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Boat Systems:Electric's

I have lived on a boat for many years since retiring from an electronic / engineer career. I now realize that there are a lot of misunderstandings about electricity and its power. As a user it is important to have at least a basic understanding in order to rectify a fault. After all the cause of a problem may well be something quite simple.

There are many books about the subject. The one I find easy to understand is by Alastair Garrod called 'Electric's Afloat'.


Electricity is GENERATED by using moving coils of wire and magnets. Power Stations and Alternators generate electricity. The more energy you put in the more electrical power you get out.

Electrical POWER is measured in WATTS. POWER is the product of 'pressure' measured in VOLTS and 'current' measured in AMPS.

Think of this in terms of water pressure in pipes and the flow of water from a tap is the current.

Power Stations supply POWER as ALTERNATING CURRENT at a 'pressure' of 240 VOLTS as 'mains' to houses and marinas. 'Mains' is dangerous because of the current it can push through your body.

Alternators supply POWER as DIRECT CURRENT at a 'pressure' of 14 VOLTS to supply power and charge batteries for use on mobile cars and boats. Sometimes the 'pressure' can be 28 VOLTS.

Batteries are used to STORE electricity. They do NOT GENERATE electricity. Think of them as containers full of water.


Power is the RATE of doing WORK. The faster you run up a hill the more powerful you are.

Electrical and electronic equipment like LIGHTS, TV's, FRIDGES and WASHING MACHINES consume POWER. A kilowatt is a thousand watts. A kilowatt consumed in an hour is one unit of mains electricity.

The power required by an item of equipment is on a label fixed to it. I have found that it is often not stated in the shops and the only way is to look for the label! The information may state the VOLTAGE and CURRENT or VOLTAGE and WATTS.

It is of course important to supply the correct VOLTAGE to the equipment. High POWER equipment consume more CURRENT measured in AMPS. WATTS = VOLTS X AMPS.

The more power you use the more energy you have to use to generate it. So it costs more. Your batteries will need charging more often.


The ALTERNATOR on the engine is your prime source of DC POWER.

Ideally two are required. One to charge the engine start battery and one to charge the domestic battery bank. The two systems should be kept separate.

Where only ONE ALTERNATOR is on the engine its power is wired to charge both engine start battery and the domestic battery bank. But in such a way as to keep both sets of battery separate. This is achieved by using a Split Charge Relay or the more reliable Diode Block.

A 100 Amp alternator generating 1400W of power at 14 Volts would be used to charge the domestic battery bank and supply power to consumers while the engine is running.

When just charging the domestic batteries the alternator should be rotating at about 3,000 Revs Per Minute to generate the rated power. As the engine idles at about 1,000 RPM you need a pulley ratio of 3:1. My Beta engine has a 9 inch pulley driving a 3 inch pulley on the alternator.

Internal Alternator Regulators

The standard voltage regulator built into the alternator is designed to charge a car starter battery. It will not fully charge 'Domestic' deep cycle batteries. A lack of temperature compensation causes under charging in the winter and over charging in the summer.

External Alternator Regulators

These ensure that batteries are charged to 80% or better by sensing the battery voltage and usually have temperature compensation which ensures correct charging voltage winter or summer. The charging voltage when the batteries are cold is higher. (14.0V at 20c to 14.6V at 0c). The sensor should be on or near the battery.

The ADVERC Battery Management System works by cycling the output voltage between 14.0 and 14.4 Volts at 20c.


Batteries store the energy by using a chemical process which needs time to react. Battery capacity is measured at a 20 hour rate. In other words a 100 AMP HOUR battery can deliver 5 AMPS for 20 HOURS to empty. But you can only use 30% of the stated capacity because you must not discharge below 50% and generally cannot charge above 80%. That is 5 AMPS for 6 HOURS. Discharging at a higher rate will give less capacity.

Engine Start Battery

These are designed to produce quick bursts energy and are poor on capacity. They have a large number of thin plates which can deliver the high current needed to start an engine.

Leisure Batteries

These are designed to deliver low current for a long period of time. They have a small number of thick plates. The thick plates are high in capacity and require time to deliver and accept the current. These are similar to a true deep cycle Traction battery.

Absorbent glass mat (AGM) batteries

In AGM sealed batteries the acid is absorbed between the plates and immobilized by a very fine fiberglass mat. This glass mat absorbs and immobilizes the acid while still keeping the acid available to the plates. This allows a fast reaction between acid and plate material. Because they are completely sealed they can't be spilled, do not need periodic watering, and emit no corrosive fumes, the electrolyte will not stratify and no equalization charging is required.

Their unique construction also allows for the lead in their plates to be purer as they no longer need to support their own weight as in traditional cells. The AGM battery has an extremely low internal electrical resistance. This, combined with faster acid migration, allows the AGM batteries to deliver and absorb higher rates of current than other sealed batteries during discharging and charging.

Battery Maintenance

Occasionally check the electrolyte levels in each cell of every battery. If necessary top up with distilled water only. If they are persistently dry check that the charging voltage not too high. If that is correct then the batteries may need replacing. They are designed to charge / discharge about 300 times. It is always best to replace all the batteries in a domestic bank because one 'bad' battery will become a load to the others. Like leaving a light on all the time. As the batteries get old the effect may not noticed. It's just that you find they need charging for longer to maintain the voltage.


Think of this as a water pipe filling up your water tank. A bit like cooking a chicken. Light the oven to get it up to temperature then wait for the chicken to absorb the heat. A lead acid battery needs time to absorb the energy.

Lead Acid batteries prefer to be kept fully charged. Your engine start battery will usually be 100% full but the deep cycle domestic batteries will be 80 - 90% full at best and should not be discharged below 50%. The lowest point of discharge is usually in the morning so it is advisable to recharge as soon as possible in the morning. It is good practice to charge for at least 2 hours in one go. The first hour is a bulk charge where the voltage rises to 14V and most of the current is delivered. Then while the voltage is held at 14V the batteries will soak up the charge like a sponge for the next hour or so. The longer you leave a lead acid battery discharged the more capacity it will loose.

The size of your Alternator will determine the minimum battery capacity it can safely charge. Charging at too high a rate can cause heat which can distort the internal plates. More batteries will reduce the rate per battery but too many may mean that the batteries do not get enough charge.

The usual rule is 20% of battery capacity. In other words if you have a 100 Amp alternator it can safely charge a 500 Amp Hour battery bank.

My 70 Amp Alternator was connected to a 550AH battery bank. 70A / 550AH x 100 = 12% charge rate. Over time this proved to be too low and the batteries lost capacity.

I now have a 110 Amp Alternator connected to a 550AH battery bank. 110A / 550AH X 100 = 20% charge rate which is correct.

To properly charge a deep cycle battery, a voltage of between 14.0 & 14.2 is required. A voltage above 14.2 will cause the battery to boil dry. (GASSING) If you do not fully recharge the battery it will rapidly deteriorate. A battery will require the replacement of 120% of the discharged current. Lead acid batteries are only about 80% efficient.

In order to reduce the dreaded voltage drop a battery bank of four or more needs to be wired correctly. Each battery will take up to 20 Amps so even a small resistance could result in a loss of charging voltage. If the bank is fed from one end the battery at the other end could be under charged. Ideally the positive connection should be at one end and the negative at the other. This would include the feed from the Alternator and supply to the domestic circuit and the Inverter.

Plate Sulfation is the single greatest cause of battery failure. During disharge the chemical reaction causes both plates to convert to lead sulfate. If recharging is not carried out promptly (within a couple of hours?) the lead sulfate starts to harden and crystallize and is not reversible. The effect is loss of capacity. As long as some charging is taking place, even from a small solar panel, a chemical reaction is taking place and sulfation will not occur.


The IUoUo characteristic. (Victron charger)

Boost:....Voltage rises slowly to 14.2 while charge current is at maximum.

Equalize: Voltage held constant at 14.2 while charge current falls. (Equalizing charge between cells.)

Float:..... Voltage held constant at 13.2 while charge current minimal.

The true way to determine the charge state of a battery is to measure the Specific Gravity of the electrolyte. The voltage is a guide only. A battery is suspect if any cell varies by more than 0.03 (eg.1.23 / 1.26) after charging.

Percentage full

Specific Gravity

Terminal Voltage










Note that when 50% discharged the battery voltage is 12.2


These convert DC power from the batteries to 240 AC VOLTS. (Mains). They do not generate power. The best type is 'Sine Wave' like household mains power. Many cheaper types are 'square wave' and can damage sensitive equipment.

Inverters require a bank of batteries of sufficient capacity for its rated output.

Victron Energy quote:- Rated Output Battery Capacity

800 Watts 100 to 400 Amp Hours eg. 2 x 100 AH batteries minimum

1,200 Watts 150 to 700 Amp Hours eg. 4 x 100 AH batteries minimum

1,600 Watts 200 to 700 Amp Hours eg. 6 x 100 AH batteries minimum

Inverters are at best 80 to 90% efficient so 10 to 20% of the energy input is lost as heat. Most inverters will have cooling fans. The worst place to put an inverter is in the engine bay or hidden in a cupboard where it cannot keep cool.


It is recommended to run the engine for POWER above 250 WATTS because the batteries on their own will be discharged too rapidly. The Washing machine demands the most power so the Alternator must deliver that power. (100 Amps at 13 Volts)

This list gives you an idea of how much power may be required on a boat.


1,300 W
1,000 W
900 W
700 W

1 Hour
0.1 Hour
0.1 Hour
1/2 Hour






14 W
100 W
60 W
60 W
30 W
11 W
104 W
52 W
52 W
13 W
26 W
1 W
13 W

8 Hours
4 Hours
1/4 Hour
5 Hours
2 Hours
4 Hours
1/4 Hour
1/2 Hour
1/4 Hour
24 Hours
2 Hours
8 Hours
8 Hours


8.6 Amp Hours (WH X 13V)

This is only a rough guide. You would need to apply your own data. In practice my boat uses about 60 Amp Hours per day. Please note that the fridge is on all the time. The Inverter draws power when on, even when not supplying mains power.


Incandescent lights are power hungry and are yellow when voltage is low.

10 watt Halogen lights are efficient and last longer. (20 lumens).

Florescent lights use less power than incandescent lights, typically 8 or 16 watts. (65-90 lumens).

Low Energy Lights are similar to florescent. 16 watt input provides 100 watt light output. These however usually require 240AC mains power.

Light Emitting Diodes (LED) are the most efficient. A 1.8 watt cluster of 9 LED's will produce 90 lumens. While their power requirement is very low they will need a voltage regulator circuit as LED's are voltage sensitive.


These panels convert sunlight into electricity directly. There are two types of cell made from Amorphous silicone (blue) or Polycrystalline silicone (black). Polycrystalline is more efficient.

Outputs can range between 12 and 130 WATTS (10 Amps at 13 Volts) per panel. A regulator is required to limit the charge voltage if output is more than 1% of the battery bank capacity. i.e.: 4A into 400 Ah. Lower output units would not need a regulator.

However the outputs quoted are for full sunshine and maximum output. The panels can be mounted flat facing the sky or at an angle to face the sun. When flat their output would be lower but they are still capable of producing useful power through daylight hours even when cloudy.


Up to 100 watts of power. Typically 4 Amps in a 20 knot wind. A regulator is required to prevent over charging.


Be Safe. As in a house mains is dangerous. But in a boat it can be more dangerous because it is floating in water. Shore power and Inverter power are equally dangerous. The European Recreation Craft Directive states that the earth should be connected to the metal hull. The earth must not be connected to neutral on the boat.

Shore Power

It is essential to be sure that the shore supply has been tested. The live, neutral and earth wires must be in good condition. If a shore side fault exists the DC Negative will provide an alternative path through the water. All 'safe' DC equipment could become live.

Earth Leakage Protection.

The European Recreation Craft Directive dictates that a Miniature Circuit Breaker /Residual Current Device should be within 0.5m of the shore cable. This is the best way to protect circuits and people both ashore and afloat. A 100mA trip protects the circuits against fire. A 30mA trip protects people against shock.

Use of Residual Current Devices (RCD's) is reliable but nuisance tripping can be caused by :-

1. Connection of Neutral and Earth down stream of an RCD.
2. A crossed Neutral between protected and unprotected circuits.
3. Deterioration of cable insulation.
4. Water and moisture in junction boxes.
5. Absorption of moisture in heating elements. (Kettle, immersion.)
6. Tracking across dirty surfaces.
7. Intermittent arching in motors.
8. Switching off inductive loads such as motors.

Mains Installation on the boat

Both AC and DC wiring should be double insulated and kept separate. This way a dangerous situation is made difficult. All wiring should be multi-stranded copper and be capable of carrying the total load used on the boat.

Galvanic Isolator

Connecting the mains earth directly to the metal hull means that all boats in a marina share the same mains earth. Unfortunately there is now a galvanic DC relationship between the boats so some will suffer serious corrosion. Anodes on the boat will be sacrificed. The Galvanic Isolator will allow a full AC earth fault current to flow to ground but will not allow galvanic DC currents to leave or enter the boat.


Now for some thing completely different. An electric motor can be used to drive the propeller. The power comes from batteries but unlike the domestic battery set up the arrangement is very different. The following information is based on the installation in a narrowboat by Thames Electric which was featured in the October 2007 issue of 'Canal Boat' magazine.

Now before going on it must be made clear how the power is measured. Electric motor power is measured in WATTS. Remember that is Volts x Amps and diesel engine power is measured in Horse Power. Now it is agreed that approximately 745 WATTS equals 1 Horse Power.

The electric motor runs on 48 VOLTS and can demand a maximum of 200 AMPS, that is 9.6 Kilo Watts or 12.8 Horse Power. But normal cruising speed demanded no more than 50 AMPS, 2.4 Kilo Watts or 3.2 Horse Power.

A normal diesel engine in a narrowboat is rated at about 40 Horse Power or 29.8 Kilo Watts. That is a lot more than the electric motor but of course the engine also provides hot water and charges batteries while it is running. The boat featured has both electric motor and diesel engine arranged so that either, not both, can drive the propeller. The engine is provided with no less than three alternators to charge the start battery, domestic batteries and the motor batteries. The domestic battery bank is a conventional 12 volt system for lights and pumps.

The motor battery bank however is a 48 volt system. The motor battery bank consists of 32 x 6 Volt traction batteries grouped into 4 sets of eight providing a total of 720 Amp Hours. Remembering that only half of this can be used, cruising at 50 Amps for 7 hours is possible. It is claimed that a one hour electric cruise requires a one hour recharge, either by running the engine or using a land line.

The batteries are charged using a 230 volt AC alternator feeding power through a pair of 3 Kilo Watt, 48 Volt 'Victron' inverter / chargers. The pair run independently as backup and provide the domestic 'mains' power.

The net result is a very quiet boat when on the move and environmentally friendly. When waiting in a queue at the locks no power at all is being used. While in locks the power is used when needed. Unlike the diesel engine which has to be running even when not driving the prop.

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