If a lead acid battery runs out of water, meaning the electrolyte has fully dried up or the battery has been tilted or stored upside down causing the electrolyte to spill, this is the main concern.
A lead acid battery is a type of rechargeable battery that has positive and negative plates fully immersed in electrolyte, which is dilute sulphuric acid.
When a lead acid battery is drained of its acid, the wet moist negative electrodes come in contact with atmospheric oxygen, triggering an exothermic reaction that releases heat and discharges the negative plates (electrodes), oxidizing the sponge lead to lead oxide.
A lead acid battery, including flooded electrolyte types, should not have its acid completely removed once it has been filled and charged. It is important not to remove the acid. A lead acid battery consists of several major components, including the positive electrode, negative electrode, sulphuric acid, separators, and tubular bags.
Flooded electrolyte lead acid batteries do not cause thermal runaway because the electrolyte, which acts as a coolant in these batteries, helps prevent such an occurrence. Designers of flooded electrolyte lead acid batteries do not face the thermal runaway problems that are common in sealed maintenance free (SMF) or valve regulated lead acid (VRLA) batteries.
The anode is transformed into lead peroxide (PbO 2) and cathode into the spongy lead (Pb). Water is consumed and sulphuric acid is formed which increases the specific gravity of electrolyte from 1.18 to 1.28. The terminal voltage of each battery cell increases to 2.2 to 2.5V.