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Batteries at the heart of DC power

Jan 1, 2003

The demands placed on the average sailboat's onboard electrical system have grown in tandem with the rapid advances in electrical and electronics systems. Electrical equipment with relatively lower consumption — such as GPS and instrumentation — and even traditional high-demand equipment — such as autopilots and radar — have now been eclipsed as the most power-hungry devices on voyaging sailboats. The new, power-thirsty gear list is topped by the AC inverter, which extracts a heavy price in electrical power for the comforts it delivers. Next are the bow thrusters and the hydraulic power packs for the deck winches and auxiliaries, even in some cases, electric propulsion.

While some technical developments, like improved, high-efficiency inverters and energy-efficient lighting, have eased the strain, future changes will see the wider implementation of 24- and 36-volt power systems as a way to improve available power. Batteries will still be central to the primary DC power system, whatever the voltage used. Power balance assessments and battery-capacity calculations have taken on a new importance as power needs have risen significantly. The search for a bulletproof and fault-tolerant battery bank is as real as ever.

Flooded-cell, lead-acid batteries have been the most common type of marine battery. But the flooded cell's position is under siege from technology like gel cells and absorbed glass mat (AGM) types. The performance claims and counterclaims are often confusing as the various numbers are worked out, and each successive innovation claims greater cycle abilities and life expectancies than previous generations of batteries. Other motivators are at work besides the hard, technical performance figures. One important recommendation emerged recently from the inquiry into the disastrous Sydney-Hobart yacht race: All yachts in future races should have sealed batteries to prevent flooded-cell-type units from losing their filler caps and spilling acid in the event of a knockdown or rollover.Flooded-cell batteries

To start, it's necessary to revisit a few battery basics. The fundamental theory of the battery is the generation of an electrical current between two electrodes of dissimilar metal when they are immersed in an electrolyte. In the typical lead-acid, flooded cell the generated voltage is a nominal 2.1 volts per cell. The typical 12-volt battery consists of six cells, which are internally connected in series to make up the battery. Each cell consists of positive plates, which contain lead peroxide (PbO2), and negative plates containing spongy lead (Pb). These are immersed in the electrolyte made of dilute sulfuric acid (H2SO4). Current flow and discharging of the battery occurs when an external load is connected across the positive and negative terminals, and a chemical reaction takes place between the two plate materials and the electrolyte.

During this discharge reaction, the plates interact with the electrolyte to form lead sulfate and water, which dilutes the electrolyte, reducing the electrolyte density. As both plates become similar in composition, the cell loses the ability to generate a voltage. Recharging of a cell reverses this reaction, and the water decomposes to release hydrogen and oxygen, with the two plate materials being reconstituted to the original material. When the plates are fully restored and the electrolyte is return@d to the nominal density, the battery is completely recharged.

Recommended densities are usually obtainable from battery manufacturers and can vary a little between batteries. In warm, tropical locations it is common for batteries to have a reduced electrolyte density, which does not cause separator and grid deterioration as fast as temperate climate density electrolytes. Deionized and distilled water is preferred for topping up cells. Some voyagers use rainwater or water straight out of the marina water faucet, which introduces impurities and degrades the plates.

Sulfation is the single greatest cause of flooded-cell battery failure. The causes of sulfation are relatively simple, and it can be counteracted. During discharge, the chemical reaction inside the battery causes the active material on both plates to convert to lead sulfate, and if recharging is not carried out promptly, the lead sulfate starts to harden and crystallize. White crystals of lead sulfate form on the typically brown plates. The process is almost non-reversible if not corrected early. The immediate effect of sulfation is partial and permanent loss of capacity as the quantity of active material is reduced. Electrolyte density also decreases partially, since the chemical reaction during charging cannot be fully reversed. This sulfated material also introduces higher resistances within the cell and inhibits charging. As the level of sulfated material increases, the cell's ability to retain a charge is reduced, and the battery ultimately fails. The deep-cycle battery has gained an unfair reputation for excessive sulfation. However, the battery is not the cause, but rather improper and incomplete charging.

The process of providing a regular, equalizing charge assists in reducing sulfation, and there are a few battery additives that also claim to reduce or reverse the effects. The other principal causes of failure are lack of maintenance and a failure to monitor cell electrolyte levels and top them up with distilled water. The charge and discharge cycle releases water in addition to natural evaporation due to ambient air temperatures. Once the plates are exposed they become seriously damaged, leading to premature failure.

Deep-cycle batteries are not tolerant of inactivity — batteries left without charging have very high self-discharge rates of up to six percent a month. For boats that sit on a mooring or in a slip unattended for long periods, the effects are serious.

Additionally, installation factors are important for flooded-cell batteries. Since flooded cells generate potentially hazardous hydrogen gas, they must be well ventilated. They also can leak acid in knockdowns and capsizes, and when the acid mixes with salty bilge water, chlorine gas can be generated. But spill-proof caps can circumvent this problem.

One trend in lead-acid battery development was sealed lead-acid units with safety venting and built-in hydrometers. The approach was devised to eliminate the task of taking manual hydrometer readings.

y´The battery decision-making process depends on the types of loads the battery is required to carry, since starting batteries have different loading characteristics than deep-cycle batteries used for house power. For deep-cycle batteries, it is about discharge rates, charge-acceptance rates, projected cycle life, etc.; for starting batteries, it is their high cranking-current ability. In most yacht applications, inefficient charging is a major cause of shortened life and performance, and fast-charge, smart-regulator systems are essential if the full capacity is to be realized. It is also important to remember that just one serious flattening episode can ruin the battery or severely curtail its life.

While these disadvantages may appear daunting, in fact, flooded-cell batteries are bulletproof in many respects. They are tolerant to overcharging, and the use of smart alternators overcomes the lower charge-acceptance rates. There are many innovations and performance-enhancing battery design features being implemented by the various manufacturers to improve reliability and performance. This may include more and thicker plates for improved current flow, along with improved grid designs. Separator design and material is also an area of development, with some using a fiberglass matting that is bonded to the plates to reduce the shedding of active material, while there are other units that use carbon fiber. This is conductive and is bonded to the plates to stop plate material shedding and improve current flow. Further, this encapsulation of the plate with matting is claimed to eliminate sulfation. The costs are much lower than other, newer battery types, and if the maintenance and charging are kept up, they will have long service lives of several years. Quality, deep-cycle batteries that are subject to frequent service, such as products from Rolls, Trojan, Deka, etc., and are properly maintained, will probably offer the best value for money and life expectancy.Gel cells

Sealed batteries have been steadily evolving for some time, and the gel-electrolyte battery was the first innovative alternative to flooded cells. These are recombinant batteries, in that the oxygen generated from the positive plate recombines with the hydrogen given off by the negative plate to form water, thus no electrolyte replenishment is required. They are also known as sealed-valve regulated (SVR) batteries, because the oxygen is retained in the cell by sealing vents that maintain positive inter6al pressure, which is essential to the recombination process. The valve also has a safety function to vent any excess pressure that arises during the charging process; othrwise, serious damagey´would occur. Unlike normal, lead-acid flooded cells with liquid-acid electrolytes, the gel cell has a solidified thixotropic gel, which is locked into each group of plates, and one feature of thixotropic gels is that they possess a reduced viscosity under stress. The gel is manufactured from a mixture comprising sulfuric acid, fumed silica, pure water and phosphoric acid.

The construction of gel cells is different than flooded cells, as the plates are reinforced with calcium, and not with antimony, which results in a reduction in battery self-discharge rates, typically around only 1 percent per month. The newer battery types use phosphoric acid to assist in retarding the plate sulfation hardening rates, and grid designs are also undergoing change and improvement using copper calcium lead alloys. The plates are relatively thin, which is to facilitate the gel diffusion into them. This results in higher charge-acceptance rates than flooded cells, so a more rapid charge rate is achievable. The optimum life and performance requires constant, potential, voltage-regulated charging in the range of 13.8 volts to a maximum of 14.1 volts at 68° F. Because the open-circuit voltage of a fully charged battery is 12.8 volts, the charge voltage must exceed 13.8 volts to overcome internal resistance. Fast-charge regulators that charge at rates higher than 14.1 volts (corrected for ambient temperature) cannot be used. The relatively thick separators that are used also increase the distance between the plates and reduce the high current-transfer rates.

Gel electrolytes also have lower densities, reducing the charging voltages and resulting in better low-temperature performance than flooded cells. Like any technology, however, there are downsides to gel cells. While low self-discharge rates, high charge-acceptance rates and no maintenance requirements are all advantages, gel cells are intolerant of high charging voltages, which unlike the flooded-cell battery, will seriously damage gel cells. In addition, the cycling capability compared to quality deep-cycles is less, along with considerably higher capital costs.

The expected life span of a gel cell in comparison to a quality, deep-cycle, flooded-cell, lead-acid battery is in the range of 800 to 1,000 cycles of charge and discharge (to 50 percent capacity), where a quality deep-cycle has a life of up to approximately 2,500 cycles. Gel cells, however, do have a much greater cycling capability than normal starting batteries, and in many applications they are ideal. If safety valves malfunction, the cells are also easily damaged by oxygen contamination, although in quality gel cells this is uncommon.Absorbed glass mat

AGM batteries, like gel cells, are classed as sealed-valve regulated batteries. The electrolyte is held within a very fine, microporous (boron-silicate) glass matting that is placed between the plates, which absorbs and immobilizes the acid while still allowing rapid plate and acid interaction. Another term used for AGM batteries is starved-electrolyte batteries, because the glass matting is only 95 percent soaked in electrolyte. The operational principle is called recombinant gas-absorbed electrolyte, as the generated gases recombine within the battery and significantly reduce hydrogen emissions. They emit less than 2 percent hydrogen gas during severe overcharge (4.1 percent hydrogen release is a flammable level). This recombination process reduces water loss by more than 98 percent, in comparison to wet-cell batteries, so the elimination of maintenance is obvious. The recombination process takes place within the separator in a molecular state, with the cells being sealed and the relief valves providing a safe, positive pressure during charging. There are variations to traditional, flat-plate manufacturing techniques, and the Optima AGM batteries have a spiral-cell and dual-plate construction.

Another important claimed feature for AGM is a greater shock and vibration resistance than gel-cell or flooded batteries. They also have extremely high, cold-cranking amp (CCA) values of up to 800 amps at 0° F; however, the one drawback that kept me from installing these units in my boat was the limitation in rating sizes. They are, on the other hand, a very good option for smaller boats.

Charging of AGM cells has few limitations, and no special charge settings are required when using fast-charge regulators. The batteries have a very low internal resistance, and there are no heating effects during heavy charge and discharge. Since they have a high charge-acceptance rate, they can be bulk charged at very high currents, typically by a factor of five compared to flooded cells, and by a factor of 10 compared to gel batteries. They also allow 30 percent deeper discharges, recharge 20 percent faster than gel batteries and have good recovery performance from a fully discharged condition. Self-discharge rates are only 1 to 3 percent per month at 77° F, which is better than either flooded cells or gel cells. If you are a weekend, harbor or river sailor, who does limited motoring, or if you leave your boat unattended for long periods, the AGM battery is a viable option. Because charge-acceptance rates are very high, and charging is in the range of 14.4 to 14.6 volts, a fast-charge regulator charging an AGM battery has the capacity to burn out a low-output alternator. This is because undersized alternators will run at full output for considerable time periods and overheat. There is a good case for installing high-output alternators to maximize charging, with the added load on the engine coming as a bonus. I have recently installed AGM batteries on my boat and will be watching their overall performance closely.

John C. Payne is a professional marine electrical engineer with 27 years of experience. He currently works for a major Houston-based company as an electrical supervisor for several dynamically positioned offshore drilling rigs. He is author of The Marine Electrical and Electronics Bible, and his website is www.marine electronics.org. He lives aboard his 35-foot classic sloop.