What happens if a lead-acid battery runs out of water?
By the author of the Amazon Bestseller Book 'Batteries Demystified', Podcaster, & Expert in Lead Acid Battery Manufacturing Processes and Machines
A lead-acid battery has positive and negative plates fully immersed in dilute sulfuric acid electrolyte, which is crucial in the battery's charge and discharge reactions.
The concentration of electrolyte is defined and specified for different battery applications. The concentration is based on the application and aligns with national and international standards. This concentration is measured and typically expressed as specific gravity, though it is also expressed as a percentage in some rare circumstances.
This specific gravity is usually ensured at the design stage by battery manufacturers who also consider the volume of electrolyte that can be accommodated in a cell. The specific gravity is always in a defined range, with the maximum specific gravity specified in such a manner so that it does not accelerate corrosion of battery components & allows for release of sulphate from the plates when a battery is on charge. The minimum specific gravity is also such that it retains the electrolyte's conductivity, so there are no problems charging a fully discharged battery.
Thus, the volume & concentration of electrolyte form part of the battery design.
The active materials of battery plates are decided based on the quantity of active materials and surface area of the plates, which decides battery capacity.
When the level of battery electrolyte reduces to an extent that the top portion of the plates is exposed, a situation is created wherein a particular portion of the plates does not take part in the reaction. This leads to a reduction in battery capacity. This is undesirable; hence, allowing the battery to run out of water is not recommended.
Regular topping up with distilled or demineralized water maintains the electrolyte level. The evaporation of the water component of the battery electrolyte has to be compensated for by topping up with water at periodic intervals.
Another effect of reducing electrolyte due to water evaporation is increasing the concentration of electrolytes, i.e., increasing their specific gravity. An increase in the specific gravity of the electrolyte, especially when the plates are not fully immersed in it, results in heating the cell during charging. The battery can get damaged since corrosion of internal components used in battery manufacturing is accelerated in the acidic electrolyte at elevated temperatures.
A physical effect of reducing water is heating up, especially during the last stages of charging or undesired overcharging.
Electrolyte also acts as a coolant, though this is often not considered the primary purpose of their presence in a battery.
Ventilated flooded electrolyte batteries can better deal with the consequences of water loss than a sealed maintenance-free (SMF) battery. These flooded design batteries sometimes have abundant electrolyte and surely have sufficient electrolyte to withstand a certain amount of abuse as a result of water loss.
The problems of thermal runaway faced by sealed maintenance-free (SMF) or valve-regulated lead-acid (VRLA) batteries are not a phenomenon to be addressed by the designers of flooded electrolyte lead-acid batteries. The sealed maintenance-free batteries are starved electrolyte batteries and are sensitive to the extent that they can get damaged beyond repair if the battery is overcharged or abused.
Finally, I'd like to address the moot question of what happens when a lead-acid battery runs out of water, totally, i.e., the electrolyte has thoroughly dried up, or the battery has been tilted or stored upside down, causing the electrolyte to spill.
Please note that we must not remove the acid completely from flooded electrolyte lead-acid batteries once they have been filled with acid and charged.
A lead-acid battery consists of a few major components: the positive electrode, the negative electrode, sulphuric acid, separators, and tubular bags. In a charged condition, the positive electrodes are lead dioxide, and the negative electrodes are sponge lead. Sponge lead is highly reactive to moisture and oxygen and is converted to lead oxide. It is discharged and heated in the conversion to lead oxide.
Hence, it is necessary to ensure that the acid is not spilled or drained from a wet battery once it is filled and charged. This is very important, and draining an electrolyte from an acid-filled Lead-Acid battery can harm battery life.
When a battery filled with acid is drained of acid, the wet, moist negative electrodes come in contact with atmospheric oxygen. An exothermic reaction takes place, releasing an enormous amount of heat, discharging the negative plates (electrodes), and oxidizing the sponge, leading to lead oxide.
During this exothermic process of heating the negative electrodes, the other components within the cell, i.e., separators, tubular bags, plastic components like bottom bars, vent plugs, cell covers, and rubber bushes fitted to cell covers, get deformed, degraded, or damaged.
It is possible to revive such batteries if they are not fully damaged, but their life and performance are adversely affected. Hence, ensuring no acid spillage from fully charged batteries is crucial.
However, the loss of electrolyte from the top of the plates in the normal course without exposing the electrodes is different and cannot be equated to spilling.
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