Battery and power management onboard a sailboat

Almost every sailing or motor boat has on board a more or less complex electrical system and a variable number of batteries. This depends on the size of the boat, the instruments installed and the equipment on board. Whatever the level of complexity, the same basic principles apply.

The electrical circuits of a sailboat and the batteries

First and foremost, electrical circuits on board a boat differ from domestic electrical circuits in the type of current used. Current is continuous rather than alternating and differs in voltage. The systems of the boats are at 12 or 24 volts, instead of the 220 of the domestic circuits in Europe.

On smaller sailboats we usually find 12 volt electrical circuits. On sailboats from 15 meters upwards and on motor boats we find 24 volt electrical circuits. Therefore, when you buy a new instrument or electrical device, check its compatibility with your on-board system.

Much electronics such as a chartplotters will have an automatic mechanism for selecting the correct voltage. Other types of equipment, such as a bilge pumps or watermakers, are supplied separately for 12 or 24 volt systems.

When we first analyse the electrical circuit of a new boat what is the greatest difficulty? It is not so much in understanding the basic principles of an electrical system. Often, the most difficult task is to untangle masses of electrical wires without labels.

If you get your hands on your electrical system, learn how to label each wire. Not so much for elegance but to be able to solve a problem quickly in an emergency.

To get to understand an on-board system we will first have to understand the basic circuits, simplified as much as possible, separated into categories.

Marine Inboard Engine

Batteries for the inboard

From the positive pole of the inboard battery we will find a very thick red electrical cable that reaches the starter motor of the engine. From the engine, a black cable will return to the negative pole. A second wire of smaller diameter will be connected with the key on the control panel.

When you turn the key this secondary circuit will cause a relé to switch and the primary starter circuit to power the starter which by rotating will start the inboard. Of course this is a simplification, on a diesel we will find many other circuits and wires that are part of the basic circuit.

Such as cooling water temperature and oil pressure sensors. As well as the circuit of the pre-heating glow plugs of the combustion chambers of each cylinder.

If we eliminate everything else on board the boat this is the minimum circuit that allows our propulsion system to start. Knowing which cables refer to the starter motor will save you a lot of time if something goes wrong at sea.

The most frequent cause of problems on this circuit is oxidation of contacts. Oxidation prevents starting by reducing the amount of peak current flow. In fact, when we start a diesel engine, the initial peak absorption is very high.

Any contact oxidation problem can jeopardize the correct operation of the system. Other common problems are caused by malfunctions of the starter motor. Unfortunately on modern inboard models there is little you can do, it is not possible to start an inboard by hand if the starter motor fails. So take care to check the status of your starting system regularly.

Alternator with smart regulator

The alternator circuit

Continuing with our extreme simplification of a boat’s electrical circuits, let’s move on to the alternator circuit. On a small diesel engine we will find a distribution belt driven by a pulley on the camshaft axis. This is responsible for running both the cooling water pump and the alternator.

The alternator is conceptually just another electrical motor. But instead of being stimulated by the current to spin, this will produce current when it is turned by the engine.

As long as the alternator is turning it will help recharge the batteries. The alternator is a rather delicate and complex component and if you are not an expert it is unlikely that you will be able to solve problems at sea.

For this reason it is advisable to have a charging system for starter and service battery banks with two alternators. Especially for long navigations. The alternator circuit is parallel to the starting circuit but here too cables start and returns to the engine battery bank or to the split charger. These two circuits are the basis for having a functioning inboard.

Services battery bank

The services circuit

The services circuit is totally separate and independent from the two circuits described above. The only common point is the charging phase. In a typical system of a small sailboat, the alternator will first recharge the starting battery.

Then through a split charger it will begin to deliver the additional energy produced to the services battery bank. This will power the entire electrical system of the instruments and electrical equipment on board. If we can identify these three circuits, we are already one step ahead.

The energy to start the inboard

The engine battery is usually only one and must be of suitable size and characteristics to provide the necessary starting peak current to start the inboard. For example on a typical sailboat with a 30 horsepower inboard engine you will find a battery of 80 amps capacity or thereabouts.

The characteristics of the engine battery are the same as those required of a car starter battery. So a common acid-lead battery of the suitable size and of the sealed type is adequate.

The services bank

The services battery bank is used in the exact opposite way to our dedicated inboard unit battery. The engine battery must be capable of delivering a very high peak current for a short moment.

The services bank, on the other hand, must release relatively little current for very long periods of time. Especially when we are sailing or at anchor.

You have to imagine the services battery bank as a container of current with an open tap. The container empties slowly until you can replace what is consumed by refilling the container with a source of energy.

The sizing of the services bank

In order to determine the minimum size of your services battery bank you will need to calculate the expected consumption over a predetermined period, for example 24 hours.

Regardless of the type of circuit, 12 or 24 volts, calculate the amps your onboard equipment uses with this formula.

For example, an old style incandescence navigation light with a 12W bulb consumes, on a 12V circuit, one ampere per hour. On average, in summer, the anchor light will be on for 8 hours at night, consuming 8 amps every 24 hours.

Repeat the exercise for all the on-board systems, you can make two hypotheses, calculation of consumption when the boat is stationary, or at sea. For example:

  • LED navigation lights: 0.5A x 8h = 4A
  • Radar: 2.2A x 24h = 52.8A
  • Autopilot: 2.5A x 18h = 45A
  • Internal LED lights: 1A x 2h = 0.5A
  • GPS AIS: 0.75A x 24h = 18A
  • Laptop PC: 2.5A x 4h = 10A
  • Tablet phones: 2A x 4h = 8A
  • etc.

When you are done, add the column of amps to obtain a total daily consumption. Your service battery bank must be at least double or triple this amount.

This is because we do not have to look at the nominal capacity of the bench, we need to calculate how many amps are actually usable.

AGM battery

Battery charge and discharge cycle

Batteries, when fully charged, should have the capacity indicated by the manufacturer, for example 100 Amps. A battery is defined as completely discharged when it reaches 40% of its capacity. So with a 100 Ampere battery, the maximum effective capacity that can be used is only 60A.

Another principle to remember is that the capacity of a battery to be recharged, the absorption capacity, is inversely proportional to the state of charge. A very discharged battery absorbs much more than a nearly charged battery. Therefore it will be difficult in navigation to imagine recharging the batteries up to 100%.

It will be more efficient to discharge the batteries to 40% and then recharge them up to, for example, to 80% of capacity. Therefore the real usable capacity of a battery is only 40% of the nominal capacity.

On a cruise boat it is common to find battery banks of hundreds of Amps. Precisely because the actual available capacity is not much and must be commensurate with consumption. If you consume 200 Amps per day you would need a 500 Amps service battery bank. This is simply to allow you to recharge the batteries only once a day.

If you can, optimize your onboard power usage by always turning off anything you don’t need. Otherwise, you will be forced to run your inboard several times a day just to load the services battery bank. Alternatively you will have to integrate your electrical system with other charging systems, such as solar panels, wind, hydroelectric etc.

Types of batteries

There are many types and models on the market, the most common being those with acid-lead. There are also GEL technologies and AGMs with much higher costs. The top of the range batteries are the super light lithium batteries used on some racing boats.

Traditional sealed lead acid batteries, which you find installed in a RV or truck, are often inexpensive and perfectly adequate. Before you spend huge sums on batteries with supposedly highly superior performance, make a list of your spending priorities.

For example, you can mistreat lead acid batteries and replace them frequently. New batteries, even inexpensive lead-acid batteries, will always be efficient. Installing Gel technology, on the other hand, always implies being afraid of damaging them. And you will find yourself keeping them installed for many years to amortize the triple cost of the initial purchases.

Standard alternator regulator

The use of the inboard as a recharging system

Before reviewing alternative and more ecological methods, let’s talk for a moment about the inboard.

On a sailboat or motor boat you are carrying hundreds of kilos of inboard engine and in some cases as many of diesel. It is not difficult to come to the conclusion that this is the most powerful system you have available for recharging your batteries banks. An inboard with only 30 horsepower has a nominal power of 23 kilo Watts.

These would correspond to over 1900 Amps, if we were able to convert this power into electrical energy and store it immediately. In theory we would be able to generate our 200 Amps that we need daily in less than 10 minutes of engine per day.

Unfortunately the charging system is not so efficient. If you have only one alternator regulated by the factory regulator the reality is that it will take you 5 or 6 hours. This maybe split into several sessions to get your 200 amps, an incredible waste.

External alternator regulator

External alternator regulator

Before investing in any other ecological technology or not, it is therefore important to make the most of what you already have. Install a regulator on your alternator, an inexpensive electronic device that controls the magnetic field of the alternator.

By controlling it, it maximizes the current produced and manages the charge cycle of your batteries. With this simple installation you will immediately cut charging times in half.

Instinctively you would think that by installing an even more powerful alternator you can further reduce charging times. Hoever, remember, maximum absorption is dictated by the size of your batteries bank. So to be able to recharge faster with a larger alternator you will also need a larger services batteries bank.