Maintenance

12-Volt Electrical Systems

Empower your boat with a reliable electrical system! Assess power requirements, choose batteries, ensure proper wiring, optimize charging.

January 29, 2024

The majority of boats on the water today depend on electricity to some degree. Unfortunately, too many of those boats are operating with inadequate and poorly installed electrical systems. This can result in unnecessary problems that drain your patience and pocketbook and that can cause safety issues. A quick fix may alleviate the problem for the short term, but if the system remains inadequate, problems will continue to plague it.

To begin taking the right steps to achieve a good electrical system on your boat you need to:

Determine your power requirements
Provide adequate power storage and starting capacity
Provide the proper charging that you need
Be sure your wiring is adequate and approved for marine use and connections are sound
Engage a qualified professional if needed
Be sure that all safety requirements, such as the relevant ABYC standards, are met

Your boat’s batteries are a cornerstone of a good electrical system. Power boats that are used simply to run from dock to dock on weekends may need only one (always two are preferable) starting battery which we’ll discuss below. These are less expensive and lighter than batteries designed for deep cycling, which is what you’ll need if you stop for periods of time to anchor or drift or, far more importantly, if you plan to anchor overnight. Boats that are used for long term anchoring, as in serious cruising, need batteries capable of deeper discharges and of greater storage capacity. Such batteries include wetted lead acid, gel and AGM deep cycle types. However, even if you just plan to run from dock to dock or dock to fishing ground or beach and back, it’s important to realize that you may be using electricity during those times that the engine is off and be prepared with the right type and size of batteries.

Batteries need good wiring to deliver the power supply to the gear onboard. Since batteries are essentially fuel cells, charging the batteries is another important element. For additional detail on that subject go to our Battery Chargers article. There are both DC (direct current as from 12 volt batteries) and AC (alternating current like what you use in your house) systems and issues on many boats, although some boats, particularly smaller ones, only use DC. Here we’ll discuss primarily DC. AC options are discussed in some detail in the section “Inverters“.

Determining Your 12-Volt Power Requirements

First, calculate your daily (24-hour) average power consumption for all of the electrical loads you place on your system. List all of the appliances and their amp draws. If amps are not listed on the appliance, you can figure amps with the following formula:

amps = watts / volts

Next, estimate the normal daily usage for each in hours, so that you have a list of appliances and their daily draw in amp-hours (Ah). Now total them all up. But don’t cut it close. As battery voltage lowers with us amperage draw will go up. Also, amperage draw increases with wire run. This means that the total of the power consumption numbers on your equipment will probably be less than your actual usage.

What Size Battery Do You Need?

Let’s assume your daily power consumption totals 100 Ah. A 100-Ah battery won’t do it. Why? Because battery capacity is determined in part by the intervals between battery charges, and the discharge level. A 100-Ah battery might in theory meet your daily energy requirements, but would have no reserve. And a battery should never be fully discharged. It must be able to store and deliver the full 100-Ah between charges. Further, with each partial discharge of your battery its capacity will gradually diminish.

Automotive starting batteries are made for starting engines, with the quick release of a big burst of power. They discharge only about five percent, and are intended to be immediately recharged by the alternator. They cannot handle the repeated deep discharges typical of marine use and they may suffer damage from the constant pounding they receive at sea. Conventional wet (lead acid) deep cycle batteries or deep-cycle gel cells or AGM batteries are best at withstanding the deep discharges, rapid high power recharging, and the physical pounding of the marine environment. Look for batteries with the greatest number of life cycles at 50% discharge and do not mix batteries of different construction type (such as, for example, wetted lead acid and AGM) in the same bank – use one or the other. There are also dual purpose batteries claimed to provide relatively high engine starting power for their size but also relatively good deep cycling capability. (Don’t count on these for cruising with repeated heavy deep cycling.)

You will possibly double the life of your battery if you don’t discharge it below 50% capacity. Consider also, for example, that lead acid batteries recharge rapidly only up to around 70-80% of capacity. If you don’t want to spend a lot of time recharging that last 30%, plan on using only about 30% of the battery’s full capacity. And it’s better to consume even less than that because no battery operates at 100% over its full life. Also, give yourself a little power in reserve. Given all this you, in theory, will need a 400-Ah wetted lead acid battery to meet your 100-Ah daily energy habit. In general, a battery rated at four times your daily usage will be adequate. But, as noted, this is only theoretical. Other factors, such as, for example, voltage drop (and therefore amperage increase) caused by long wiring runs, will affect the usage and size requirements. If you’re thinking that all this sounds like a real drag; that you’re going to need to be boating with a calculator to deal with batteries, relax. With good modern battery chargers and alternators and a little common sense and observation on your part, your equipment will handle much of these issues. There’s more on this below.

Battery Banks

Reserve an adequately-sized, fully charged battery or bank of batteries solely dedicated to starting your engine. Some prefer to use what are commonly referred to as “starting batteries” dedicated to engine starting. These typically have thinner plates but more plates and therefore can deliver more Cold Cranking Amps (CCA) for size and weight. CCA is the measure of how many amps the battery will supply to the starter motor for 30 seconds continuous at 0°F while maintaining its voltage above 7.2 volts. Marine batteries may also be rated in Marine Cranking Amps (MCA) which is similar to CCA except at 32 degrees F. Starting batteries are typically not as tough in construction as batteries designed for deep-cycling and less expensive. These will not serve well if you need them for deep-cycling because such will quickly deplete them, leading to early failure. A deep-cycle battery can be used for engine starting, as long as it provides enough cold-cranking amps (CCA) to easily start your engine.

Use a separate bank of deep-cycle “house” batteries to supply the rest of the boat’s electrical needs. Or, you can use two banks of deep-cycle batteries, each with enough cold-cranking amps to start the engine. Parallel the two banks with a dual-purpose battery isolation/selector switch for tough engine starts and then alternate between the two banks for “house” use. Always, however, keep one bank fully charged for starting.

Battery Charging

If you don’t replace what you use, your batteries will eventually go dead, no matter how much battery capacity you have on your boat.

The rate at which you can recharge your batteries depends on a number of factors including how much you discharged them, the condition of the battery, the temperature, the alternator’s power (in amps) and its output (in volts) as well as an AC battery charger’s capacity and features.

Many boats charge their batteries with an engine-driven alternator. If your engine running time is minimal, you want to charge as quickly as possible, without damaging the battery. Battery damage begins when the internal temperature becomes too high, causing it to gas and heat up. If it feels warm to the touch or if you’re smelling gas, it’s getting too hot. A smart voltage regulator for the alternator should control and taper off the charge to prevent this and can also be set, within limitations, to accommodate your needs. More on this below.

The marine alternator is generally more suitable for use on a boat than the typical automotive alternator sometimes found as original equipment on new gas and diesel engines. Alternators are typically rated in amps; the rating refers to the maximum output in one hour at a certain temperature and rotation speed. Typical automotive alternators can’t run continuously at full load in the warm temperatures found around boat engines, or in warm climates. Further they have internal voltage regulators that are set to deliver a rapid charge to a starting battery, not to deliver sustained tapered intelligent deep cycle recharge for house batteries. A suitable marine alternator may be rated at full continuous load at temperatures up to 200°F. Some marine alternators have special grease in the bearings and other special construction to withstand the higher running temperatures and loads as they continue to put out at higher levels during deep cycle recharging. They are also more robust in many other respects.

A good rule of thumb is that you will need at least 120% of the energy you used to restore the battery. And to prolong the life of your alternator, plan on running it at less than full output. Take into account any other power-draining loads you might be adding to the system as you are recharging, such as refrigeration or inverters. If you install an oversized alternator, you can recharge efficiently while at anchor, with the engine at idle and the alternator operating below its rated speed and output. (However, it isn’t helpful to the engine to run it for long periods of time under low load. Many prefer to charge batteries while at anchor using an AC generator and AC battery charger. More on this below.) Again, a “smart” regulator which you can set to perform optimally for your parameters is needed.

The speed of your alternator’s rotation is a function of engine RPMs and your alternator’s pulley wheel size. Once you have determined the maximum alternator output you require, add around 25% or more so you won’t have to operate it at full bore to achieve the required results. Now check how many alternator RPMs it takes to reach that output. Then determine the minimum engine RPMs at the engine’s power take off wheel at which you will be charging. You need a pulley ratio that gives you maximum required output at your minimum engine speed. If you need 110 amps, and it takes an alternator RPM of 4,000 to generate it, and your engine is running at 1,000 RPMs, then you need a 4:1 pulley. Make sure that if you punch your engine up to 3,000 RPMs, thereby increasing your alternator speed to 12,000, you are not exceeding maximum safe alternator speed. Typically, the manufacturer of a good marine alternator should provide this information for your application and also the alternator pulley wheel that you need.

Voltage Regulation

A voltage regulator determines the rate and manner which an alternator or AC charger will charge your battery. Normally we think of voltage regulators specifically for alternators, but even AC chargers have internal components that determine rate and manner of charge. In better units these internal components can be field adjusted. If charging isn’t done properly your batteries can suffer failures ranging to total depletion to overheating, gassing and essentially self destructing.

The basic alternator voltage regulator maintains voltage at a certain level by matching alternator output with the load and the charge level of the battery. Voltage drops when a load is placed on the power system, or when the battery discharges. The regulator then increases the amperage output of the alternator until the voltage level is restored, and then tapers output to a level that will sustain that voltage.

You should have a regulator that is external, and field adjustable, so that you can adjust the alternator’s performance, tailoring the alternator’s output to the type and size of battery(s) that you are charging. A “smart” voltage regulator can also be set to charge specific types of batteries in a way that will not only charge them well, but also give them longer life. A good marine adjustable smart voltage regulator should be able to charge at three stages, often called “bulk, absorption and float.” Bulk puts a lot of power into the battery rather fast, to bring it up rapidly to a certain point. Then the regulator will switch to absorption mode before the battery gets too hot or gasses or becomes otherwise damaged. At absorption, charging occurs more slowly to provide more subdued charge commiserate with the state of charge of the battery. Once the voltage is at the desired level the regulator shifts into float which essentially maintains the battery, making adjustment for usage. Some regulators even have heat sensors on the bank(s) so that they can compensate for that factor. Of course all of this must be set for the parameters of your battery and/or bank. Different sizes and different types (such as lead acid, Gel and AGM) of batteries require different types and rates of charging, as well as different voltage levels. Usually instructions with the voltage regulator will help you with this.

Your 12 volt system can also be used to supply AC current, like you have in your house, with the addition of a marine approved inverter which will invert the DC current (normally obtained from your battery bank) to AC. For further information see our article on Inverters.

Inverters increase the load on the battery and bank size and alternator charging must compensate for this. Of course, the greater the AC draw from the inverter, the greater the inverter’s DC draw from the battery. Also, as the inverter is converting at higher amps, its efficiency will diminish, again causing higher rate of battery depletion. This makes it even more important to have a robust marine alternator controlled by an external adjustable voltage regulator. With suitable equipment, you can use AC current while you are running and keep the batteries topped up. If the battery banks are large enough for your consumption and properly charged, you can have quiet times at anchor with AC power available without running the generator all the time.

There are now AC powered battery chargers on the market that can charge and maintain not only different banks but also banks that have different construction techniques, such as wetted lead acid, gel and AGM. These essentially jump from bank to bank in sensing and do what’s needed for that bank. Of course, you must set these up appropriately for your system. When you charge your batteries when AC is available (from the dock or from an onboard marine generator) through a battery charger, it should be rated for marine use and also have “smart” charging with similar charge settings and charge characteristics discussed above as to the alternator’s voltage regulator.

With multiple battery banks many people prefer to have a device which automatically shifts charging output to different banks so that no one bank is overcharged. However many prefer to monitor bank status manually and use a battery selector switch to direct charging current to the appropriate bank. Failure of automatic devices, often hidden away in an engine space, can result in improper charging leading to discharge or overcharge which will “cook” a battery, ruining it, and which could cause explosion or emission of large amounts of corrosive explosive gas.

Wind And Solar Charging

Many people also use wind generators and solar cells to keep their 12 volt system charged. Solar cells generally have less output for the money than wind generators and require special mounting racks or cabin or bimini top space. Wind generators can usually put out more current, but only if the wind is up. They make noise, which some find objectionable but others find soothing. Usually people become accustomed to it. Great care must be taken to avoid being hit by a propeller and also to shut it down when the wind gets too high. Better wind generators have a self contained automatic dampening mechanism to prevent over-speed in high winds. One or both systems are often used by those who don’t consume much electricity or who want to keep their batteries up while they’re off the boat, particularly as when there’s no dock power available as when they’re at anchor or on a mooring. Even with these systems you must provide overcharge protection. Devices that do this usually come with the wind generator or solar cell.

We can only scratch the surface of this subject, which could easily fill a large book. If you want to be hands on (and that’s good) read some of the many books on the subject. In any event, it never hurts to involve a well trained competent professional when dealing with any kind of electricity on a boat.

Technology, knowledge and practices change almost daily therefore it is prudent to research for the very latest up to the date information and seek qualified professional assistance when needed.

BoatUS Editors

Contributor, BoatUS Magazine

About Vessel Vanguard

Vessel Vanguard is a leading marine safety and maintenance management software provider dedicated to revolutionizing the maritime industry. With a commitment to innovation and excellence, Vessel Vanguard delivers cutting-edge solutions to streamline operations and enhance vessel performance and safety.