 10-15% excess electrolyte volume above plate immersion requirements. The acid-to-capacity ratio averages 10-12 ml per Ah in SLI batteries and 15-20 ml per Ah in deep-cycle designs.

Advanced monitoring techniques include in-situ density measurement using ultrasonic sensors that detect sound velocity changes correlated with acid concentration. Some industrial systems employ conductivity probes for continuous state-of-charge estimation, though these require compensation for temperature and plate sulfation effects.

End-of-life electrolyte analysis provides diagnostic information. Normal aging shows gradual decrease in specific gravity due to water loss and plate sulfation. Abnormal patterns such as rapid density loss may indicate internal short circuits, while metallic contamination appears as accelerated self-discharge between measurements. Proper electrolyte management remains essential for maximizing lead-acid battery performance and longevity across automotive, industrial, and stationary applications.

The sulfuric acid system in lead-acid batteries represents a mature but carefully balanced electrochemical environment where proper maintenance directly influences energy delivery, cycle life, and operational reliability. Understanding concentration dynamics, stratification effects, and temperature relationships enables optimized battery use across diverse operating conditions.