Bank management: Properly configured and maintained batteries are the heart of a voyaging boats electrical system

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Batteries are normally the key element in a voyaging boat’s electrical system. Except on boats that are constantly hooked up to shore power or have constantly running generators, it is the size and nature of the battery bank that defines the capacity of the system overall. If the battery bank is undersized, it will be chronically overloaded, thus chronically undercharged, and will quickly die. If properly sized, it will be subject to moderate loads, can be kept well charged, and will have a long and fruitful life.
When sizing a battery bank you must consider its effective range of operation. You cannot, for example, install a 100-amp-hour battery to service a routine 100-amp-hour demand. Even deep-cycle batteries, which should be used on any cruising boat to service house loads, cannot be fully discharged without suffering harm. Instead, to stay healthy, they should not be regularly discharged to much below 50 percent of their total capacity. Conversely, because it takes a very long time to recharge a battery back to 100 percent of capacity, on active voyaging boats that don’t spend much time plugged into shore power, batteries are often not recharged to more than 80 percent of capacity. This effectively means approximately 30 percent of the battery’s total rated capacity is available to service working loads.
Thus, as a general rule, battery capacity should exceed routine demand by a factor of three, though four is preferable, to allow a comfortable margin for undercharging. A boat with a routine daily demand of 100 amp-hours, for example, should have at least 300 amp-hours of battery capacity. On boats that are often hooked up to shore power, so that their batteries are routinely pumped up to a full charge, battery capacity can be safely kept at just double the routine demand. But should the useage pattern change — when the owners, for example, decide to take a very long voyage — things will need reconfiguring. Either battery capacity or onboard charging capabilities should be increased, or demand should somehow be decreased.
On small and midsize boats the physical size and weight of batteries can be a limiting factor. A large battery bank can weigh hundreds of pounds and take up many cubic feet of space. It may not be possible to significantly enlarge battery capacity on some boats without remodeling some portion of the interior. In some cases it may be impossible to fit in the desired battery bank. Note, too, batteries should be mounted in a cool location (heat can seriously degrade battery performance), but close to the engine, and low in the boat (for the sake of stability and performance), though preferably not so low that they can be easily flooded should the boat take on water.
Types of batteries
Reduced to their basic components, lead-acid batteries consist of lead plates immersed in a sulfuric acid solution, referred to as the electrolyte. Automotive cranking batteries, which are rarely deeply discharged and can be quickly recharged, have quite thin plates. Heavy-duty deep-cycle batteries, used on boats to service house loads, have much thicker plates, thus have more total capacity and can recover more easily from deep discharges, but also take much longer to recharge. In all cases, the quality and quantity of lead in the battery determines its performance.
Otherwise, distinctions between lead-acid batteries have to do with the nature of the electrolyte. In a flooded wet-cell battery, the electrolyte is a liquid solution of acid and distilled water. These batteries require ongoing maintenance, yet also provide excellent potential performance in that they can run through thousands of discharge-recharge cycles. The battery cells must be topped off with distilled water on a regular basis, as the water tends to gas off when batteries are fully recharged. Also, you need to equalize the batteries from time to time, so as to reclaim sulfated portions of lead plate that have become electrically inactive through chronic undercharging. Equalization is a carefully controlled overcharging of the battery, takes a great deal of time, and requires special equipment. It is therefore rarely performed by most boat owners; as a result, many wet-cell batteries lose capacity over time and never fulfill their performance potential.
During the last 20 years, sealed maintenance-free gel-cell and absorbed glass mat (AGM) batteries have become increasingly popular. In a gel-cell battery, the electrolyte is a soft, paste-like gelatin substance that is slathered over the battery plates. In AGM batteries the electrolyte is liquid, but is absorbed in spongy sheets of glass fibers that are pressed against the battery plates. Because the plates in both cases are not entirely immersed in electrolyte, they must be somewhat thinner than in wet-cell batteries. As a result, sealed batteries theoretically have shorter life spans than well-maintained wet-cell batteries, but they also can be recharged more quickly. Sealed batteries have other important advantages: they hold a charge much better over time, as they do not self-discharge as quickly as wet-cell batteries when left idle; they can sit idle for long periods without suffering damage; plus they can be submerged and flipped upside-down without suffering ill effects. The most important disadvantages are that sealed batteries are more expensive than wet-cells and are more easily damaged by over-charging, as there is no way to replace electrolyte once it gasses off.
In practice, sealed batteries are often better suited to power-useage patterns on cruising boats that are not hooked up to shore power on a routine basis. Because they recharge more quickly, suffer neglect more easily, and are much less susceptible to environmental damage, they in many cases have a longer useful service life than poorly maintained wet-cell batteries.
In addition to conventional lead-acid batteries, there are other more advanced technologies now emerging in the market. Thin plate pure lead batteries, as well as nickel-cadmium and lithium-ion batteries, can all suffer deeper discharges and also recharge much faster than conventional batteries. Cutting-edge lithium-ion batteries, for example, are reputedly 70 percent lighter, have three times the life span, and have 20 percent more charge efficiency than wet-cell batteries. They are also amazingly expensive. At this writing, a single 160 amp-hour lithium-ion marine battery costs more than $5,000.
At prices like this, high-tech batteries won’t be appearing on most voyaging boats anytime soon. A few well-financed race boats and performance cruisers are utilizing them, however, and as superior battery technology evolves and trickles down from the automotive industry it seems likely they will become more common (and more affordable) in the future.
Battery bank configuration
Battery banks can be configured in numerous ways. On most older and many contemporary small and midsize boats, the most common practice is to split the bank in two, with two equally-sized deep-cycle batteries connected to a single selector switch that permits either battery, or both together, to satisfy all power requirements on board, including engine-cranking and house loads. Both batteries in such cases are typically fed power by the same alternator. The traditional recommended operating procedure has been to use only one battery at a time, alternating between the two on a daily basis so both get exercised. The theory being it is safest to have one fully charged battery held in reserve at all times so that it is always possible to start the engine in an emergency. The problem with this, however, is that the effective working size of the total bank is cut in half. Each battery is more deeply discharged each time it is used than it would be if both were used together, thus curtailing the life span of both batteries.
More sophisticated arrangements have evolved as electrical loads on boats have increased. More powerful banks are often created both by using larger batteries and connecting batteries in parallel to combine their amp-hour capacity. It is also very common now to install a separate automotive cranking battery for starting the engine. In many cases the cranking battery is tacked onto a dual house-bank system, creating a three-bank system, and the user alternates between the two house banks as before. The best practice, however, is to hold just the cranking battery in reserve and use all house batteries simultaneously to maximize capacity and reduce discharge levels.
As far as charging goes, it is best if the cranking battery is served by the engine’s standard automotive alternator and if the house bank has its own high-output alternator, though this is not always possible. Again, where both sides must share an alternator, a manual selector switch is normally used to parallel all batteries while the engine is running and isolate them again after it is switched off. Alternatively, isolation diodes or paralleling relays (also known as battery combiners) can be used to isolate batteries automatically after charging so as to reduce the possibility of the cranking battery being accidentally run down by house loads. Relays are preferable, as diodes induce voltage drops that may confuse voltage regulators. In cases where the cranking and house banks do have their own alternators, there should still be a switch connecting the two sides to preserve flexibility. Most particularly, you may want to use house batteries to crank the engine if necessary or feed output from the cranking battery’s alternator to the power-hungry house batteries.
Charge regulation
Batteries can accept only so much power over time while being charged. Wet-cell batteries have a maximum charge-acceptance rate of about 25 percent of their total capacity — i.e., a 100 amp-hour wet-cell battery cannot receive more than 25 amps of power at any given time. The maximum acceptance rates of gel-cell and AGM batteries are about 35 and 40 percent of total capacity, respectively. Thin-plate cranking batteries have much higher acceptance rates than thick-plate deep-cycle batteries. Charge-acceptance rates also vary depending on a battery’s state of charge. A deeply discharged battery can be fed power at its maximum acceptance rate, but the rate steadily declines as the battery becomes more fully charged. Thus, as mentioned, it takes a long time to fully charge a battery.
To charge a battery as quickly as possible you need to feed it power right up to the limit of its acceptance rate at any given state of charge without overfeeding it. This can be accomplished through the use of a “smart” computer-controlled multistep voltage regulator. These progammable regulators pump power into a battery through three distinct phases: first a so-called bulk charge is delivered at the maximum acceptance rate until the battery’s voltage rises to a certain level; next comes an absorption charge, delivered at a fixed voltage for a certain amount of time or until the battery’s acceptance rate declines to about 2 percent of total capacity; last comes a float charge, delivered at a reduced voltage level. More sophisticated regulators can also account for factors like battery temperature, have equalization programs to use with wet-cell batteries, and include fail-safe features that protect both the battery and the alternator feeding them. Such regulators are often bundled with monitoring systems that allow you to track the charging process in detail.
Multistep voltage regulators are now considered standard equipment by most voyagers who spend lots of time off the grid. They greatly increase charging efficiency by reducing charging times — sometimes by a factor of two or more, compared to a standard regulator — and increase battery life expectancy by ensuring batteries are not over- or undercharged on a regular basis. Note, however, that multistep regulators are programmed to charge specific battery banks and can damage batteries with different charging requirements. Different types of batteries should not be mixed in the same bank; nor should different batteries of the same type that are different ages and/or have had markedly different service histories. If the batteries in a bank are replaced with another type, the regulator must be reprogrammed.
Twelve or twenty-four volts?
Ever since the 1950s, when standard automotive electrical systems switched from six to 12 volts, standard marine electrical systems have likewise been 12 volts. In recent years, however, particularly in Europe, some builders have started installing 24-volt battery banks on their boats. At first this was done mostly on large boats, about 60 feet and longer, but now 24-volt systems can be found on boats as small as 40 feet. The great advantage of these systems is that the cabling needed to carry 24-volt current is one-quarter the size of that needed to carry 12-volt current. For low-power devices this is not terribly significant, but for power-hungry equipment like bow thrusters, electric mainsail and headsail furlers, electric winches, and windlasses, it can spell the difference between cables that weigh hundreds of pounds versus cables weighing mere dozens of pounds. Because lighter 24-volt cables are much more supple they are easier to install than 12-volt cables; they are also much less expensive.
There is, however, a lot of equipment that cannot run on 24 volts, including most marine electronics. Any 24-volt boat must also therefore still be a 12-volt boat in certain respects. Often the only 24-volt equipment on a “24-volt boat” is the heavy-duty gear mentioned above. All the other equipment on board — lights, small pumps, refrigeration, plus all navigation and communications gear — still operates at 12 volts.
The most common way to feed a dual 12/24-volt electrical system is straight off a 24-volt battery bank. This usually consists of two 12-volt batteries connected in series so that the bank’s voltage is doubled while amp-hour capacity remains the same. The 24-volt current coming off the bank is then stepped down to 12 volts where necessary by voltage converters. In most cases multiple converters are used and are installed close to the loads they feed. This complicates the wiring, but reduces the total amount and weight of the system’s cabling.
Converters also act as filters, delivering much cleaner power to the devices they serve; some come equipped with small internal batteries to smooth out sudden voltage drops caused by large loads coming online.
The alternative approach is to feed 12-volt loads from one of the 12-volt batteries in the 24-volt bank, while feeding 24-volt loads from the bank as a whole. This requires an equalizer to keep both batteries at an equal state of charge. When 12-volt loads pull down one battery, the equalizer allows the other battery to feed power to the now-weaker one to make up the difference. The battery serving 12-volt loads, however, is still cycled more often than the other, and this will shorten its life expectancy.
Using an equalizer is generally less expensive than using converters, but there will still be many long 12-volt cable runs to contend with.
In most dual 12/24-volt systems, the starter on the engine is fed by its own dedicated 12-volt cranking battery. Ideally, there should be two alternators — one putting out 12 volts to feed the cranking battery, the other putting out 24 volts to feed the house bank. In a single-alternator system, the alternator puts out 24 volts, and that output can be converted to 12 volts to feed the cranking battery, but this approach to a dual voltage boat should be avoided if possible.
The preceding article is an excerpt from Charles Doane’s new book: The Modern Cruising Sailboat: A Complete Guide to its Design, Construction, and Outfitting (International Marine/McGraw-Hill 2010) by Charles J. Doane. You can also check out his blog at and


By Ocean Navigator