6. Lead Acid Battery Advances
The development of the lead-acid battery has come some way since Gaston Planté’s raw invention in 1859. Progress in Planté’s design saw the inclusion of multiple cells composed of alternating negative and positive plates suspended in electrolyte with wooden walls between the cells. By 1910, lead-acid batteries were made by using asphalt-coated and sealed wooden containers, thick electrode plates, wooden cell separators between the negative and positive plates and connections between cells made through the cover using heavy lead posts and connections.
The first important change arrived in the early 1920s when a more acid-resistant, hard rubber case became de rigueur. Basic battery construction changed little during the next 30 years. Active-material performance was enhanced by use of additives (including expanders) and through improvements in raw materials.
Industrial Manufacturing Brings Advances
Increases in the efficiency of the manufacturing process were later achieved, including the introduction of machine pasting of plates. During the late 1950s, epoxy sealed one-piece covers were introduced. The case and cover material remained hard rubber, and connections between cells were still made through the cover. Lower-resistance separators made of phenolic resin-impregnated cellulose fibre came into use and significantly raised the electrical performance of cells. Battery plates stacked mechanically became common practice, reducing the manual labour, hence, lowering costs involved in lead battery manufacture.
In the early 1960s, a technique for connecting the cells within a battery in series through the cell walls was developed. Simultaneously, a method for automatically and efficiently joining plates of the same polarity within a cell element was devised. As a result, the battery’s internal resistance and the amount of connecting lead were both significantly reduced.
Battery Design Comes To The Forefront
Significant advances were also made in plate design and production techniques that gave rise to more efficient batteries with higher energy density. The late 1960s saw the introduction of injection-moulded polypropylene cases and covers providing the lead battery with a durable, thin wall, lighter weight container. The thinner outside walls and cell partitions permitted the use of more active material at no sacrifice of weight or volume of the battery. Very durable and low resistance separators became available as a final step of improvement in the flooded battery design providing for further increased battery life.
Meanwhile, a dramatic change was in the wind. The classic flooded lead-acid battery design contains the electrolyte medium as an unbounded liquid filled to a level above the top of the plates and over the busbars. Consequently, the cells must be vented to release gases (oxygen at the positive and hydrogen at the negative electrodes) liberated during charging. Not only is water lost to the venting (and thus must be replaced regularly), but the battery can be safely used only in the upright position, or spillage of the sulfuric acid electrolyte solution occurs. Also, the vented gases carry a fine mist of sulfuric acid that is highly corrosive and unhealthy in the environment.
Valve Regulated Technology Arrives
Efforts were made to develop sealed batteries not demanding topping up with water and would be safe under conditions of use which would not be abnormal for the applications in which they were deployed. These efforts culminated with the development of the valve-regulated lead-acid (VRLA) battery. The first commercial units of which were introduced by Sonnenschein in the late 1960s and by Gates Energy Products in the early 1970s.
These were the gel and absorptive glass mat (AGM) technologies, respectively. In the VRLA design, oxygen evolved during charging passes through a gas space to the negative electrode, where it is reduced (recombines) back to water. This is known as the internal oxygen-recombination cycle. There are two alternative techniques for providing the needed gas space. One design has the electrolyte immobilised as a gel within the cell; the other has the electrolyte held within AGM separators. Gas passes through clefts in the gel or through channels in the AGM separator.
Because a corresponding recombination cycle for hydrogen is not possible, and the fact that oxygen recombination is not complete (the efficiency is typically 93% to 99%), each cell is fitted with a one-way valve as a safeguard against catastrophic pressure build-up. Hence, the term valve-regulated. The VRLA battery can be employed in any orientation and is more resistant to being jostled in use.