Without a battery, the electrical system on a boat is just so much iron, copper, plastic, and miscellaneous bits of silicon. Given the variety of battery types available, what are the advantages and disadvantages of different battery systems?
The chemical systems used in today’s storage, or rechargeable, batteries include lead-acid, nickel-iron-alkaline (Edison cell), nickel-cadmium, nickel-metal-hydride, and lithium-ion. The lead-acid battery is the overwhelming choice for most storage battery applications on boats.
Although the Edison cell was once popular for electric vehicles it is not often used today. Liquid electrolyte (wet) nickel-cadmium batteries are widely used in aircraft applications. They are valued for their ability to deliver a massive amount of current for turbine engine starting (more than 1,200 amps from a 32-amp-hour battery) and then accept a charge current on the order of 450 amps. Unfortunately, they are very expensive (more than $3,000 for a typical 28-volt, 32-amp-hour battery) and require special maintenance. Nickel-metal-hydride and lithium-ion systems offer spectacular energy storage efficiency, but their costs make them totally impractical for other than the small sizes appropriate for portable computers and cellular phones. The lead-acid system is the only economically practical choice for voyaging boats.
Each type of electrochemical reaction has its own unique voltage. For example, the voltage from a fully charged lead-acid cell is approximately 2.1 volts. Batteries are composed of a number of individual cells, usually connected in series. Six such cells connected in series produces a battery whose nominal voltage will be 12.6 volts (immediately after charging, cell voltage may be slightly higher).
Each cell of a lead-acid storage battery contains two electrodes, often referred to as plates. The plates are immersed in an electrolyte composed of dilute sulfuric acid. These electrodes are not simple sheets of lead. They are mechanically complex structures that have pockets filled with active materiallead peroxide for the positive plate and finely divided metallic lead, called spongy lead, for the negative plate. The magnitude of the electrical current that the cell can deliver (maximum current into a load) depends on the total surface area of the active material exposed to the electrolyte. The amount of energy that a cell can store depends on the total amount of active material in the plates.
One of the unavoidable aspects of lead-acid chemistry is the formation of lead sulfate on the plates, referred to as sulfation. Lead sulfate is an insoluble electrical insulator, and if allowed to remain on the plates it will harden to the extent that reconversion into lead peroxide or metallic lead will be inefficient or even impossible. For this reason, prompt recharging of lead-acid batteries is always preferred, even for battery types that claim immunity to damage from being left in a discharged state.
In the event a battery becomes sulfated, it may be possible to regain a measure of future use by a recharging process in which a voltage much higher than normal is applied, often for periods approaching a week. A standard battery charger is not capable of performing this special recharging processit has to be done by a battery specialist. And it should be noted that batteries subjected to this rescue process will have a greatly diminished energy storage capacity.
Two types of battery service
The two major classifications of boat batteries are: starting, lighting, and ignition (SLI) and deep cycle (the type best suited for marine service). The defining difference between the SLI and deep-cycle batteries is the ability of the first battery to provide extremely high current for a short time for engine starting versus the second type’s satisfactory life under cycling service, where, in each use cycle, a significant amount of the total energy stored in the battery is withdrawn before the battery is recharged.
Delivery of high-surge current (engine starting) requires maximum plate surface area. So an SLI battery has a large number of thin plates. Deep-cycle cells use fewer, thicker plates that can better withstand the severe stress imposed by this type of service. The contrast between the cycling capability of SLI and deep-cycle designs is striking. An SLI battery subjected to cycling service (where 50% of the energy storage capacity of the battery is withdrawn before recharging) will rarely survive more than 50 to 70 cycles. Deep discharge actually physically damages this type of battery. On the other hand, medium-duty deep-cycle batteries will provide 200 to 500 cycles; high-quality, heavy-duty, deep-cycle batteries can yield more than 2,500 service cycles; and very-heavy-duty, deep-cycle batteries can yield more than 3,000 cycles.
Obtaining economical service life from a deep-cycle battery is best achieved by limiting the depth of discharge on each cycle. A nominal limitation of 50% discharge is a reasonable compromise between lengthening the time between recharges and the battery’s ultimate service life. In addition, although many of the electronics on a boat may function properly with an input voltage as low as nine volts, the operation of radio transmitters, incandescent lights, and motors will be inefficient below about 12.2 volts.
This is not to condemn the SLI battery. It simply is not designed for deep-cycling service. Similarly, the fact that the heavy-duty deep-cycle battery can outlast a medium-duty battery by a factor of two or three does not mandate the exclusive use of these more costly batteries in all marine applications. For boats used only 50 to 100 days per year, a medium-duty deep-cycle battery can be the best choice. With proper care, replacement will be needed about every three to five years.
Maintaining electrolyte levels
Regardless of the type of lead-acid battery used, or the nature of the electrolyte, the cells must be kept properly filled. The desired chemical reaction cannot occur unless the plates are immersed in the electrolyte. Any portion of the plates not immersed cannot engage in the electrochemical reaction and if allowed to dry will suffer permanent damage. This need for maintenance of electrolyte level has been a driving force in the development of various types of sealed, no-maintenance batteries.
Eliminating or minimizing the need to inspect and maintain the electrolyte level in the conventional, flooded-cell battery has long been the desire of many battery users and manufacturers. Water loss from the electrolyte during the final stages of the charging process is the result of hydrolysis, in which the flow of current separates the water into its component parts, oxygen and hydrogen. In a battery, this process is called outgassing. A degree of outgassing is inevitable in the final stages of charging of all lead-acid batteries. Some batteries have special cell caps, called Hydrocaps, that incorporate a catalyst capable of recombining the hydrogen and oxygen gases into water. (If water does need to be added to a battery, only distilled water should be used.) Although these Hydrocaps and, in some cases, automatic watering systems, can solve some maintenance problems, a need still exists for truly low-maintenance batteries.
For the SLI battery, a change in the alloying of the lead in the plates, switching from antimony to calcium, greatly reduces outgassing and also decreases the self-discharge rate of the battery (all batteries slowly lose charge over time due to internal chemical reactions and the slight leakage current that may flow across a top surface that is less than totally clean). The change to calcium in the plate alloy, coupled with mechanical designs that provide for a greater depth of electrolyte over the top of the plates, allowed the introduction of “low-maintenance” and, later, “no maintenance” SLI batteries for cars and trucks. These batteries have vented cells and do lose some water during use. However, the amount of electrolyte installed when they are manufactured is usually sufficient for the life of the battery.
Gel cell batteries
The need for a no-maintenance, light- to medium-duty deep-cycle or float-service battery (in which the battery is constantly on charge at a low current until it is required to deliver energy, as in an emergency lighting system) was initially addressed in the 1930s. The loss of water from the electrolyte is minimized by use of low-gassing designs and by mixing the electrolyte with a gelling agent, which promotes the recombination of emitted gas into liquid water within the cell. Since it is possible for a malfunctioning or mistreated battery to emit gas in quantities greater than its ability to recombine them into water, pressure relief valves are fitted to each cell. The battery industry refers to this type of battery as a valve-regulated lead-acid type (VRLA). Since, in normal operation, the valves never open, the VRLA gel-cell battery may be used in any orientation, although a terminal-up position is recommended by virtually all manufacturers.
The electrolyte in gel-cell batteries may contain a small amount of phosphoric acid. This strengthens the positive plate and reduces the shedding of active material, which is a part of the aging process for all lead-acid batteries. The gel cell’s chemistry provides superior resistance to damage from progressive hardening of sulfate on the plates if they are left in a discharged, or sulfated, state. Often, gel batteries are built with a greater number of thinner plates than are used in conventional, flooded-cell deep-cycle batteries. The additional surface area reduces the internal resistance of the battery and allows higher recharge current than is normal for deep-cycle batteries. As with most technical advantages, there are corresponding limitations. As will be seen, no gel cell can match the life-cycle performance of heavy-duty or very-heavy-duty deep-cycle flooded-cell batteries.
Absorbed glass mat units
An alternative type of VRLA battery has been developed that does not use a gelled electrolyte. This battery is usually referred to as an absorbed glass mat (AGM) design. All of the electrolyte is absorbed in the material that separates the plates of the battery. The separators are made of a very fine fiberglass filament material. This material is an excellent electrical insulator, is mechanically strong, and acts like a sponge, retaining the electrolyte in the spaces of the material. AGM batteries offer a number of the operational advantages of the gel-cell types while avoiding some of the limitations imposed by the use of the gelling agent. In the gel cell, the mobility of the ions in the electrolyte is slowed by the presence of the gelling agent. The higher mobility of the electrolyte in the AGM reduces the internal resistance of the cell, allowing delivery of high peak currents. The lower internal resistance of the AGM design reduces the heating effect that accompanies rapid charging, permitting the application of a charge current larger than usual without risk of damage to the cell.
In gel-cell batteries, the presence of the gelling agent reduces the active chemical content of the electrolyte in the cell by about 10% when compared with the AGM design. Note that both the gel cell and the AGM must operate with somewhat less electrolyte than used in the flooded cell. Voids must be left within the cell’s electrolyte to allow space for the gas recombination process.
All AGM cells are not equally suited for the deep-cycle service required on a boat. The majority of the AGM batteries produced by the major manufactures are specifically intended for service in applications such as non-interruptible power supplies for computers, where they are on a float charge except when the main power source is interrupted. Other applications for these AGM batteries include telephone systems, security systems, emergency lighting, radios, and as engine starting batteries for emergency generator sets. These AGM batteries are not suitable for use in marine deep-cycle applications.
One manufacturer of AGM batteries, Concorde, markets an AGM battery that is suitable for marine use under the Lifeline name. These batteries are the result of work done by Concorde in developing a lead-acid battery that could replace the liquid-electrolyte nickel-cadmium batteries used in some civilian and military aircraft. The requirements for this service included robust mechanical construction, the ability to deliver high peak current, the ability to accept high recharge current (approaching but perhaps not equal to a ni-cad), and reasonable cycling life. The result of this engineering effort is a battery that the manufacturer claims can provide approximately twice the cycle life of a typical gel battery (claimed life of 1,000 cycles at 50% discharge versus approximately 400 cycles for the gel battery) and can withstand a charging current approximately equal to the battery’s amp-hour rating (initially, 100 amps for a 100-amp-hour battery).
It is also interesting to note that unlike gel-cell batteries, which prohibit equalization charging, the periodic equalization charge recommended for flooded-cell batteries is also recommended for this specific type of AGM battery. (An equalization charge is intended to bring all of the cells of a battery up to the same state of charge, a condition not always achieved in normal charging. The performance of a battery is limited by the condition of the weakest cell in the string. During an equalization charge, the battery is charged at a voltage above its normal gassing potential, using a current-limited power supply. Equalization is an important part of proper care for flooded cell batteries.)
A variation on standard, flat-plate AGM batteries that uses cylindrically wound plates is produced in England for marine use. In this type of battery, the flat, individual plates are replaced by winding a pair of long strips of plate material, with a glass fiber separator, into a cylindrical form. This design approach offers excellent density and large plate surface area. It is possible that batteries using this type of construction will become popular in the U.S. market.
All lead-acid batteries are sensitive to operating temperature. Batteries are normally rated at 77° F. Ideal operating temperatures are between 70° F and 80° F. Battery life, especially for most VRLA gel types, is adversely affected by temperatures above 76° F. Temperature effects can be critical when the battery is being charged. All batteries have a certain amount of internal resistance. Typically, as the temperature of the battery increases, the internal resistance decreases. With flooded, open-cell batteries, excessive charge current will result in excessive gassing, often accompanied by a strong acid odor that may be noticed before significant harm is done. If charging at an excessive rate continues, the electrolyte will literally boil away, leaving a ruined battery.
Assuming the battery box provides suitable ventilation for removal of the gases produced (oxygen and hydrogen), there is small risk of fire or explosion. Where high temperatures are created during charging, any sealed battery, particularly of the gel VRLA type, presents a distinct risk of battery explosion. U.S. Coast Guard Boating Safety Circular no. 78 specifically cautions that sealed batteries, referred to as Sealed Valve Regulated (SVR) batteries may explode if connected to battery chargers that do not have automatic, battery temperature sensing, charge current controls. According to the manufacturer, the Lifeline AGM battery’s very low internal resistance safeguards it from presenting the overtemperature explosion risk.
Regardless of the type of battery used, provisions for cooling and, in the case of flooded-cell batteries, removal of evolved gases, is an important consideration. Separating adjacent batteries by about half an inch is good practice.
It is also important to ensure that adequate mechanical strength is incorporated in the design of the battery box and the battery hold-down. Keeping batteries in place, even when the vessel is being tossed about, is critical. The possibility of one or more 100-pound boxes of lead and acid flying about the interior of a boat is unacceptable. Designing the battery box and the retainers to withstand a minimum acceleration of two Gs is good practice.
Choosing the most appropriate type of battery for a boat is a complex task. In a comparison of probable cycle life, using manufacturers’ data, medium-duty flooded-cell deep-cycle batteries can be expected to yield 300 to 500 cycles at 50% energy withdrawal. Comparable gel-cell batteries are typically rated by their manufacturers at 300 to 400 such cycles. In its literature, Lifeline, distributor of the Concorde AGM battery, claims 1,000 cycles. When purchase cost is compared, the Lifeline AGM costs 16% more than a similar-capacity gel-cell and 85% more than a comparable flooded-cell battery. However, cycle life, although important, is not a sufficient criteria in selecting a battery type. The attraction of the zero electrolyte maintenance afforded by the gel-cell or AGM products must be weighed against the impossibility of checking the condition of individual cells in the sealed batteries. It is important to remember that, in a series circuit (the cells of a battery), failure of any one part of the circuit (one cell) can fault the entire circuit. Where ultimate life is desirede.g., ocean voyages of long duration or for living aboard away from any shoreside power facilitiesthe best choice will likely be a very-heavy-duty, flooded-cell deep-cycle battery, such as one of the Rolls Marine, Surrette brand products.
Many sailors’ voyaging is primarily coastal and, unfortunately, of limited duration. In this case, a two-battery house bank, composed of medium-duty, deep-cycle, flooded-cell batteries, with a separate, dedicated engine-starting battery, may be the most appropriate choice. When needed, charging intervals can be modified to compensate for unusual power demands without risk of depleting the engine fuel supply.
The use of modestly priced, quality batteries is justified by the fact that most boats used for coastal cruising rarely see more than 100 days of use per year. With proper care, these batteries will likely provide three or more years of service. In addition, in the event of premature failure, warranty replacements are usually available nearby. Installing premium batteries, designed for heavy-duty use over an eight- or 10-year period, on such a vessel makes little practical sense.
For voyages of longer duration, heavy-duty flooded-cell deep-cycle batteries may be the most logical choice. The use of gel-cell or AGM batteries is clearly justified for those installations or for those users for whom inspection and maintenance is impossible or undesirable. Assuming the claims for the Concorde AGM product are accurate, its price premium over the gel variety seems well justified. It may also prove worthwhile in special applications where absolute minimum recharging time is critical.
Contributing editor Chuck Husick is a sailor, pilot, and Ocean Navigator staff instructor.