Some amount of electrical energy is required on most voyaging boats. However, for all practical purposes, it isn’t possible to store energy in an electrical form. Fortunately, however, it is possible to convert electrical energy into chemical form and store that.
After more than 100 years of development, the lead acid battery has become an efficient, reliable, widely available, and economical means for storing electrical energy in a reversible chemical reaction.
Today, the choices of lead acid battery types available include the lighting, starting, and ignition battery found in automobiles and trucks; medium- and heavy-duty, deep-cycle batteries designed for service on boats; and special-purpose batteries, such as traction batteries designed for electrically-powered vehicles. Of these types, the deep-cycle battery, designed to provide 400 to 2,600 discharge/charge cycles (at 50% depth of discharge per cycle) is of primary interest for vessel use. Although these three types of batteries differ in some important respects, they all share the use of lead alloy plates (actually grids, which hold the active materials), lead peroxide, spongy metallic lead, and a liquid electrolyte composed of water and sulfuric acid. Some, such as an automobile starting battery, may have sealed cell caps and carry a “no-maintenance” designation. These flooded-cell batteries are constructed with plate alloys that minimize water consumption and with a supply of electrolyte judged to be sufficient for the probable life of the battery.
In recent years, a different type of battery has become available for marine use: the sealed, unvented gel or absorptive cell battery. When initially introduced, numerous claims were made for the performance of these batteries; some were reasonable and realistic, others that were slight to extreme exaggerations. With the passage of time and the entry of additional manufacturers to the market, sealed storage batteries have won a place in the marine world. Understanding the operating mechanism of these batteries can be useful in choosing a battery appropriate for a particular application. Lead acid batteries with immobilized electrolyte have been available for more than 60 years. Safety concerns about flooded cell units in applications such as aircraft spurred this early development since the use of a gelling material prevented spills. When two electrodes, one composed of lead peroxide, the other of lead are immersed in a sulfuric acid solution, an open-circuit electromotive force (emf or voltage) of approximately 2.1 volts will be present. The precise voltage will depend on the temperature of the electrolyte and the concentration of sulfuric acid in the electrolyte. A typical six-cell battery will provide an open-circuit voltage of 12.6 volts when fully charged.
During discharge, lead sulfate (PbSOis formed at both plates as the electrolyte is converted into water (HO). When the battery is recharged, lead sulfate at the negative plate is reconverted into spongy lead (Pb), while the lead sulfate at the positive plate is reconverted into lead peroxide (PbO).
Recombining gasses
Although the basic electrochemical reaction in the gelled-electrolyte battery is little different from that in the flooded-electrolyte battery, there are significant differences in the function of the cells during the discharge/recharge process. In a flooded cell, oxygen will begin to evolve at the positive plate when the cell is approximately 70% charged, with hydrogen being evolved at the negative plate when the cell is approximately 90% charged. The evolution of these gases results in a gradual water loss and lessening of electrolyte volume. Open cells, therefore, require the periodic addition of water to make up for the amount lost as gasses during the charging process. Sealed cells cannot be permitted to evolve any significant amount of gas during normal cycling. Outgassing would result in a loss of electrolyte leading to a gradual failure of the cell as the amount of electrolyte became insufficient to completely cover the active areas of the plates.
A solution to the problem of preventing outgassing includes the use of antimony-free alloys in the construction of the plates or grids of the battery. Antimony is used in the lead alloy of flooded deep-cycle cells since it both strengthens the plate or grid and contributes to long life in deep cycle service. The problem with antimony-containing alloys in batteries is their contribution to the premature evolution of hydrogen gas at the negative plate. In flooded, open-cell types it is possible to add water to make up for the loss of electrolyte volume. In the sealed cell it is necessary that the oxygen evolved at the positive plate migrate to the negative plate through the electrolyte and the plate separators and react with the spongy lead of the negative plate. The reaction works like this: O + 2Pb > 2PbO, PbO + H + HO. Successfully done, this reaction insures that the water remains in the sealed cell. The plates of sealed, gelled-electrolyte batteries are normally alloyed with calcium (PbCa) or lead calcium tin (PbCaSn). The sensitivity of the sealed cell to even trace mounts of metal contaminants requires that the lead oxide used in the positive plate be made from pure virgin lead.
Early in the development of gelled electrolyte cells it was observed that gas recombination, where the evolved oxygen and hydrogen recombined and formed water (as outlined above), was occurring. This lead to the development of an alternative to the gelling of the electrolyte. Cells could be made in which the electrolyte was absorbed in the pores of the active material of the plates and in the electrical insulating separators between adjacent plates. The use of separators made of randomly oriented borosilicate glass microfibers contributes to the practicality of almost 100% efficient recombinant absorbed-electrolyte cells.
The material used for the plate separators can be critical to the life of the cell, especially when deep discharge conditions are encountered. When lead acid cells are discharged, some of the lead sulfate can dissolve in the water of the electrolyte, putting lead ions into solution. When the cell is recharged some of these ions are deposited on the negative plate. After repeated deep discharges these lead ions may create short circuits between the negative and positive plates. Although many sealed cells are advertised as being immune to damage due to being left in a fully discharged condition, such treatment can shorten a battery’s life. Manufacturers typically incorporate excess sulfuric acid in the electrolyte to guard against formation of significant amounts of pure water, thus preventing the lead sulfate from going into solution.
The ratio of active materialspositive plate (lead peroxide), negative plate (spongy lead) and electrolytediffers among the three types of lead acid batteries: flooded cell, gelled electrolyte, and absorbed electrolyte. In a typical flooded, deep-cycle battery, an excess of electrolyte will be present, both to insure that an adequate amount is always available for the electrochemical reaction and to prevent the active areas of the plates from being exposed to the air. In a recombinant cell the volume of electrolyte is usually about 95% of that used in the open, flooded cell. The gelled cell contains even less electrolyte, on the order of 92% of that used in the flooded cell. Due, in part, to the decreased mobility of the acid in the electrolyte in the gel or absorbed cell, a manufacturer may choose to use an increased number of thinner plates compared to a deep-cycle, flooded cells.
Lower self-discharge
One of the advantages of the gelled- or absorbed-cell construction, largely resulting from the exclusion of antimony from the plate alloy, is a reduction in the loss of stored energy due to self-discharge. A typical sealed-cell battery will self-discharge only 3% to 6% per month when stored at a temperature of 68° F. Storage at elevated temperatures, such as 95° F, can result in capacity losses reaching 10% per month. These self-discharge rates are small compared with those encountered with flooded-electrolyte cells, which can reach levels in excess of 20% per month at 95° F.
As with all physical systems, there are trade-offs when comparing various lead acid battery systems. When a gelled electrolyte or recombinant cell is compared with a flooded cell in extended discharge service, such as over a five-hour period, the relative capacity will likely show that the flooded cell performs best. A typical absorbed-electrolyte cell will provide approximately 75% to 80% as much output while the gelled-electrolyte cell yields 68% to 75% as much as the flooded cell. When high rates of discharge for short time periods are needed (engine starting), the absorbed-electrolyte cell will perform best, with the flooded electrolyte cell likely second, and the gelled cell a close third in output.
Any sealed cell construction is particularly sensitive to overcharging. When a flooded cell is subjected to excess charging, outgassing will occur at both plates, with resulting loss of electrolyte volume. Since the cell is open, it is simple to add pure water, restoring the electrolyte volume. So long as the active area of the plates is not allowed to dry and the temperature in the cell is kept low enough to prevent mechanical damage, the cell will be reasonably tolerant of an occasional overcharging. With a sealed cell the possibility exists for evolution of oxygen and hydrogen at rates which cannot be recombined rapidly enough to prevent an increase in pressure within the sealed cell. Should the pressure become sufficient, the pressure-relief valve fitted to the cell will open, venting the excess pressure and likely expelling some of the electrolyte. Since the cell has no filler caps, it’s impossible to replace any expelled electrolyte. An electrolyte deficit will seriously diminish the life of the battery. Because of this, sealed cell batteries require care to prevent damage and premature failure.
Even with the danger of overcharging outlined above, absorbed-electrolyte- or gelled-cell batteries can still be recharged more rapidly than a flooded deep-cycle battery. When charging currents flowing into fully discharged batteries are compared, at identical temperatures and charging voltages, the sealed cell battery (gelled or recombinant type) may initially accept 1.6 times the current accepted by the flooded cell battery. After 60 minutes of charging, the sealed types will likely accept charging current equal to or perhaps slightly less than that accepted by the flooded battery. Sealed-cell batteries, therefore, offer advantages in speedier recharging. Another advantage of the sealed cell battery is the elimination of need to periodically apply a finishing or equalization charge. In fact, since this type of charge requires application of a charging voltage considerably above that normally used when charging, exposure to such a process would likely lead to failure.
Cells unavailable
One of the unavoidable consequences of most sealed-cell battery constructions is one’s inability to check on individual cells. Since all six cells in a 12-volt battery are connected in series, a single weak or inoperative cell can compromise the capability of the entire battery. Open cell, flooded batteries may be checked with a battery hydrometer that measures the specific gravity of the electrolyte in each cell, and thus its state of charge. Where batteries are constructed of individual cells, with accessible intercell connections, it is possible to measure the voltage of each cell, thereby obtaining knowledge of the condition of each cell. Unfortunately, sealed-cell batteries offered for marine use do not offer access to each of the cell interconnections. Sealed units will operate in any position, an asset if it is impossible to install the battery with the terminals on top. The sealed battery will accept a recharge more rapidly, but its service life will likely be shorter than that of a properly maintained flooded battery. The impossibility of checking the health of individual cells of the sealed battery may rule out its use in some applications (although it is possible to construct a 12-volt sealed battery from six individual sealed cells, thereby permitting checking of individual cell state with a digital voltmeter). Proper choice, maintenance, and use of a battery can be critical to maritime operations. It is best to carefully consider all aspects of each type of battery performance before making a choice. Choosing a product from a quality manufacturer is also vital to success.
