The history of the lead acid battery has been one of constant improve-ments — very rarely has it been in huge leaps forward but mostly it’s been slow and steady modifications. Or that was until the VRLA battery arrived and the challenges it threw up. By David Rand
Moving on from one iteration to the next in lead battery performance
Gustave Planté’s invention of the lead acid battery came at an opportune time, the availability of industrial-scale electricity was accompanied by a rapid expansion in lead acid manufacture.
A decisive step in the commerciali-zation of the lead acid battery was made by Camille Alphonse Faure who, in 1880, coated the lead sheets with a paste of lead oxides, sulfuric acid and water.
On curing the plates at a warm tem-perature in a humid atmosphere, the paste changed to a mixture of basic lead sulfates which adhered to the lead electrode. During charging the cured paste was converted into elec-trochemically active material (or the active mass) and thereby gave a sub-stantial increase in capacity compared with the Planté cell.
Soon the idea developed of cut-ting rectangular holes out of the lead plates to lighten their weight and also to provide receptacles into which the paste could be packed. So was born the modern pasted-plate battery which is by far the most common type of lead acid battery in use today.
The first major market was for stand-by batteries to provide emer-gency power to essential equipment in electricity-generating stations and at other critical sites. For such large battery applications, it is notable that no other battery chemistry has been able to compete on cost grounds with the lead acid system. Towards the end of the 19th century, electric cars ap-peared on the roads and were pow-ered mostly by lead acid.
Batteries also began to be used for illumination in railway coaches as well as for powering railway signal-ling systems, the electrical equipment of ships, and radio receiving-transmit-ting equipment. With the advent of the internal-combustion engine, the lead acid battery was first employed in road vehicles for lighting, then later also for engine starting, and now ad-ditionally for the whole range of elec-trical duties expected in the modern vehicle.
The market for off-road traction batteries has also expanded and in al-most all cases it is the lead acid system that predominates when the require-ment is for stored energy of more than a few hundred watt-hours.
By 1910, the construction of lead acid batteries involved the use of an asphalt-coated and sealed wooden container, wooden separators, thick plates, and inter-cell connections made through the cover by the use of heavy lead posts and links.
The first important change came in the early 1920s when the more acid-resistant, hard rubber case was devel-oped and came into use. During the next 30 years, basic battery construc-tion changed little, although active-material performance was enhanced through the use of additives and through raw material improvements.
Significant advances were also made in grid technology — it’s worth not-ing that in 1881, J Scudamore Sellon had demonstrated the appreciable me-chanical and electrochemical benefits to be gained by replacing the pure-lead grids of Faure plates with lead antimony counterparts).
Increases in the efficiency of the man-ufacturing process were also achieved during this period, especially following the introduction of machine pasting of plates. During the late 1950s, one-piece covers that were epoxy sealed to the cases were introduced. The case and cover material, however, remained hard rubber and inter-cell connections were still made through the cover.
Lower-resistance separators, which were made of phenolicresin-impreg-nated cellulose fibre, also came into use and obviously raised the electrical performance of cells. Machine stacking of battery plates became common and thereby reduced the level of manual labour involved in battery manufacture.
In the early 1960s, a method was de-vised for automatically joining plates of the same polarity within a cell element. Simultaneously, a technique for connecting the cells within a battery in series through the cell walls was devel-ped. This markedly reduced both the internal resistance of the battery and the amount of connecting or ‘top’ lead needed.
Major advances were also made in plate design and production techniques that gave rise to more efficient batteries with high specific power. In the late 1960s, the injection-moulded polypro-pylene case and cover were introduced and gave the lead acid battery a dura-ble, thin wall, lightweight container.
Moreover, the thin outside walls and cell partitions permitted the use of more active material without increas-ing the external weight or volume of the battery. Finally, the performance and life of the batteries were both enhanced through the availability of low-resistance, highly durable, plastic separators.
Meanwhile, a technological explo-sion was waiting in the wings. Classical lead acid batteries are flooded systems. That is, the electro-lyte medium is a free liquid to a level above the top of the plates and above the busbars. This has the disadvan-tage that the cells have to be vented to release the gases liberated during charging, namely, oxygen at the posi-tive electrode and hydrogen at the negative.
As a consequence, not only is water lost (and thus has to be replaced by reg-ular maintenance operation), but also the battery may be used only in the up-right position, otherwise leakage of the sulfuric acid solution takes place.
Also, the released gases carry a very fine mist of sulfuric acid that is highly corrosive. Thus efforts were made to develop sealed batteries that would not require topping up with water and would be safe under all conditions of use and abuse. At first, such attempts revolved around the catalytic recombination of the gases within the battery, but this idea proved to be impractical.
Success came, however, with the in-vention of the valve-regulated lead acid (VRLA) battery. The first commercial units were introduced by Sonnenschein in the late 1960s and by Gates Energy Products in the early 1970s.
These were, respectively, the gel and absorptive glass mat (AGM) technolo-gies. In the VRLA design, oxygen evolved during charging transfers through a gas space to the negative electrode where it is reduced (recombined) back to water.
This process is known as the in-ternal oxygen-recombination cycle. There are two alternative techniques for providing the gas space. One cell design has the electrolyte immobilized as a gel; the other has the electrolyte held within an AGM separator. Gas passes through fissures in the gel, or through channels in the AGM.
A corresponding recombination cycle for hydrogen is not possible because oxidation of the gas at the positive electrode is far too slow. This feature, together with the fact that oxygen recombination is not com-plete (the efficiency is typically 93% to 99%), requires each cell to be fitted with a one-way valve as a safeguard against excessive pressure build-up — hence, the term valve-regulated.
The VRLA battery can be employed in any orientation, and thus gives equipment design engineers a much greater degree of flexibility. Antimony is not included in the grid alloys of VRLA cells because this element low-ers the hydrogen over-potential and therefore encourages gassing at the negative electrode during charging.
Care must be taken against the intro-duction of other elements that might act similarly. Today, lead calcium tin alloys are preferred by manufacturers of VRLA batteries for float duties, and lead tin for cycling applications. Initially, there was a vexatious prob-lem to be solved, namely, the propen-sity of batteries employing non-anti-monial grids to suffer a rapid loss of capacity early in the projected life of the cell, particularly under deep-dis-charge conditions.
It was found that the adverse be-haviour, loosely termed the ‘anti-monyfree effect’, originated from the positive plate. Given the serious commercial ramifications of the ef-fect, both the lead suppliers and the battery industry soon recognized the need for a consolidated programme, coupled with a forum for the global exchange of ideas on how to eliminate the problem.
This prompted the successive for-mation of the Asian Battery Confer-ence in 1986 (organized by the Aus-tralian lead industry), the European Lead Battery Conference in 1988 (organized by the Lead Development Association, LDA), and the LABAT Conference in 1989 (organized by the Bulgarian Academy of Sciences) — all of which continue to this day.
The knowledge and advice gathered via the scientific and technological network that evolved from the above three conferences proved invaluable to the International Lead-Zinc Research Organization (ILZRO) in establishing the Advanced Lead Acid Battery Con-sortium — ALABC — in March 1992.
Its prime purpose was to combat the threat from the alternative battery sys-tems that were to be developed under the management of the US Advanced Battery Consortium (USABC) follow-ing legislation in California for the im-plementation of zero-emission vehicles (ZEVs). The battery specifications for such vehicles were set by the USABC and included a target life of 500 cycles under the Simplified Federal Urban Driving Schedule (SFUDS) — a performance that VRLA batteries could not achieve at that time.
Consequently, in 1992, a joint ILZ-RO LDA meeting was held during the Third European Lead Battery Confer-ence to develop a consolidated strategy in search of a remedy for the antimo-nyfree effect (which had, in fact, first been diagnosed by Jeanne Burbank way back in 1964!
Since it had been found that batter-ies using lead-antimony alloys with antimony contents < 2 wt.% were also subject to the phenomenon, it was de-cided to refer to the effect as premature capacity loss (PCL), a term that had been recommended by CSIRO’s Tony Hollenkamp in the previous year. The ALABC subsequently formed a PCL Study Group, which first met at the Second LABAT Conference in 1993.
After robust discussion, the compet-ing theories of PCL were defined and grouped under two categories, namely:
• PCL-1, caused by deleterious events at the positive grid | active-material interface;
• PCL-2, caused by gradual inactiva-tion of the active-material itself.
Eventually, a unified explanation of PCL was developed, in which capac-ity loss falls on a continuous scale. The position where a cell lies on this scale is determined by the rate and location at which the connectivity of the active material (ie the apparent density) de-clines to the critical value where con-ductivity is compromised.
The final key to solving the PCL puz-zle, therefore, was to squeeze more life into cells via a controlled level of separator compression to minimize positive-plate expansion. Apparatus such as the CSIRO piston cell was developed to determine the optimum conditions for a given type of AGM separator.
The results of the studies, together with other improvements in cell design, enabled VRLA batteries to meet cycle-life targets. Thus, in 1995, Pat Moseley, the manager of the ALABC, was able to state confidently that: “PCL is in re-treat.”