Nautica Digitale
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Following the previous issues on the electrical plant, lets start with some useful advises to optimize this essential part of the boat


  1. Remember, when leaving the boat, to switch off all electrical appliances, to open the main batteries switch and the shore line switch. Please keep in mind that "to open a switch" (usually this position is indicated by "OFF" or "0") means to break the circuit, while "to close a witch" means to connect a circuit (usually shown on electrical panels with "ON" or "I"). It may seems obvious but, in everyday life, some misunderstanding may occur.

    Before opening the main batteries switches, be sure you shut off all the electrical appliances which are in use or which have current. The battery switch is not a common switch and can be seriously damaged by high electrical loads.

  2. Shut off all circuits and magnetotermic switches which are not in use, when you are on board as well. Unused circuits under tension can improve the possibility of damages.

  3. Remember to worm up the generator set for 2-3 minutes, especially during the winter, before supplying the electric plant. This is, on the other hand, an excellent practical rule for all engines.

  4. Check, just after starting, that the generator sea water cooling system is running. In this case, the sea water flow is visible at the exhaust pipe outlet.

  5. Shut off all AC current appliances before stopping the generator: stopping a generator under load may cause damages to the generator itself.

  6. Remember to run the generator without any load for a couple of minutes before stopping it, especially if the generator's engine is turbocharged. In this way the oil will cool down and it will cool down the generator as well.

  7. Check the ammeter (the instrument which quantify the amount of energy being used) every time you turn on or off an appliance. This simple habit will allow you to detect immediately every inefficiency which may occur.

  8. The ammeter is the instrument which quantify the amount of energy being produced by the engines' alternator. Once the engines are started, the instrument will first display an high value because the batteries are being charged. This value will constantly reduce till the batteries are completely charged. From this point on, the instrument will show the same amount of energy which is being used on board (light, instruments, pumps etc.). In this case the batteries absorb from the alternators the same amount they give to the on board plants.

  9. The automatic battery charger employ the same criteria. First it will provide the energy required to re-charge the batteries, and then it will deliver the energy required by the on board plants.

    Some battery charges have a manual option which allow to keep on charging the batteries even after they have reached their maximum capacity. This operation is, however, extremely dangerous, because it cause batteries overheating which can seriously damage the batteries themselves.

    The building yards should, in my opinion, supply those type of battery chargers with a "safety switch", so to be sure that the "manual" option is available only when required. The battery charger must be set on "manual" only if the automatic circuit is damaged or in case the batteries are completely down thus requiring a particular charging method: in both cases a constant inspection is required.

  10. Each engines drive one alternator (on twin engines installation). Every alternator is connected to a batteries set. Do you know which alternator give current to the auxiliaries batteries?

    It is very important because, when you start the engines, you have to start first the one which is connected to the engines' batteries, so that the other engine will surely start.

  11. Before you start the engine you have to disconnect the battery charger so to avoid interference with the alternator . This is a good practical rule although most boats are fitted with circuits especially designed to prevent this problem.

  12. The generator set has its own starting battery, which is charged by the generator itself. If this battery is down, you can only start the generator by connecting it to another battery with an electrical wire (just like those used on cars). Check, when you buy them, that they are long enough to reach one of the batteries on board. Connect them properly, and remove them with care right after the generator's start. Be aware of short circuit, when removing the wires.


Batteries, used on boats today, are of different types, such as: car batteries, industrial batteries or nautical batteries. The performances required depends on the job they have to do. In fact, a battery for main engine starting must support short but heavy loads, while auxiliaries batteries must supply energy constantly and for long time. Therefore one should expect different types of batteries on board, according to their purpose. Economical requirements, however, usually influence the yard's batteries choice; sometimes the batteries employed on board are not those especially designed for the required application.

All batteries, anyway, need maintenance, even many of those certified as "maintenance free".

All batteries are manufactured in a similar way: an external chassis contain the "elements" which are drown into the "electrolyte" . The elements are divided one another by cells which are electrically connected: each cell has its amount of electrolyte and its level must be constantly checked. On liquid electrolyte batteries (the most common), the solution is made , for the majority, of distilled water. Its evaporation reduce the level which need to be filled up once in a while.

Evaporation occur due to overheating: this happen because of the location of the batteries (the engine room is a real hot place, especially during the summer) and because of the periodical re- charge of the batteries, which produce heat. If the charging current is high, in particular, the electrolyte may evaporate quickly and if the elements are dry (even partially) the battery may be seriously damaged. Some of the most modern batteries have a creamy electrolyte (gel), which is less exposed to evaporation problems.


First of all, and in case the shore line is connected or the generator is running, you have to shut off the battery charger,. Then you have to disconnect the DC plant (which is the one fed by the batteries) opening the batteries switch. Then we will disconnect first the negative wire followed by the positive. Particular care must be exercised in storing those two wires, in order to avoid accidental contacts between them or short circuits with other batteries. Covering each terminal with insulating tape is, however, good practice. Afterward the top surface of the battery can be cleaned with a wiper, so to avoid dirt entrance when opening the caps. It is a good safety rule to wear protection gloves (those usually used for dish-washing are fine) in order to avoid contact with corrosive solutions.

Then we will check the electrolyte level, filling it up, if necessary, with distilled water only. The electrolyte must, however, be clean, pure and transparent: if the liquid is cloudy, then the element is surely damaged. The inspection of a battery condition must be carried out with a densimeter, which is an instrument measuring the density of the electrolyte. It is, essentially, made of a graduated glass tube, a float and a hand rubber pump. The latter allow the entrance of the liquid, while the float indicate on the scale the amount of charge of the element. The working principle of the densimeter is based on the fact that the electrolyte density is proportional to the charge of the element.

After each measurement the liquid will be replaced in its cell, which is then closed.

Attention: the charge condition of a battery can not be measured only by checking the voltmeter on the main electrical board. In fact, a damaged battery can show a normal "empty voltage" (meaning the voltage shown when the battery is not supplying power) but can not be able to deliver voltage to the electrical appliances.

The next step is to remove the batteries from their housing: particular care must be exercised, because they are usually heavy and, therefore, dangerous.

Attention: the electrolyte is highly corrosive. Always wear safety glasses and protection gloves before handling the batteries. In case of skin contact wash immediately with water; in case of eyes contact consult a doctor immediately.

We can, then, clean the batteries' holder, washing it with fresh water, so to remove every acid remaining. Dry carefully the entire area afterward.

The battery holder may not be watertight. If this is the case, the water will surely drain to the bilge. Please remember to dry out the bilge before the cleaning operations start, and to dry it again once the operation is completed. Remember that is forbidden to discharge overboard polluted waters: and this is good practice as well!

Clean the battery with fresh water (better if you use distilled water) and dry it out with care.

The electrodes, those poles to which the terminals are connected, must be sanded to remove oxidation residues.

Check the terminal to wire connection and replace it if this is the case (ask to a professional electrician). All the above operations are necessary to ensure a perfect connection between the batteries and the electrical appliances they supply.

I want to be clear on the fact that a poor connection will compromise the overall performances of the electric plant (the lower the voltage the higher the consumption) while it is dangerous as well, because the improved "connection resistance" lead to terminals overheating causing fire on board in the most extreme cases. On the other hand this is true for all electrical connections.

Vibrations on board are responsible for connection loosing and, therefore, I suggest to check connections and terminals habitually. Once you are done with all these inspections, you can place the batteries in their housing and re-connect the terminals, being sure they are fastened properly. It is good practice to cover the connection with Vaseline, in order to avoid air contact which is responsible for oxidation.

Remember to check constantly the liquid level, in orger to understand the evaporation rate of your batteries and to schedule the filling period. All the elements must be covered by the electrolyte.

If the batteries discharge too fast both the batteries and the electric plant may be responsible: in this case call immediately an electrician.

The batteries may need a charging period: it is usually performed discharging completely the battery and then re-charging it slowly.

If the problem depends on the charging devices it is necessary to check the engines' alternator or the battery charger.

Dispersion may cause fast battery discharge rates as well. In this case, the best thing to do is to inspect the electric plant with a professional electrician.

Here are some practical advice to keep in mind:

  1. The batteries' holding box serve the purpose of storing the batteries and preventing them from moving: it must be always easy to reach.

  2. The box cover serve the purpose of keeping clean the batteries, of avoiding short circuits and water entrance. It must be easy to remove without any particular tool. In case your battery box do not have a cover, remember to shelter the terminals with rubber protections (just like on cars)

  3. Be sure that the batteries' position is high enough to ensure a perfect working condition even with some water in the engine room: twice the water level needed to start the bilge pumps is a good reference point.

  4. The batteries are among the most important part of your boat, supplying energy to all on board appliances: do not forget them in their dark tiny space in the engine room and consider them as "essential components".