When people refer to “battery acid,” they mean the electrolyte solution found inside a lead-acid battery, which powers most vehicles. This solution is a mixture of water and sulfuric acid. While the term “evaporation” is often used to describe fluid loss, the highly stable sulfuric acid component does not easily dissipate under normal operating conditions. It is the water component of the electrolyte that leaves the system, and this loss occurs primarily through a chemical process far more significant than simple thermal evaporation.
Understanding the Electrolyte: Sulfuric Acid and Water
The liquid within a conventional flooded lead-acid battery is an aqueous electrolyte, typically composed of about 35% sulfuric acid and 65% water by weight when fully charged. This specific ratio is calculated to optimize conductivity and chemical reactivity with the lead plates, facilitating the reversible chemical reactions that store and release electrical energy. The two components possess vastly different chemical properties that dictate their behavior within the battery environment.
Water is a volatile compound, meaning it readily transitions into a gaseous state, especially when temperatures rise or when subjected to electrical current. Sulfuric acid, however, is a dense, non-volatile mineral acid with a significantly higher boiling point.
Under the typical temperatures experienced by a car or deep-cycle battery, the sulfuric acid component remains securely liquid. It requires temperatures well over 550°F (288°C) to vaporize, which is far beyond the normal operating range. This confirms that any significant decrease in volume is almost exclusively the loss of water from the electrolyte mixture, not the acid itself.
The Process of Water Loss: Gassing During Charging
The primary mechanism for water loss in a flooded lead-acid battery is not simple thermal evaporation but a process called electrolysis, which is commonly referred to as “gassing.” This event occurs specifically when the battery is being recharged, particularly as its voltage rises and it approaches its fully charged state of 100%. Once the battery is near full capacity, the electrical energy delivered by the charger is no longer efficiently converted into chemical energy for storage on the plates.
Instead, the excess electrical energy begins to decompose the water molecules within the electrolyte solution. This process splits the water into its constituent elements: hydrogen gas is released at the negative electrode, and oxygen gas is released at the positive electrode. This decomposition reaction requires a certain minimum voltage, often around 2.4 volts per cell, and is a direct result of the continuous application of current, especially in scenarios involving overcharging.
These newly formed hydrogen and oxygen gases then rapidly bubble up through the electrolyte solution. Since they are light gases, they escape into the atmosphere through the battery’s vent caps, leading to a measurable and often rapid decline in the overall electrolyte volume. This gassing phenomenon causes much faster water loss than the comparatively slow process of simple thermal evaporation due to ambient heat.
The rate of gassing is directly proportional to the charging voltage and the internal temperature of the battery. For example, a battery that is consistently overcharged or one operating at an elevated temperature, such as 113°F (45°C), will experience a significantly greater rate of water loss compared to one maintained at a standard 77°F (25°C). While this loss is a normal byproduct of charging a flooded lead-acid battery, excessive gassing indicates an overcharge condition or a regulator malfunction that must be corrected to preserve the battery’s longevity.
Effects of Reduced Electrolyte Levels on Battery Health
When the water loss through gassing is left unaddressed, the level of the electrolyte drops below the top edge of the lead plates and separators within the battery cells. The portion of the plate that is no longer fully submerged in the electrolyte becomes chemically inactive and is exposed to air. This exposure leads to the hardening of the lead sulfate crystals, a condition known as permanent sulfation.
The sulfated area of the plate can no longer participate in the reversible chemical reactions necessary for storing and releasing electrical energy, which results in a permanent and irreversible reduction in the battery’s overall capacity. Furthermore, a lower electrolyte level means that the fixed amount of sulfuric acid is now dissolved in a smaller volume of water. This significantly increases the specific gravity and concentration of the remaining acid solution.
Highly concentrated sulfuric acid is far more corrosive and aggressive toward the internal components than the properly diluted solution. The increased concentration accelerates the corrosion rate of the positive lead grid structure, leading to premature mechanical failure and the eventual complete death of the battery. This corrosive effect is compounded by the fact that the exposed plates can also suffer from rapid oxidation.
The combination of permanent plate sulfation and accelerated grid corrosion severely compromises the battery’s ability to hold a charge and deliver power, thereby drastically shortening its service life. To mitigate this damage, the appropriate action is to replenish the lost volume by adding only distilled or deionized water. It is important to add water and not more sulfuric acid, since only the water component was lost through gassing. This restores the electrolyte to its intended operating specific gravity and ensures the entire plate surface is utilized.