The further away you sail from the umbilical shore power cord of the marina, the more bullet proof your electrical system will have to be.

I’m often asked technical questions about batteries and electrical systems, and I thought I'd spend a bit of time and share what I've learned over the years about this topic, both from my 20 years of sailing, as well as my 30+ years as a professional electrical engineer.

I’ve been a participant in a sailboat owner e-mail list for many years. In all those years, I don't think there was a more controversial topic than the issue of batteries, chargers, inverters, and electrical systems in general. It has been my observation that the marketplace for products addressed to this area is full of more spin than the average political campaign, and the victims are often those sailors who are not technologists (the polite work for 'geek like me'). While I don't presume so much arrogance as to claim to be any sort of ultimate expert, I think I do have a bit of knowledge that I can share with everyone, and hopefully, it will be helpful to you.

The Energy We Need The most fundamental issue for most sailboat owners ought to be an assessment of just what their electrical power needs are. Few people bother to actually analyze their requirements to determine the right gear to buy and install. First, let me state clearly that most of what I have to say applies to average recreational sailors. My boat is in the water for 6-to-8 months per year, I do some occasional daysailing, frequent weekend trips to Block Island, some 36 miles from my slip in Warwick, RI, and a couple of one-to-two week vacation cruises per year. While not using the boat, it is tied to a slip and plugged into shorepower. If your own sailing patterns are markedly different, then your electrical requirements may be different, so some of my judgments may not apply to you.

While at the slip, whatever electrical power we need is supplied from the dock, and the battery charger essentially becomes little more than a power supply to keep the refrigeration going; the battery is more or less irrelevant. The batteries only become an issue while we're sailing, so that's the basis of my analysis of power needs.

Computing electrical loads will vary from boat to boat and it’s equipment and how power hungry a crew is on board.

What are the loads? The biggest load for the typical sailboat in the 36-foot and up category is refrigeration. While it may vary considerably, the typical sailboat refrigeration system, using a compressor/evaporator plate design, draws roughly 5 amps while running, and cycles approximately 50% of the time. This totals out to 60 amp-hours in a 24 hour day. All other loads are typically far lower.

Navigation electronics like the depth sounder, knotmeter, wind indicator, and the VHF radio while transmitting may draw as little as 0.5 amps; GPS/Chartplotters draw 1 amp, and navigation lights, radar, autopilot, etc. can be 4 amps or more. These things are obviously only drawing current while sailing, so the average draw can be estimated based on the number of hours under sail. An autopilot, for example, may draw 4 amps, but is cycling intermittently, so the average draw is likely to be 2 amps or less. Some measurements I did, on my own boat a few years back, indicated no more that 15 amp-hours for all these items in a typical day.

Other house loads include cabin lights (1 amp each), stereo (1-2 amps), small TV set (2-3 amps), and so on. Pumps can draw 5-7 amps while running, but are usually intermittent in operation, running for only a minute or less at a time (water pump, bilge pump, etc).

I once did a total analysis of the typical draw for a day, based on the idea of 7 hours of sailing, followed by an evening at anchor. The analysis included the operation of the navigation stuff while sailing, followed by the use of cabin lights, stereo, TV, etc., in the evening. My total was in the range of 80 amp-hours per day, so we'll use this number as the basis for discussion.

Batteries Now that we've established the power requirements for our 'typical' cruise, we can next consider what sort of batteries we might need to support the needs. The most common battery type used in a cruising sailboat is a 'deep discharge' flooded wet cell battery, designed to allow for considerable current draw over an extended period of time. These are different than 'automotive' batteries in that the plates of a deep discharge type are thicker and less subject to warping and distortion during heavy discharge. The tradeoff is the amount of instantaneous current they can deliver; automotive types can provide enormous current for very short periods of time, but can't tolerate high steady loads.

In the last 20 years, there have been a couple of newer technologies for battery construction that have caught the sailing community's eye such as AGM (Absorbed Glass Mat) and 'gel cell' types. These batteries may offer some unique advantages for certain very specific requirements, such as the ability to be positioned independently. Personally, I think these types of batteries are not worth the huge premium in cost that they command; I figure that if my sailboat is upside down, I've got bigger problems than whether my battery is operating! To be fair, the AGM types are claimed to have a greater 'acceptance rate' (rate at which they can be recharged), but this presumes that the boat is equipped with a charger and/or alternator capable of supplying the larger charging currents. Whether the high cost is worth it depends on the situation--I've never found the tradeoff worthwhile.

The most common battery sizes used in sailboats (smallest to largest) are Group 24, Group 27, Group 31, '4D', and '8D'. These standard sizes are comparable to automotive and truck batteries, although they're available as 'deep discharge' types, rather than the 'automotive starting' types. If you have refrigeration, then the smallest type that is worth considering is the Group 31 size, which is typically rated for 90-95 amp-hours... a bit more than your typical daily requirement. To complicate matters, most experts recommend that for the longest service life, a battery should not be depleted to its rated capacity; you'd like to retain some margin, anywhere from 30-50% or so. Therefore, you probably don't want to draw more than 50-60 amp hours from a Group 31 battery, and so on. '4D' batteries, with a typical rating of 180 amp hours, seem to be a very practical size, and can be bought relatively inexpensively. Here's a hint: manufacturers like Exide make '4D' batteries for both the commercial truck market as well as the marine market, the difference being the style of terminals on top, yet, the one they sell to the commercial truck market is half the price! For a few bucks, you can buy conversion terminals to convert the automotive posts to threaded studs with wing nuts to match the typical boat.

An even better alternative, presuming you have the space for them, are golf cart batteries. These are 6-volt batteries, and you would use them in pairs, wired in series, for 12 volts total. They're typically rated for 225 amp hours, and a pair can be found for less money than a 'D' battery, in some cases. They're very rugged, and designed for lots of cycles and brutal use.

I think that conventional flooded wet cell batteries, with removable caps for replenishing the electrolyte, are the best type to use. The so-called 'maintenance free' batteries, while being a great idea for your car, are a lousy idea on a boat. If a short or an unusual load condition should occur, the seals on a maintenance-free battery will blow, and the battery will be ruined, whereas a conventional wet cell battery can be topped up with distilled water and salvaged.

When is a battery depleted? A battery is a nonlinear, monotonic device, so the definition of 'depletion' is a bit arbitrary. Manufacturers usually define depletion for a wet cell battery as 10.7 volts under load, a value which will still operate many electronic devices, although things like cabin lamps may be noticeably dim. This point represents a greater depletion that recommended, for best service life.

It should be noted that voltage alone is not any indication of battery condition, except for a very badly depleted battery. At room temperature, the electrochemical potential of a lead acid battery is 12.6 volts, almost irrespective of the state of charge. There is one, and only one way, to make an accurate measurement of state of charge, and that is with a hydrometer, presuming the battery is a flooded wet cell type with caps that can be opened, that is. Some years back, I arranged a group purchase of high quality professional hydrometers for the Catalina email group--at $30 each, it was a good investment.

Mostly forgotten about until it stops working, neither sexy and often grimy with belt dust and the source of heated debates among sailors, the alternator is still the key player in getting the most charge into a battery bank.

Recharging: Basic principles Batteries are recharged by forcing current in to them until the reversible electrochemical reaction restores the plates and electrolyte to a 'fully charged' condition. The amount of current used during recharge depends on the type of charger, the length of the recharge, and the specified capacity of the battery. Battery manufacturers will often recommend a maximum recharge rate of 'C/5', where 'C' is the rated battery capacity; therefore, the recharge of 185 amp-hour '4D' battery should not exceed roughly 37 amps, initially. Battery manufacturers also agree that a slow tapering charge results in longest life, so your desire to get the thing recharged quickly is at odds with the best interests of battery life. The best kind of recharge, hard as it may be to believe, is a standard alternator with a voltage setpoint of 13.4 volts. This results in an exponentially tapering charge which will not exceed the maximum charge rate. Until a few years ago, you could easily buy a shorepower charger which would do the same thing, but they're hard to find now.

The hot topic in battery charging, these days, is 'multiphase' or 'three phase' recharging. The basic idea of these charging systems is that a battery can accept a higher than normal rate of charge for a limited period of time (in some cases, C/4 or even C/3), after which the charge rate can be reduced to 'finish' the charge, and finally, the charger can 'float' the battery to maintain it. The basic objective is to get the entire job done quicker. It is important to note that the battery itself has no concept of 'multiphase' charging; it is a nonlinear monotonic device with no specific definition of 'phases'--the multiphase idea is purely an invention of the manufacturers.

Do these things work? Sure, but at a price. I did an exhaustive survey of battery manufacturers a few years back, virtually all of whom agreed that a modest constant voltage charge was better for the batteries than a hot 'multiphase' charge, in terms of service life. The manufacturers couldn't agree on just how much service life would be lost with aggressive charging, though.

Recharging with an alternator If you’re cruising, then the most probable recharge source will be the alternator. Most stock alternators provide a constant voltage charge, with a set point of 13.4 volts, which is gentle to your batteries, but not as fast as a 'multiphase' charger. You can replace the voltage regulator in your alternator with a 'multiphase' charge controller, in which case, you can get a faster, hotter charge--at the expense of battery life (although no one agrees just how much service life is lost).

Alternator ratings are often misunderstood. The typical stock engine alternator is likely to be made by Motorola, Delphi, or some other Far East manufacturer, and carries a current rating. On my last boat, the alternator was rated for 72 amps.

However, this rating is somewhat akin to the power rating of a car stereo--it's only valid under a precise set of circumstances. In the case of alternators, the rating usually applies to a cold alternator running at maximum RPM., which is not the conditions under which you'll be recharging your battery. Your RPM isn't likely to be at maximum while cruising, and the output of the alternator will drop off substantially as it heats up, because the windings are made of copper wire, and copper has a strong positive temperature coefficient of resistance. If you convert your alternator voltage regulator to a multiphase type, the situation gets even worse, because you're then asking the alternator to put out even more total power, at a higher voltage, which can only mean that the current must be reduced. You can't squeeze blood from a stone!