Multihull sailors attempting to make and manage their own electrical power supply must have some way of storing electrical power for later use. With renewable power sources such as solar, wind and water chargers it’s essential to take advantage of favorable charging conditions while they last. And with engine-driven charging sources it’s best to be able to charge at the highest rate possible while the engine is running, then tap your stored electricity until it’s time to generate again. Although many energy storage mediums have been proposed over the years, lead-acid batteries remain the most practical alternative for boaters.

Batteries have traditionally been high on the list of nautical gear that multihull sailors love to hate-they are bulky and heavy, can potentially be a safety hazard, and seem to let you down when you need them most. But the cause of dead or dysfunctional batteries lies most often with the system operator, either in their choice of batteries or in the way the batteries are charged and discharged. Cheap batteries won’t give the service or dependability of high quality models, and poor charging techniques and negligent monitoring can quickly turn a great set of batteries into nothing more than expensive ballast, the last thing needed on a multihull. Understanding their operation and the differences between various models, as well as the best size and configuration for your needs, may give you a whole new appreciation of your batteries.

The electrolyte in a lead-acid battery is the material surrounding the internal lead plates that allows batteries to chemically store or release electrical energy. The electrolyte in Wet lead-acid batteries is a sulfuric acid solution in liquid form. Distilled water must be added to liquid-electrolyte batteries periodically to replace losses which normally occur during performance charging. They must also be periodically equalized-charged at a higher voltage under controlled conditions-to prevent sulfation deposits from decreasing battery capacity. Wet batteries that have been neglected, or used regularly in the lower portion of charge capacity instead of the upper 50 percent, will require several equalization cycles to help bring them back into good service, as long as they haven’t been severely damaged.

During an equalizing cycle voltage is allowed to rise to over 15 volts temporarily under controlled conditions, which creates active gassing of the electrolyte. Vigorous gassing removes sulphate from the battery plates. Equalizing should only be done with a charger or charge control with an equalizing feature, the batteries at full capacity, the battery caps taken off, the battery space well ventilated, and current no more than 3-5 percent of total battery capacity. Some solar and wind charging controls allow the operator to implement a manual equalizing cycle for wet batteries. Solar and wind chargers are ideal for equalizing since the have relatively low current output for extended periods of time. Conversely, gensets or main auxiliary engines are not very efficient for equalizing.

Despite some maintenance considerations, good quality wet batteries such as those from Rolls Engineering (Surrette Battery Co. in Canada) are hard to beat for good performance and long life.

Batteries that are permanently sealed, with the electrolyte immobilized, are increasingly popular with boat owners. There are two basic types of sealed batteries to choose from, AGM (absorbed glass mat) and Gel (gelled-electrolyte). Both of these types are lead-acid construction (lead plates and acid electrolyte), they just have their electrolyte in different form. (Note: these high quality hybrid deep-cycle batteries should not be confused with inexpensive “no-maintenance” SLI batteries).

In an absorbed-electrolyte battery the electrolyte is contained in thick, felt-like glass fiber mats that are compressed between the plates. During construction, some of the electrolyte is also absorbed by the battery plates. The mats serve as receptacles for the electrolyte as well as plate separators. Compressing the plates and mats together lowers the internal resistance of the battery and allows for higher charge and discharge rates.

In a gel battery the electrolyte is contained in gel form, allowing these batteries to also be completely sealed and mounted in any position (as long as the safety vents are allowed to operate if necessary).

AGM and Gel batteries are considered hybrid deep-cycle batteries with high performance characteristics, allowing them to be used in power systems with heavier electrical loads. They typically don’t have the service life of true deep-cycle batteries, but they work well and have many advantages over wet batteries.

Part II – Battery Applications

There are three types of lead-acid batteries appropriate for use on board a multihull, each designed and constructed for a specific task and categorized by their ability to deliver current and stand up to repeated discharge. The basic difference between them is the thickness and number of the positive and negative plates, the strength of the lead alloy in the plates, and what type of electrolyte is used.

1. Starting-Lighting-Ignition (SLI)

Starting-Lighting-Ignition (SLI) batteries have a higher number of thin positive and negative plates, creating a large total surface area capable of producing high cranking power for the few seconds it takes to start an engine. However, they can’t maintain high discharge for very long and they have a relatively high self-discharge rate (power loss even when no electrical load is connected). SLI batteries should always be electrically isolated from the batteries and charged independently through one several means 1) bank-to-starter bank trickle (ie. echo-charge from xantrex), similar device, 2) separate output multi-bank ac charger (from shorepower), or 3) standard on an auxiliary if a second, high-output alternator is added engine for rapid charging of house bank. deep discharging will greatly shorten sli battery life; the plates simply aren t thick enough to handle it.

2. True Deep-Cycle

The plates in a true deep-cycle battery are thick and heavy, trading surface area for strength, and starting power for reserve capacity. Deep-cycle batteries have the ability to withstand repeated deep discharge without harm, have lower self-discharge rates, and are used to supply typical house loads. they often come 6-volt configurations easy transport, and connected in series series-parallel to achieve the desired system voltage. true deep-cycle batteries can also be used for engine starting if you have enough total capacity (several hundred ampere-hours or more).

For supplying house loads it may be tempting to buy relatively inexpensive 6-volt golf cart type deep-cycle batteries. This is fine as long as you understand that these batteries typically have only a three-year life expectancy (less in the tropics). If your boat is a long term proposition and you have the money to invest, you’re better off with high quality marine models.

Well worth investigating are the heavy duty deep-cycle batteries from Rolls Battery Engineering. These wet batteries cost more initially but easily last 8 to 15 years in normal service (up to 10 years for their 12-volt models, up to 15 years for their high capacity 6-volt models). Rolls batteries have heavy-duty plates that are individually wrapped with a protective envelope, eliminating short-circuiting and cell damage due to sediment build-up or faulty or misaligned plate separators, and a large liquid electrolyte reservoir over the plates. The new Rolls 12-HHG-325 low maintenance, modular 8D replacement battery is a great choice for multihull sailboats. This battery consists of six easy-to-transport, high capacity 2-volt cells integrated into a plastic battery container. In the same footprint of a standard 8D it provides up to 65{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} more capacity (325 ampere-hours) and is much more manageable. With the use of efficient appliances and a modest degree of energy conservation this single battery can supply the house loads on even larger boats.

3. Hybrid Deep-Cycle

In between the SLI and true deep-cycle batteries are the gel and AGM hybrid deep-cycle batteries with plates of medium thickness. Commonly used as house batteries where loads are moderate, they also have cranking power for engine starting, can be moderately discharged repeatedly without harm, have a relatively low self-discharge rate, and come in 6- or 12-volt configurations. Because of their plate thickness these batteries typically don’t have the service life good quality deep-cycle models. Immobilized-electrolyte (gel or AGM) batteries are multi-purpose, but are also more sensitive to voltage, which means that all charge controls on board must be properly set and they should include temperature compensation to make sure the charging voltage stays within acceptable limits. However, AGM batteries have the reputation for being able to handle higher current charges and discharges than wet batteries.

Part III – Ratings & Performance

Batteries are rated according to their construction and how they perform, allowing multihull owners to make an intelligent selection for their needs. The various battery ratings are:


Batteries are composed of a series of 2-volt cells packaged into a single container. Batteries for marine use are typically available in 6- volt or 12-volt models, although some high quality batteries are composed of individually packaged, replaceable 2-volt cells. Batteries can be connected together to create the desired system voltage and capacity. They can be connected in series, in parallel , or in series-parallel.

Multiple batteries connected together are called a battery bank. Only batteries of like size, type and age should be connected into a single bank. When batteries are connected in series (positive post of one battery connected to the negative post of another battery) the voltage of the bank is the sum of the voltage of the individual batteries, while the ampere-hour capacity remains the same as for a single battery. When batteries are connected in parallel (positive posts of the batteries connected together and the negative posts of the batteries connected together) the ampere-hour capacity of the bank is the sum of the capacity of the individual batteries, while the voltage of the bank remains the same as that of a single battery. When connecting multiple batteries in parallel it is important to take the main positive and negative leads from opposite corners of the bank.

Marine Cranking Amps (MCA)

This rating tells the current that a battery at 32 degrees Fahrenheit can deliver for 30 seconds while maintaining a minimum cell voltage of 1.2 volts. Gasoline engines require about 1 CCA per cubic inch of displacement, diesel engines about 2 CCA per cubic inch.

Reserve Capacity

This refers to the number of minutes that a fully charged battery at 80 degrees Fahrenheit can be discharged at 25 amperes while maintaining a minimum cell voltage of 1.75 volts. Reserve capacity can also be expressed for other rates of discharge such as 5, 10, or 15 amperes. The higher the rate of discharge, the lower the total reserve capacity rating.

Size and Ampere-hour Rating-Marine batteries are most often marketed according to their case size and corresponding ampere-hour capacity (see chart below). Ampere-hour capacity is an energy rating similar to reserve capacity. It refers to the amperes a battery can supply at 80 degrees Fahrenheit in a specific period of time while maintaining a minimum cell voltage of 1.75 volts. Many battery manufacturers use a 20-hour rate. In this case a 100 ampere-hour battery could supply 5 amperes for 20 hours. When comparing batteries, it is essential to make sure they use the same hour rate.
Chart of typical battery sizes and ampere-hour capacities

Battery Type Typical Size Typical Ampere-Hour Capacity
Group 24, 12V 11″ x 7″ x 9″ 85-90
Group 27, 12V 12″ x 7″ x 10″ 100-105
4D, 12V 21″ x 8.5″ x 10″ 160-180
8D, 12V 21″ x 11″ x 10″ 220-250*

*This is standard; the Rolls 12HHG325 actually provides 325 amp-hours in an 8D configuration.
Here are a few rules about battery performance: Ampere-hours and Energy

The rating of ampere-hours is really an indicator of the amount of usable electrical energy the battery can provide. Remember that volts x amperes = power, and power x time = energy. This means that ampere-hours x battery voltage = watt-hours, a true measurement of electrical energy. A 6-volt battery rated at 100 ampere-hours has half the total energy available as a 12-volt battery with the same rating.

Useable Battery Capacity

The rated ampere-hour or reserve capacity of a battery is quite different from the amount of energy you can actually store and retrieve on a daily basis. Deep-cycle battery life can be greatly extended if you discharge to only about half of its rated capacity, or 50 percent charged. Frequent deeper discharges will shorten battery life dramatically. And because of the low rate of current a battery will accept during the final charging stages, it is likely that with an engine-driven charging source you’ll most often charge the battery to only about 90 percent of it’s rated capacity. In effect you have about 40 percent of the total battery rated capacity at your disposal as usable electrical energy. (Note: The relatively constant, quiet output from solar panels and wind- and water-powered generators easily completes the final stages of charging, making that extra 10 percent of battery capacity available) For long battery life, it is important to bring the battery to a full state of charge periodically.

Recharge Energy Losses

Some of the electrical energy used to charge a battery becomes lost as heat or wasted on hydrolysis, the breakdown of water into hydrogen and oxygen gas. Hydrolysis in a battery is referred to as battery gassing. It begins when the charge rate exceeds what the battery can naturally absorb. Gassing can be limited by using good performance charging techniques, but you still lose about 15{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} of the charging energy. In other words it essentially takes 115 ampere-hours to completely charge a 100-ampere-hour battery.

How Charge and Discharge Rate Affect Capacity

The rate of charge and discharge greatly affects the amount of usable electricity you can get from a battery. As a rule the higher the charge rate, the less efficient the battery is at absorbing all of the energy available. In like fashion, the higher the discharge rate, the lower the total amount of usable energy. This has definite implications when choosing high-draw appliances for your independent power system.

Capacity and Temperature

Battery temperature will also affect capacity, as it does corresponding specific gravity and voltage readings. While chemical reactions are accelerated at higher temperatures, improving battery performance, hot ambient temperatures most definitely shorten battery life. This is why it is best to keep batteries in a cool, dry location.

How Age & Use Affects Capacity

Batteries eventually lose their capacity to store energy, although higher quality batteries have a greatly extended life expectancy. Proper charging methods and routine care will also extend battery service life.

Number of Cells & Life Expectancy

Using high-capacity 6-volt batteries connected in series can reduce the total number of cells in your battery bank that can potentially have problems, increasing life expectancy. For example, to get 320 ampere-hours of capacity you could use three 110 ampere-hour 12V batteries in parallel (18 cells total), two 320 ampere-hour 6V batteries in series (six cells total), or the Rolls 12HHG325 8D replacement battery that has 8 individual, replaceable cells.

Part IV – Sizing and Selecting House Batteries

In the last issue we reviewed battery ratings and performance. In the last part of this series we’ll focus on selecting and sizing batteries for your house loads on board a multihull.

Determine Total house battery capacity

The first thing to do is determine your total “house” battery capacity, which relates to two things: 1) your total electrical load drawn from the house batteries (NOT those supplied by direct AC power sources-ie. from shorepower or a gen-set), and 2) the reserve battery capacity you’d like to have (the amount of time you can live solely off battery power before needing to recharge). The reserve capacity you choose determines the time between regular engine-charging cycles. If your contribution from renewable chargers is small or nonexistent, you’ll have to adhere to this regular engine charging schedule.

If renewables are making a significant contribution, you can extend the length of time between regular engine charging, or even eliminate the need for it altogether.

Rules of thumb when sizing batteries:

Your USABLE battery capacity is the amount of energy available when the batteries are between 50 and 90 percent of full charge. This means that your usable capacity is about 40{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} of the total capacity; battery life is extended if you don’t discharge below the 50{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} level on a regular basis, and topping up the last 10 percent of charge usually takes too long with an engine-driven charging source since charging current drops significantly during the latter stages of charging.

Not all of the charging power that reaches the batteries actually gets stored as electrical energy; some is lost in the process. It’s a good idea to make provisions for about 15{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} battery losses.

In addition to these rules, I usually recommend sizing total battery capacity as if no renewable chargers were present. That way the more power renewables produce, the less you’ll need to be concerned with a regular engine-charging routine. For example, assuming an electrical load of 110 amp-hours per day and a one-day reserve capacity, sizing total battery capacity would be as follows:

110 amp-hours per day load x 1 day of reserve capacity = 110 amp-hours of usable battery capacity needed.
110 amp-hours of usable capacity = 40 percent of total battery capacity, therefore110 amp-hours divided by 0.4 = 275 amp-hours of total battery capacity before losses.

275 amp-hours of total capacity x 1.15 (accounts for 15{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} battery losses) = roughly 320 amp-hours of battery capacity required.
Increasing your electrical load or the length of your reserve capacity increases the total battery capacity required. For instance, if you wanted two days between regular engine-charging cycles, you’d need to roughly double your total battery capacity.

Before proceeding, check to see if the total battery capacity you’ve calculated is suitable for your charging sources. To make the most efficient use of your alternator, total battery capacity should be at least four times the amperage delivered during bulk charging (when your alternator is producing the most current). With 320 amp-hours of total capacity, you could easily have a 100-amp high-output alternator, which would produce about 85-90 amps or so when it’s warmed up. If you want to use a larger amperage alternator, technically you should have more battery capacity to keep the alternator output current from dropping prematurely, although the only harm done if you don’t have more capacity is a loss in alternator efficiency.

Now check to see if you have enough storage capability for your renewable chargers. You should be able to store all the charging current produced during a full day of maximum output. This is usually not a problem with solar panels, but a high-output wind- or water-powered generator could produce upwards of 200 amp-hours per day. With a 320 amp-hour battery bank you could only store 200 amp-hours if the bank was discharged to 50{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} at the beginning of the day. 160 amp-hours of charging would bring the batteries back up to full charge and the remaining 40 amp-hours could supply house loads directly. If the batteries were more fully charged to begin with, or you used less power during that time, or you had optimal charging conditions for several days, some of that hard won electrical power would go to waste. Some multihull sailors would want to increase their battery capacity if space, carrying capacity and budget allows.

Select The Number of Banks

For most multihull applications a single house bank of batteries with a designated engine-charging battery or two makes the most sense and is the easiest to control and monitor. With this arrangement you can track charging performance, load draw, and battery condition with a low-cost system monitor, and there’s no need for switching banks to supply your loads. With the addition of an echo-charge from Xantrex, even the charge distribution to the house and engine-starting banks will be automatic.

An alternative on larger catamarans is to run dual house banks-one each hull-and use these banks for engine starting as well. This can simplify wire runs, but adds complexity to the charge control, distribution and monitoring.

Select the Battery Type & Performance Level

As we discussed in the last issue, you’ll need to choose between deep-cycle, hybrid deep-cycle, or starting batteries for your application, between wet, gel or AGM battery type, and between batteries that are inexpensive but with modest life expectancy or more expensive models with increased life and performance. Batteries strictly for house loads should be true deep-cycle or hybrid deep-cycle, and batteries strictly for engine starting should be starting or hybrid deep-cycle. It’s important that you don’t mix batteries of different size, type, or age in one bank-this can lower performance and life expectancy of the bank.

Total battery capacity can be achieved in various ways. For example, in a 12-volt system 320 amp-hours of total capacity can be supplied by:

a single Rolls 12HHG325 battery (8 cells total),
two 6-volt batteries of 320 amp-hours each wired in series (6 cells total),
by two 4D 12-volt batteries of 160 amp-hours each wired in parallel (12 cells total), or
by three Group 27 12-volt batteries of 105-110 amp-hours each wired in parallel (18 cells total).
The more cells in a bank the more potential for problems. On the other hand, smaller single batteries are much easier to handle and find space for than larger batteries. In the example above I would choose the single Rolls battery with individually packaged 2-volt cells if space were available to accommodate the footprint. The second best option would be two 6-volt cells connected in series.

Once you’ve sized and selected your house and engine-starting batteries, make provisions in the way you install them for simple charging and discharging sequences and proper monitoring so you can visually track battery performance.

Part V – One Bank or Two?

You’re about to go cruising and you want the electrical power system on board to be reliable and easy to manage.  You’ve got the charging side of the system figured out—your charging sources can produce enough power to comfortably keep up with your electrical demand on a daily basis.  But what about the battery set-up?  Should you have one house bank or two, and what size should the bank(s) be?

Sorting out the best way to store electrical power on board can be a confusing business, and the conflicting views of dockside “experts” don’t help matters.  In this article I’d like to present the case for maintaining a single house bank for most cruising applications, with a smaller auxiliary bank (usually a single battery) for engine starting (both the main auxiliary and other engine-driven gear such as gen-sets).

I believe that the dual house bank set-up is a remnant from the days when sailors didn’t understand their power system and didn’t have the gear to effectively manage it.  A typical scenario was that one bank would be used for house loads until the lights started to dim, then house loads were switched to the other bank.  The reasoning was that one battery would always have some juice in it to start the engine.  Not only did this treatment reduce battery life significantly by repeated deep discharge, but the whole dual-bank approach was used more for security than convenience, since it takes much more effort to keep track of where the charging current is going and where the electrical loads are being supplied from.  With this set-up the user is constantly adjusting the battery switch between 1, 2, BOTH, and OFF.  System monitors connected to the various house banks can usually handle this temporary change in bank capacity without being confused, but the skipper may not fare as well. 

A single house bank with proper monitoring and current control eliminates the confusion, costs less, and keeps the batteries in good shape.  The first step in setting up this type of system is to estimate the capacity of the house bank.  Describing how to size batteries constitutes an article on its own, but suffice to say that your usable battery capacity will be approximately 40{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} of the total capacity in your bank.  This is because you don’t want to drop below a 50{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} charge level very often, and it’s hard to get the top 10{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} of of battery charge with an engine-driven charger since the charging current drops off as batteries become charged, and who wants to run the engine any more than necessary?

Once you know the size you need, you can select the right batteries to make up the house bank.  I encourage boaters to use good quality 6-volt batteries as the building blocks for the bank, since you can reduce the number of battery cells in the bank, and that reduces the potential for problems.  For example, if you want to create a 350-amp-hour bank at 12 volts, you can either use two 350-amp-hour @ 6 volt batteries connected in series, or two 175-amp-hour @ 12 volt batteries (4D types) connected in parallel.  The former has only 6 cells to maintain, while the latter has 12.  A single, good quality starting battery as a separate bank will ensure you’ll always be able to start your engine, since the only load on it will be the starter.

The need for a multi-position battery switch can be eliminated if: 1) all house loads and the renewable charging sources are connected to the house bank, 2) the engine starter is connected to the engine starting battery, and 3) a two-battery “link” device is employed to temporarily link the house and engine-starting banks together when the alternator is producing power.  This way each bank receives the power it needs without affecting the other bank, and the only thing the user has keep track of is when it’s time to recharge the house bank.

A single-bank system monitor can be used to track amps and amp-hours in or out of the house bank, plus give you information about battery voltage and the time remaining until the batteries are fully charged (if the amp display is positive) or until they are at 50{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} discharge (if the amp display is negative).  Some models also allow you to track the voltage of the engine-starting bank.  These simple yet sophisticated devices provide all the information boaters need to know about their power system—essentially they’ve taken the mystery out of electricity.

If you need to have two house banks to handle larger loads that you’d rather keep separate, you can use the same basic approach described above, only with a three-battery link device (for two house banks and an engine-starting bank) and a two-bank system monitor (for the two house banks; some models will also display voltage of an engine-starting bank).

About the Author

Kevin Jeffrey is a long-time multihull sailor, independent energy consultant, author and book publisher.  He is the author of Independent Energy Guide, a valuable resource for cruising mutihull sailors, and is the publisher of Adventuring With Children by Nan Jeffrey and the first three editions of the Sailor’s Multihull Guide.