A lead acid battery consists of electrodes of lead oxide and lead are immersed in a solution of weak sulfuric acid. Potential problems encountered inlead acid batteries include:
Gassing: Evolution of hydrogen and oxygen gas. Gassing of the battery leads to safety problems and to water loss from the electrolyte. The water loss increases the maintenance requirements of the battery since the water must periodically be checked and replaced.
Damage to the electrodes. The lead at the negative electrode is soft and easily damaged, particularly in applications in which the battery may experience continuous or vigorous movement.
Stratification of the electrolyte. Sulfuric acid is a heavy, viscous liquid. As the battery discharges, the concentration of the sulfuric acid in the elecotrolyte is reduced, while during charging the sulfiric acid concentratin increases. This cyclicing of sulfuric acid concentration may lead to stratification of the electrolyte, where the heavier sulfuric acid remains at the bottom of the battery, while the less concentrated solution, water, remains near the top. The close proximity of the electrode plates within the battery means that physical shaking does not mix the sulfuric acid and water. However, controlled gassing of the electrolyte encourages water and sulfuric acid to mix, but must be carefully controlled to avoid problems of safety and water loss. Periodic but infrequent gassing of the battery to prevent or reverse electrolyte tratification is required in most lead acid batteries in a process referred to as "boost" charging.
Sulfation of the battery. At low states of charge, large lead sulfate crystals may grow on the lead electrode as opposed to the finely grained material which is normally produced on the electrodes. Lead sulphate is an insulating material xxx.
Spillage of the sulfuric acid. If sulfuric acid leaks from the battery housing it poses a serious safety risk. Gelling or immobilizing the liquid sulfuric acid reduces the possibility of sulfuric acid spills.
Freezing of the battery at low discharge levels. If the battery is at a low discharge level following the conversion of the whole electrolyte to water, then the freezing point of the electrolyte also drops.
Loss of active material from the electrodes. The loss of active material from the electrodes can occur via several processes. One process that can cause a permanent loss of capacity is the flaking off of the active material due to volumetric changes between xxx and lead sulphate. In addition, xxx. Improper charging conditions and gassing can cause shedding of active material from the electrodes, leading to a permanent loss in capacity.
Depending on which one of the above problems is of most concern for a particular application, appropriate modifications to the basic battery configuration improve battery performance. For renewable energy applications, the above problems will impact the depth of discharge, the battery lifetime and the maintenance requirements. The changes to the battery typically involve modification in one of the three basic areas: