Boat Fault Finding - Marine Electrical Problems

I am afraid that as each manufacturer/boat builder tends to use their own colour codes for wires, colours do not help much.

The cable running to the sender unit usually comes from the gauge. the gauge is then fed from a suitable supply - usually from the "ignition on" position from the ignition switch. There may well be a fuse in this supply.

There should be at least three cables at the back of the gauge - more if it has an illuminating light. One being the live supply (from fuse/switch), one running to the sender unity, and one being negative.

First connect a volt meter or testlamp to battery positive and the other end to both the gauge and sender unit negative in turn. In
both cases the meter should show battery voltage or the bulb should light. If not you must find the broken negative connection.

Next switch the ignition on and test between the gauge live supply terminal and negative. You should find a supply - if not look for a blown/dirty fuse, lose connection, or a broken supply cable.

If all the above is OK use a jump wire to connect the gauge terminal that runs to the sender directly to negative - the gauge should rise to full - if not the gauge is faulty.

If it does, repeat at the sender end - if the gauge rises find the break in the cable between the gauge and sender unit.

This basic procedure should work on nearly any electrical engine gauge.

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What alternator, and what do the terminals look like or what are they marked as?

You need three connections on the alternator, or two if it has it uses the body as its negative connection. If you can find two large, bolt type, terminals one will be pos. and one neg. - you need to look at the marks to find out which is which.

The negative needs connecting with very thick wire (something larger than 120/0.30) connected between the neg. terminal and domestic battery neg.

A similar sized cable needs connecting between alternator pos. & battery pos.

Use either a relay with its coil fed from the warning lamp terminal of the engine alternator to feed the domestic alternator warning lamp terminal from the domestic battery, via another warning lamp, or a rising oil pressure switch to do the same thing, but I would not advise the latter.

The alternator warning lamp will be a small one. If there is only one small terminal, that's it, if not you need to wait a few weeks until I get the electrical course notes on the website, then you can look up the markings (or buy a text book).

You could also consider taking a thin wire from ign.sw. on/instrument position, via another warning lamp, a (say) 10 amp diode to the alternator warning lamp terminal -

With this configuration you might get a slight glow on the warning lamp if you get a very flat engine battery.

If your alternator only has three blade terminals - two large and one small - the small one is the warning lamp terminal, the case is negative, and the two large ones are most probably both pos. output terminals.

If the case is negative or if you are feeding the warning lamp from the ignition switch, you need to use a thick wire to join the eng. & dom. battery negatives together.

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I am a bit careful about the 127 series because I think the largest one may actually be a re-badged Bosch machine and different from the rest. Also without the machine to check I can not be 100% sure.

First may I suggest that you contact Averc because they sell modified regulators for those alternators that require them. Thus you also get a spare regulator.

If they insist the wire you require is already accessible, the only one I can think of is semi-inside the alternator and connects to the side of the regulator via an insulated 6mm lucar blade. If there is much "gunge" about the back of the alternator this can be difficult to see, but you should see it if you remover the alternator.

I always remove the regulator fixing screws before trying to remove this wire. Take care in removing the regulator - this requires tipping and lifting at the same time so that the brushes that are fixed to back clear the end bracket and do not snap.

From memory I thought this wire fed the regulator, and that the Adverc needs a wire soldered to one of the brush holders so it can bypass the existing regulator - so please check with Adverc.

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Lets assume that you have bought a 12v inverter - thus the need to connect the batteries in parallel. I could also do with knowing the power of the inverter so I can advise on minimum cable size, but lets also assume it a big one - say 1000 to 1500 watts

The BSS requires that all electrical systems be fitted with a master switch, so you should have a switch, but you make no mention of how you intend to charge the batteries especially when away from a land line.

I think that the cable you need (assuming a maximum run of two to three yards) is with about 25mm sq conductor cross sectional area - this is starter cable, a 1500 watt inverter will draw over 150 amps!

Now the charging. I think the easiest thing is to simply add the new batteries to you existing domestic bank, even though they may ell be some way away for each other.

Because of the inverter load, I would suggest that you (again assuming a 12v system) make suitable leads up of the 25mm sq cable to join all the batteries negative terminals together, and another to join all the positives together and on to the existing domestic master switch. If this is an "open water" boat, its your insurance company that might have a view on master switches. If they do not, you can ignore them!.

Just reconnect any existing cables to to wherever they were connected before.

Run a cable from the negative on the inverter to some point on the wire connecting the battery negatives, but just make sure that you use nice heavy terminals.

You may have to uprate the domestic master switch to one that rated at least 170 amps cont and possibly more it it has a "no name" switch.

Run a wire from the output from the switch to the inverter positive - again the 25mm sq cable.

You should also do a power audit (electrics, course notes, on www.reading-college.ac.uk/marine ) and check the alternator will provide enough output to cover the inverter load during your normal cruising time per day, but if you have a 55amp alternator or above it will probably be ok.

If you have another scheme for charging the new batteries, simply join the negatives together (25mm sq) and then up to the inverter, and do the same for the positive, but taking via a master switch.

Because of the cable size, a short circuit WILL cause a fire, so fit a suitably large fuse or circuit breaker (possibly 200 amp) close to the master switch (very often one is not fitted, but its bad practice.)

Please, please do not stint on cable size, if you do you will get voltdrop under heavy loads which can cause all sorts of baffling symptoms.

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I am the proud owner of a little 1923 Dutch barge. Over the last year my partner and I have made some headway with the necessary renovation and upgrading.

The pre-purchase survey that was conducted, reported that both the 240V AC and 12V DC circuits were "DANGEROUS". To this end, we employed an old shipwright to correct the problems.

Although, I was a lot happier with both the 240 and 12 Volt circuits - In that they looked convincingly safe - I was sure that there was still room for improvement.

In particular, I had read some very worrying reports about Steel boats, marinas and electricity; and I decided that our electrics should be the very best to preserve the very old and thinning hull that my partner and I now reside in.

Starting with our 12V DC circuit.
We have one engine battery and two domestic batteries (both unfortunately are maintenance free). The engine battery has an isolator switch as does the domestic battery bank. There is a split charger to look after charging from the alternator. (This all seems fine).

I understand that everything connected to the engine battery ( engine instrumentation, the alternator, and starter motor) have the neutral returned via the engine mountings back to the engine battery neutral; and subsequently ,the hull is made part of this circuit. However, on closer inspection of the domestic 12V wiring I have found that the neutral is returned to the domestic battery neutral, in addition to the engine mountings. Which means that the hull is part of this circuit also. Is this right? and could this lead to avoidable corrosion of the hull? As far as I can see the engine mount is the only point where neutral makes contact with the hull.

240V AC circuit.
In my pursuit to preserve the hull from avoidable corrosion I have invested in a Zinc saver 2 Galvanic Isolator.
Our 240V arrangement seems very simple. We have two 20 amp circuits, protected by breakers. The circuits feed a number of plug sockets situated around the boat. It would appear that the earth is not bonded to the hull in any way. It simply returns from the various plug sockets back to the boats Mains Box, then back to the shore via the shoreline connection. With this arrangement I am not able to install the Galvanic Isolator, but more worrying, is it safe for the earth not to bond with the hull? Also, while the earth is not bonded to the hull, are we avoiding corrosion since the boat is not part of the marinas earthing? Please note that I do disconnect the shore line cable before any investigation into the 240V system.

First the 12/24 volt circuits.
Ideally all marine electrical items would be insulated return - that is with insulated positive AND negative connections, then the boat could be wired so the hull was totally isolated from the low voltage electricity. Regrettable, because of cost, many, so called, marine items are modified automotive ones with an additional connection fixed to the case marked negative (in DC we use positive and negative - line and neutral are for AC). This ensures that the engine block is usually at battery negative potential.

This negative side of the circuit is usually provided by a length of "starter motor sized" cable running between the starter motor case (or a suitable bolt very close to it) to starting battery negative. The idea is to minimise voltdrop on the starting & charging circuit.

With split charging systems it is usual to link the domestic and engine battery negative terminals together with a cable no thinner then the main charging cable. If all your battery negatives are connected by "starter size" cable, then all will work, but I would advise changing the engine negative cable to the starter battery (and linking to the rest) at your leisure. If the domestic and starter batteries are linked by "charging size" cable they must be altered at once (there could be a cable fire on starting) and the competence of your contractor questioned.

As long as you only have ONE negative connection to the engine/hull the danger of corrosion by low voltage DC is minimised - most canal boats are wired this way. Just make sure ALL engine and domestic negatives return to the battery by cable and busbar.

Now the mains earthing question.
I AM ONLY LOW VOLTAGE DC TRAINED SO IT IS YOUR RESPONSIBILITY TO VERIFY THE QUALITY OF THE INFORMATION I GIVE HERE - it is for help only.

General opinion amongst the more authoritative members of the waterways news group would indicate that the best method of minimising the danger of electrical corrosion is by means of isolating transformer (ideally soft start). These are expensive & bulky. The Zink Saver would appear to be second best, relying upon voltage drop across diodes for its effect.

With no earth to the hull, there is the distinct possibility that a fault would remain undetected until you killed someone/something by electrocuting them between hull/deck and the ground/water.

You must check, but I fear your system is in breach of regulation, and as such you would be held liable for accidents. I suggest you first read up on the subject and then employ a proper marine electrician.

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I think the CAV alternator that matches the 440D is a normal 6 pole machine, but I can not be 100% certain because its years since I stripped one (even then I did not count the poles). That is 6 north poles and 6 South poles.

My diagrams for the alternator/440 regulator setup do not show a suitable phase tap connection for the tachometer.

What you need is a connection soldered to the end of one of the phases (one of the thickish wires between stator & rectifier) so that you can "pick off" the AC output of that phase.

It is not difficult to do, but you could end up overheating a diode, so unless you are familiar with soldering, it may be better to take the alternator off and taking it to an autoelectrician for modification. You will end up with a lead stuck out of the back of the alternator with a "flying" connection, but I think this mat be the only way.

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I think the CAV alternator that matches the 440D is a normal 6 pole machine, but I can not be 100% certain because its years since I stripped one (even then I did not count the poles). That is 6 north poles and 6 South poles.

My diagrams for the alternator/440 regulator setup do not show a suitable phase tap connection for the tachometer.

What you need is a connection soldered to the end of one of the phases (one of the thickish wires between stator & rectifier) so that you can "pick off" the AC output of that phase.

It is not difficult to do, but you could end up overheating a diode, so unless you are familiar with soldering, it may be better to take the alternator off and taking it to an autoelectrician for modification. You will end up with a lead stuck out of the back of the alternator with a "flying" connection, but I think this mat be the only way.

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I do not have the spec for your battery charger, but, from the name, I assume it is a vehicle type charger. If this is so, it is likely that it will only charge to about 80% of full. However this should not be a problem if the batteries are in fairly regular use.

All your batteries should have their negatives joined together by cable that is more than capable of handling the current your charger produces, so the charger negative goes to one of the batteries' negative connector.

Your engine battery (as long as its in fair condition) should always be nearly full charged, so the simplest thing to do is to connect the charger positive lead to one of the domestic batteries positive terminals, and just charge the domestic batteries.

You could connect the charger positive to the alternator's main positive output terminal, but to do so would be to cause a voltdrop across the blocking diodes so both battery banks, although being charged, will not even charge to 80%.

To maximize your battery life, and depending upon the sophistication of the charger, may mean that you should check the relative density of the batteries half hourly whilst on charge and stop charging once the relative density (specific gravity) has failed to rise over two hours. Ignore any indications the hydrometer shows as state of charge. Elecsols use a lower relative density of acid, so will never reach "fully charged" on a normal hydrometer.

I would expect any attempt to gauge the degree of charge by measuring battery voltage to give errors until long after the charging is stopped (even then you may have to take steps to remove something called surface charge - Elecsol says their batteries do not suffer from it - my voltmeter says they do, but far less that standard batteries).

If your charger has a "float" charge indicator that comes on once the batteries reach what the charger considers fully charged, the easiest thing to do is to charge the domestic bank and engine battery at different time - As I said above, you should find that the engine battery hardly needs charging.

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I regret that this is totally outside my area of expertise. I do not like the sound of "vessel" - it sounds far to big for me, and subject to rather more than the Boat Safety Scheme.

I am sure you can get frequency converters, but as they will almost certainly have to handle motor starting & heater loads, I fear they are going to be very, very expensive.

There are 12/24v inverters sold as suitable for washing machines, but using one of these should involve a power audit to ensure the battery bank and charging system can cope. I suspect this may turn out to be the most practical way to solve the problem.

There is one other thing to check. Is your 60hz from a generator or inverter. If its a generator, it may be possible to slow the engine down to give 50 Hz, but I would consult a professional over this - try posting on uk.rec.waterways or try to contact the generator's manufacturer or agent.

If it is invertor driven, is it a sine wave output one? There are a number of items, washing machine controllers amongst them, that take very badly to a modified sinewave output.

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One relay energizing contact is connected to earth with the other one connected to the D+ side of the ignition warning lamp. The theory is that the relay is energized when the alternator starts to operate, and sends a small current to cancel out the ignition warning lamp. But it doesn't work as designed, instead the relay energizes as soon as the ignition is switched on, so I am assuming that the ignition current passes through the lamp and activates anything connected to the other side of the lamp. Is this normal?

In one way it doesn't matter once the engine has started, because the alternator will send the appropriate amount of charge to the two battery banks. But what worries me is that in the depth of winter my elderly engine may need more than a minute pre-heating and a long wind on the starter before it will start. Under these circumstances with the relay already energized, there is the possibility of the engine battery trying to draw a high current from the cabin batteries, which could cause a wiring burn out.

I have studied circuit diagrams for charge splitting relays in several books and they all show the same connections as those described above, I have also spoken to several people who inform me that their relay energizes as soon as the ignition is switched on. So what are we doing wrong?

For information I have a 55 amp Lucas A127 type alternator (modified to incorporate a separate field wire), a 170 amp Cabin Battery bank and a 90 amp Engine battery. I also have a Sterling Digital Alternator Controller fitted with the sense wire on the cabin Batteries.

One of your replies on the Web site suggests exactly the same wiring connections as those on my boat, so any suggestions would be appreciated

My first reaction is that you have connected the relay to the ign. sw. side of the warning lamp - so if you made the connection behind the warning lamp, try changing the connection to the other side.

If you made the connection at the warning lamp terminal on the alternator or into the wire just make double check that the connection is what you think it should be.

I suppose it is possible that someone has fitted a warning lamp with far too high a wattage bulb, but this is unlikely, but it may work as you describe if they have.

How warm is the alternator FIRST thing in the morning before starting? If it has the chill off it, you may find you have a short circuited main + diode, although this would reduce the output. In this case you would also tend to have a discharged starter battery.

This would tie in with the blown charge splitting diodes - but how a 55 amp alternator (self limiting for currant) can blow 70 amp diodes is beyond me - as long as the diodes ar adequately cooled. I would expect the warning lamp to be on at all times under these conditions though.

If all else fails you could get a phase tap fitted (this is a connection to one of the stator winding outputs) and power the relay from there. As far as I know the Lucas unit does not have a phase tap (sometimes marked W or AC).

I note from my text books that Lucas do not use the designation D+, they use the designation F for the warning lamp terminal.

I have never known a blocking diode that does not work as expected when it is connected to either the warning lamp wire to the alternator or the phase tap.

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The first thing you need to do is a power audit for your typical period of inverter use between chargings.

Take the wattage of all the inverter driven appliances and divide it by 10 - this will give you the amps they will draw at 12volts and also allow for inverter inefficiency. Having done this for each appliance, multiply each figure by the number of hours or fractions of an hour you will be using it for in each period. Add all these figures together to find the raw amp hour capacity of batteries you need. For all sorts of reasons (see www.reading-college.ac.uk/marine  in course notes) double this figure if you do have an advanced alternator controller or triple it if you do not. This will then give you the capacity of batteries the inverter will require.

NOTE - I assume the Sterling controller is an advanced alternator controller (its supposed to give you 99% fully charged batteries and makes the alternator battery sensed), but Stirling market a lot of kit, so check to be sure.

Now the charging. Assume for a period of three hours or so hat your alternator will provide an average of 25 amps. This will allow you to provide about 75 amp hours of charge to cover your loads.

Do an energy audit of your non-inverter, domestic loads and apply the double or triple rule to get an idea of the capacity of domestic batteries needed.

Now for the charging.

Charging the battery bank - you can ignore the engine battery - will, ideally be done at about 10 to 20% of and hour capacity. This gives minimum heating and optimum battery life. We may exceed this level, but at the expense of more frequent topping up and battery life.

This would suggest that you can safely charge about 550 amp hours worth of batteries - I suspect you are thinking of 2 X110 ah domestics and 2x110 inverter, so this is about right. As you will only be discharging them by 30 or 50% you should be able to get the maximum charge back into them within about 3 to 5 hours of engine running time. If this is too long a time, then you need a larger alternator and expect shorter battery life.

You can not count time when the inverter and engine are running together as part of the charging time.

You will also need to "charge split" the inverter batteries, otherwise they will just become part of the domestic system. The simplest way is to use a split charge relay to connect your inverter battery and engine batter positives when the alternator is charging. Feed the relay coil from the alternator's warning lamp (small) terminal.

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If there is a physical size difference between the alternators, make the larger one the domestic one.

Take a wire from the ignition switch side of the existing warning lamp and take it, via a small, new warning lamp, to the warning lamp terminal on the unwired alternator.

Throw the selector switch away (I suspect this has an off position, so is also a master switch) and purchase two DIESEL master switches.

Leave the battery negatives connected together.

Split the positives so you have one pair (the domestic batteries) and one single (the engine battery). Keep the heavy engine wiring on the single battery. Make sure the main output lead on one alternator runs to the "live" starter terminal on the starter motor. Fit one master switch in the thick lead between battery and starter and take any thin wires that may be on the battery terminal or the selector switch to the output side of the master switch.

The thickest of those wires should feed your domestic fuse/circuit breaker box. This one is run back to the other master switch. A wire of similar thickness to the one above is run from the positives of the domestic batteries to the other side of the second master switch.

All circuits should now work, apart from the second alternator.

Take the main output wire from the second alternator and run it to the switched side of the second master switch.

Both alternators should now charge their respective battery banks.

If you turn either master switch off, or fail to turn the domestic master switch on whilst the engine is running you are likely to damage your alternators.

I am sure a search on Google or Amazon would turn up some sort of book. Also look in the electrical section of course notes in www.reading-college.ac.uk/marine.

If you intend getting involved with the electrics, why not email me your snail mail address, so I can send you details of our "Electrics for Boaters" course which should cost about £65 for 2 days and run next March - I am sure your system should be OK until then.

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Is this a simple problem of the nearly completely discharged battery demanding so much charge through the relay that it burns out? In which case I assume charging the battery, replacing the relay and switching the sensor wire back to the starter battery should work. If not is it worth investing in a more modern system or do I need a marine electrician?

In my experience split charge relays very rarely burn out, but there must be a problem with your system.

I trust you have the marine version of the relay (two nut & two small blade terminals) rather than the automotive one (two large blades & two small blades).

I suspect some cells are dead in which ever battery the relay is switching into circuit. This is normally the domestic ones, but one can never tell. If your main alternator lead is connected to the engine battery (possibly via the master switch) then its the leisure battery in trouble, if it goes to the leisure batteries, its the engine battery with problems.

When these relays switch in, they simply connect all the batteries together, so if one bank is very flat, or with shorted cells, the full alternator output, plus as much current the "other" battery bank can supply flows through the relay, burning it out - I suspect this is your trouble.

As soon as you get on your boat measure the voltage on both battery banks. Any with less than 12.5 need topping up if required and charging - not with the alternator.

When this is done measure the relative density of all cells on both banks. on any bank they should all be within 0.05 of each other. If they are, then suspect a constant discharge on one bank (probably the domestic one or a lack of alternator output) or low output on the alternator - this would need a 100 amp ammeter and competence to test. If the cells are more than 0.05 of each other, I fear you have a faulty battery.

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Did you charge the batteries? The most usual cause of the click is a very flat battery.

If the battery is charged up and serviceable the engine may have seized up. You do not tell me what type of engine it is - are we talking outboards here?

Try to find a way of turning the engine by hand or spanner (wrench). If it refuses to turn you have a problem and it should go to a professional, but if it is an outboard (or petrol engine) you could try removing the spark plugs and squirting plenty of penetrating oil into the cylinders for a day or two, the rock the crankshaft, again by hand and it may free. If it does turn it over a few times by hand, squirt some engine oil down the pug holes and spin it (with plugs still out) on the starter.

Refill tank with new petrol (gas), replace the plugs (having heated them on a stove) and with a bit off luck it may start. Set it to fast idle for about half an hour before applying any power.

If its a diesel, the same thing can be done as far as removing injectors, applying oil etc, but do not heat the injectors before refitting them and you should not need to change the fuel.

The other thing it could be is a faulty starter motor, but I would expect it to batteries or seizure.

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I am sorry to say this, but I just can not get a picture of exactly what you are on about - its the shortcomings of words.

Points which may or may not help.
1. How are you measuring the voltages - where is the meter negative connected?
2. Voltages from 1.5 to 12 ish sent to other circuits - this suggests "back feeding" to me. Have some wires burnt together within a harness somewhere, so when you turn one circuit on, other are fed via short circuits in the harness?
3. Regulator failure and (do you mean 2 sq mm cross section) cable expanding till it failed. It will not expand and fail. The wire will get hot, bubble and expand the insulation (just before it bursts into flames) and then the conductor may melt and fail

Are you sure the regulator failed first?

If something shortcircuited it is quiet possible for the alternator to go to maximum output as the short pulled the voltage down, however the voltage should have remained at about 12.5 at the alternator output (assuming it is the engine alternator regulator you are talking about).

I feel there is a lot of looking at wiring harnesses to be done. If a + wire overheats and melts its way through to a -, then it will cause cable failure somewhere along the harness.

4. I still have a feeling that something (s) have failed and you are getting back feeds, but it is possible that some terminations have overheated and gone resistive, so as soon as they are loaded they give rise to a very high voltdrop and no/low current flow.

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Does the alternator have a plastic end cover? I rather suspect that it does not, and that it may be a Lucas 10 or 11 AC. If I am right the alternator uses a dedicated warning light "putting out" device. This may have failed.

These alternators also used an external energising relay, again this may have failed or become disconnected.

If I am right, the easiest way out for you may well be to get a modern alternator off a car (as long as your boat engine is wired negative to block). As long as the alternator is not too large (a mini one would be fine), use the existing main output + wire, if you have a main - wire connect to one of the alternator mounting bolts, and run a new wire from the ignition switch (on terminal) to the warning lamp, and then from the warning lamp down to the small terminal on the alternator.

If you do not want to do the above, or if the alternator does have a plastic end cover, I am afraid you need to to see an experienced auto-electrician - try to make sure they are over 40, otherwise they might not have seen your setup before ;-)

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Thanks for giving me a chance to air one of my pet subjects.

The question is why? - and just because all the marinisers now do it is not a good enough reason.

Before you even begin to think about alternators and batteries you must do a projected power audit to decide battery capacity.

This is too complicated to explain here, but you will find full details about this in the electrical section of the course notes on www.reading-college.ac.uk/marine.

Having done that you will know the capacity of the battery bank you are going to install - make sure you install a separate starting battery, but ignore it in your calculations.

For long life you should not charge your batteries at much more than 10% of the amp hour capacity (although most of us work on 20% or so). So if you needed a 330 AH battery bank you should not charge them at much more than 66 amps (33 ideally). Thus a "normal" 60 amp alternator should suffice.

90amp alternators are available from car scrapyards!

You should also fit an advanced alternator controller to prevent you loosing 30% of you battery capacity every winter to sulphation (again see the website). This should also convert the alternator to "battery sensing" which you will also want. I would advise a large split charge relay to split the charge between engine and domestic banks.

If you fit two alternators your engine battery one will spend most of its time doing nothing, but you will looses about 30% of its starting capacity every year to sulphation unless you fit an advanced controller to that one also. Even with a bowthruster I would have difficulty in recommending another alternator.

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There are so many different types of equipment, that it is impossible to answer. There are two main type of ammeter, three types of tacho (depends how they sense the speed), myriad different makes of "ignition" switch and ditto for the alternator. The gauges might be mechanical or electrical.

All I can advise is that generic diagrams should be in most marine electrics books, there is a website which has workshop manuals which may or may not be available in English, or - and I am not to happy with saying this, we hope to run a boater's electrics course next March time.

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Although I am an autoelectrician by training (long story) I have just spent some £30 on a text book to cover just the sort of thing you ask about. I am no expert in mains stuff, and its propensity to eat through steel hulls scares me.

ALL BELOW GIVEN AS GENERAL GUIDANCE AND NOT IN ANY WAY AS ENSURING COMPLIANCE WITH STATUTE AND REGULATION.

The boats mains wiring must be isolated from, and run separately from all 12/24 volt stuff, fuel and gas supply pipes. If close it should be put in trunking. All small boat wiring should be in multistrand, flexible cables of suitable size for the current flowing.

The boat's wiring must be "earthed" to the hull (or ground plate if wood or grp), but the PD between true ground and the supply earth may be enough to cause galvanic corrosion to the hull. To overcome this ideally you would use an isolation transformer (soft start if possible) or a galvanic isolator as advertised in the inland boating press. I understand that if this was not done and a fault made the hull live, without tripping, you could be prosecuted if you killed/injured anyone either in the water or stepping on/off the boat - do you have professional indemnity insurance? If not, for goodness sake research the project to make sure you comply with best practice.

As you will be grounding the boat's system and isolating it from the mains supply, the boat must have its own circuit protection.

Do not use the "earthing" of the 12/24 volt negative as the point to ground the mains (or lightning grounds). If you do, and the fixing becomes detached, you could put the mains fault current through the low voltage equipment.

Take care with any inverter or generator equipment - it might have its own ground, and ideally there should be only one ground point per "circuit" e.g. 12/24v, mains, lightning = 3 grounds.

This whole topic was discussed on googlegroups, uk.rec.waterways a while ago, so look in the google archive. Some of the people who replied were very knowledgeable.

I am afraid you will have to do some studying and getting your local library to order in textbooks for you. One of the popular ones was described as being wrong in respect of what you ask.

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We already have 10" TV/Video combo that is converted to 12v and runs off our 12v ring. However, to run the Playstation I'll have to invest in a small inverter (300W), connect it across the domestic batteries and run the Playstation from it. I have borrowed one of these before and run a mobile phone charger from it without a problem - obviously the Playstation is a little bigger.

My question is do you think my 3 x 110 amp domestic batteries will cope with the 12v TV being switched on, the Playstation running off the inverter and other bits and pieces that run from these batteries, e.g. fridge. Should I restrict the operation to just when the engine is running as the drain on the battery will be offset by the alternator.

The only major problem I can see is if you buy a modified (square) sine wave inverter. These are the cheapest, but some (and only some) electronic equipment either will not run correctly, buzzes, or produces interference with TV pictures if run off one. I understand that if the Playstation uses a "switched mode power supply" it should be fine - now having just tried to get Nokia to tell me what type of power supply their digi-box uses, I wish you the best of luck in finding out.

It would good if you can borrow an inverter, or access to one to try it before you buy it. Are you also sure a "car lead" is not available for it - it saves on inverters. It might be a good idea to give some thought about future AC needs ( I hear Freeview produces much better pictures on much of the system that analogue reception), then there is the use/charging of power tools like drills etc. It might be worth while investing in a larger unit so you have capacity to spare. A larger unit should not draw significantly different current for any given load than a smaller one.

Without doing an energy audit, I can see no reason why everything should not be fine. It might be a good idea to limit its use once the engine has been switched off - say till 8pm, but that would be mainly to ensure they do not try using it in the wee hours when you are asleep.

FWIW I have no problems with an electric fridge and 2 X 110 AH batteries and leaving a phone and Dustbuster on inverter fed charge overnight.

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