Boat Fault Finding - Marine Electrical Problems

I regret that in view of boat safety scheme requirements and the possibility of legal action if there is an accident relating to such a modification I must decline to answer your question.

Electrolux are now called Dometic of 99 Oakley Road, Luton, Beds, LU4 9GE

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I am afraid I can not give specific answers to your questions - in both cases the answer is "it all depends".

As far as battery bank size goes, you must do a power audit as per the Course notes on www.reading-college.ac.uk/marine . Assume all inverter loads will be at 120% of the theoretical load to allow for inverter inefficiency. Having obtained the wattage of each "mains" appliance divide it by 10 (not 12) to get the amps drawn.

Having calculated the number of amp hours you intend to draw from your batteries per day, you can then decide if you already have enough capacity, or how much more you need to add.

Do not forget to also calculate if your typical cruising time/engine running time per day will be enough to replace what you will have taken out of the batteries.

Also remember that without an advanced charge controller you only have 81 amp hours effective capacity at present, or with a controller about 121 amp hours effective capacity. Again the explanation as to why is on the website.

I rather suspect that if you confine your inverter use to when the engine is running (apart from the TV) and do not use the vacuum for woodwork dust extraction, your battery bank will probably do, but you should do the power audit to be sure.

The question about possible problems with a modified sine wave inverter depends upon both the internal design of the inverter and the internal design of your equipment. The only advice I can give is to use a supplier who has the facilities to let you take your equipment to them and actually try before you buy, or at the very least agree to refund if there is a problem.

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Maybe you can make some suggestion for my new narrowboat purchase.
The electronics leaves a lot to be desired. The existing system is set up with a 230v mains ring that connects directly onto the marina power supply - there is no connect between this and the batteries. The engine has its own starter battery and there is a 12V system that runs the fridge/water/pump and lights. An alternator runs off the engine and the only other way that the 2 standard CAR batteries are charged is by a standard car battery charger that runs off the mains supply and starts using a domestic timer. Unsurprisingly fixing this, what I shall generously call, electrical "system" is my first job after I get on board later this month. I need the system to be available for both marina living cruising and I feel that since the 230V wiring is there I might as well use it. So what would you suggest I need to do to end up with a functional electrical system (apart from the obvious fusebox to separate my boat from the National grid!!! :)
With great interest and thanks in advance for any reply. Simon L-R

First the 240 volt supply. My expertise is in 12/24v DC and I, personally, have concerns about 240v on a steel boat. This relates to two issues. First, when connected to the shore line, with the boat wired for 240v in accordance with the basic Codes of Practice, it is possible and/or likely to get a current flowing - through the water - between the hull, pontoon, other boats, and land. This does cause excess corrosion very quickly. There may be ways to overcome this, but they involve extra equipment. Secondly, I have seen 12v eat a starter motor away, when submerged in bilge water, in a few months. 240v would do this much more quickly if it could escape from the wires. Boats and damp appear to go hand in hand to some degree so the possibility of leakage is ever present.

My own boat is, and is likely to stay a 12v system, although I accept many people use 240v without apparent trouble.

If you want to go down the 240v at all time route you need to purchase an invertor to convert 12v dc to 240v ac and do a power audit (as described on www.reading-college.ac.uk/marine  under electrics in Course Notes) to decide how many extra batteries you need to purchase and if your alternator/average running time is adequate. Increase the current demanded by the 240v appliances by 20% to allow for invertor inefficiencies. Please make sure you fit the invertor in the driest place possible - I have friends who's invertor caught fire, and I am fairly sure mounting it in a cupboard in the bathroom did not help the situation.

On the battery charging front. Again you need to do a power audit as described on the website above. Assuming your alternator is adequate to match your running time to current demand you have a choice of three battery charging systems:-
1. Manual switch to connect battery 1, battery 2, or both batteries to the boats electrical system - this is simple, but easy to forget to select the appropriate position, so the batteries go flat.
2. Splitting the alternator output with a blocking diode between the alternator and the two batteries. This is fully automatic, but there is a cooling issue (many need to be in a 4mph air stream) and it will cause a voltage drop which will prevent the batteries ever becoming fully charged. The latter can be overcome by converting the alternator to battery sensing and/or fitting an alternator controller - more extra cost. (see above website)
3. Fitting a split charge relay to connect the domestic batteries to the engine battery only when the alternator is charging - so both battery banks get charged. This is the cheapest option, and in my view the one most likely to be cause the least problems. You might need to change the relay every 5 years or so to prevent volt drop across the contacts. Make sure the relay you get is for boats and has two screw connections, rather than four blades. The four blade ones are designed for caravans and are not "man enough" for marine use.

Hope this helps. Whilst on the website, use the link to Power Pages for more info.

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When I try to run any 'major electrics', such as pushing 'up or down trim' to raise/lower the drive or trying to start the engine, I hear a sound like the 'click' of a relay, and then everything dies (i.e. all gauges go to zero and main light goes out as if there is a blown fuse) BUT THEN COMES BACK TO LIFE AFTER ABOUT 2 MINUTES! This happens every time I try it, i.e. almost as if there were a 'circuit breaker' in action, except that there are no circuit breakers.

If I run something 'lightweight', like the bilge pump, when the electrics come back on that is, it works fine. So trim and starter give me a 'click' and then all power dies, but then comes back to life after approximately 2 minutes, and then the same thing happens all over again. Battery is brand new, and have probed around with a pocket meter and checked all wiring and fuses, but can't see anything obvious. Any thoughts on what this might be?

This is typical of a bad connection somewhere, the "click" being arcing as a resistive connection fails under a heavy load.

I would suspect:-
Dirty battery clamps (these should be cleaned to bright metal and treated with petroleum jelly when connecting to clean battery posts.)
Resistance between battery lead and battery terminal - are these clamped on with screws?
And then trace all the main wires back and test each terminal & junction.

You need to check the connections by putting a (20v) voltmeter across the "joint" and operating the load, this measures voltdrop, which should be no more that 0.25 volts at any individual point or 0.5 volts across the whole + or - side of the circuit. You might have to put a pin through the cable insulation to get a good connection.

Note - Two days later I got a reply, thanking me for the advice & confirming it was a "bad earth".

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I want to install a 3 way fridge, am nervous about using gas, and would prefer to power it on 12 volts. However I expect this will drain the batteries very quickly.

We are not on the boat all the time. Switch on the engine now and then to keep the batteries topped up and then go home at night.

Any suggestions as to how i can provide 24 hour 12 volt power without running out? Should I get more batteries, different batteries? How much does power does an Electrolux 3 way fridge use?

Any help or advice where to seek help deeply appreciated, regards, Peter Simpson, restaurant boat " Zazou."

First a note about your batteries. I very much doubt if they are in series - if they were it would give
you 36 volts & 36 volt equipment is very hard to come by. I suspect you have the common setup, with one 12v battery for starting, and two in parallel (to increase capacity) to give 12v for the "domestics" - interior lights, pumps, fridge etc.

Some form of charge splitting will be used (or two engine alternators) - see the electrical section of course notes on www.reading-college.ac.uk/marine . This will also tell you how to do a power audit which you should do before spending any money.

Please also remember, the deeper and more often you discharge a battery (of whatever type) the shorter its life.

Now about the fridge. This is an absorption fridge, and absorption fridges are not the most efficient in respect of input energy, so whatever you do you have to accept the fridge will demand rather more energy than a compressor type.

I would expect to find its power/current consumption on a sticky, silver lable near the connections. I also think it now has a time limited exemption under the BSS, & will have to be replaced within a few years (check with BW on this).

I am afraid that, as far as I know, this fridge was designed for the caravan market and the only supplies that have a thermostat are the 240v & gas. The 12v being designed to keep it cold whilst being towed to the site, the car's alternator would easily cover the drain & the pitching of the caravan would make it an advantage to keep the cooling going all the time.

The above means that if you do connect it to 12v, it will never turn itself off by the thermostat & would thus be very bad news for your batteries.

I operated a fleet of about 40 hire boats, all with gas fridges, and never felt there was a real safety issue, except when the burners became blocked. This caused them to burn with a yellow flame which created a lot of soot & consequent smell. This scared the customers, but in reality, was mainly just unpleasant.

I would ask if you have done a power audit, to see exactly what your battery and alternator requirements are. You will find this in the electrical section of course notes on above web site.

I think you have a number of ways of going forward, because buying more batteries only puts off the day they will go flat unless you ensure adequate charging.
1. Ensure the fridge burner is working correctly and use gas.
2. Get a small inverter and run it from 240 volts, but this would require almost daily battery charging (see audit & increase the fridge rating by 20% to allow for inverter inefficiencies).
3. Get a land line & run it on 12v when under way & 240v when moored.
4. Get a modern compressor 12v fridge, but this will still demand about 36amp hours of capacity a day & requires large, expensive, cables.
5 Get a domestic fridge (highest energy saving rating you can) & run it via an invertor. This might be the best route for a catering establishment, because of the large range of sizes available. Again
I think it would demand almost daily charging unless a land line is available for when moored.

From what you tell me, I think I would go with option 1 for now and work towards option 5 plus land line within the next few years. I would give different advice to a private, leisure boat, based on
availability of fridges to fit the old fridges "hole".

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You do not say, but I assume you are talking about a boat. If that is the case I am afraid that there is very little chance of getting a marine wiring diagram for your specific engine. It is an old engine, and there has been too much opportunity to change components e.g. ignition switch, alternator etc. I also need to know what alternator you have. 2.2s were originally fitted with dynamos, so you could well have the original "mass market" Lucas design 10 or 11AC, these are very different from the more modern ACR range or 127A.

Either use the link from www.reading-college.ac.uk/marine  or do an internet search on "POWER PAGES". It could be that Colin Marsh has some information on his website - its url take a bit of remembering.

I will try to help you, I assume you have a mechanical stop control and that the boat is wired "negative earth" (some engine of this age put the controls and instruments in the negative line).

You say the ignition switch has 4 connections, this does not sound like the original Lucas unit, because they had some connectors with two or three terminal on. I suspect this is has been changed at some time.

You need to identify the feed terminal, I suspect this will be a large one. If one has 30, bat or + by it, you are lucky This needs a large cable (say 65/0.30mm) from the engine battery master switch. If there are no markings, all you can do is to use an ohm meter to try to find the off position, and the main input terminal .

Having connected this cable and turned the master switch on (make sure all other cables are insulated or clear on metal), connect a volt meter (20V range) or test light to the engine battery negative. Test each switch terminal in turn, and between each series of tests alter the position of the key. Eventually you will find a key position that isolates all the other connections - this is the OFF position. This position could be fully anti-clockwise, or one position clockwise.

If fully anti-clockwise:-
1. Turn the key one position clockwise and find the terminal that is energised. This is the "ignition on" position. Feeds to the following items need connecting here. One side of the oil pressure warning light. one side of the voltmeter (the other side goes goes to battery negative), and assuming it is not a 10 or 11AC alternator, one side of the ignition warning lamp (the other side goes to the small alternator terminal). Also connect anything else here you want to come on and off with the ignition. All these wires can be quiet thin, say 28/0.30mm or 14/0.30mm. If you are also feeding a tunnel light, nav. lights, horn etc. from this terminal increase the wire thickness between the ignition switch and fuse box (that should be there to protect these circuits). Things like fuel gauges are also fed from this terminal.
2. Turn the key another position clockwise - you may have to hold it in position against a spring - and locate the terminal that becomes live. This is the heater position. A thick wire, 44/0.30 or 65/0.30, should run from here to the glow plug fuse, then the glow plug resistor (a coil of wire under a steel shield), and then to one end of the glow plugs on the engine. Some times a thin wire is used here, feeding a relay or solenoid mounted close to the engine, This solenoid/relay is then used to switch the thick wire, but ensure the unit is has an earth (negative) connection to engine battery negative it one is used.
3. Turn the key another position clockwise - again there is likely to be a return spring, the terminal that comes live needs connecting (again with a thick piece of wire) to the small terminal on the starter motor solenoid cap. Again, it is not unknown to use a thinner wire and a remote relay or solenoid (see 2 above).

If one position clockwise:-
Turn one click clockwise, locate the terminal that comes live, follow 1 above.
Turn one more click clockwise, if this is the last clockwise position the follow 3 above, if second from last follow 2 above.
Turn the key fully anti-clockwise & locate the terminal than becomes live (you may have already connected to if from the above) follow 2 above.

The ammeter would ideally be a shunted type, but I bet it is not. If the wires for the ammeter are thin, and when traced back to a point between the alternator and master switches are connected to what looks like a slotted brass bar, it is a shunted ammeter. Simply connect the two wires, and if, when you run the engine, the ammeter reads "backwards" change the wires over.

I suspect the ammeter is of the moving iron type, these are not accurate and MUST be wired using thick wire. Again I have no information about the type of boat and layout, so I will assume you have an aft cockpit boat with the panel close to the engine, if otherwise save up and buy a shunted ammeter. You need thick wire, I would use 120/0.30mm cable for an alternator. With master switches it is difficult to get a charging and discharge reading, so I would connect a wire from the large, alternator + terminal to one side of the ammeter and from the other side of the ammeter to the switched side of the master switch. This will indicate a rough amount of charging, again, if it shows discharge with the engine running, change the wires over on the back of the ammeter.

If you have a 10 or 11AC alternator with separate control box and warning lamp control, I would advise a visit to a scrap yard an the purchase of a modern car alternator. If you choose a Montego type vehicle you are likely to get the vehicle equivalent to the 127A - the difference will be that the marine version has screw terminals and provision for connecting a negative, earth wire - you just make sure the engine is connected to the battery negative.

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No it is not correct. In an ideal world no part of the 12/24 volt system would be connected to the boat's hull. In theory there can be problems with electricity eating holes in a steel boat, in practice (as long as we are talking 12/24v) it is unlikely unless the boat is wired positive to hull. This might have been done in the 1960s to allow an automotive, positive earth alternator to be used.

There are, however, large numbers of starter and alternators in use that are sold as "marine", but are actually modified automotive units. Many of these do have a post for connecting to battery negative, but if you strip the unit, you find it is just connected to the case, and the unit is, after all, a standard negative earth unit.

Once this is understood it makes less economic sense to buy an expensive "marine" unit, so using automotive ones demands a negative connection to the engine block. This connection is likely to also bring the hull to battery negative via the shaft and control cables.

I note you have a yacht and surmise that you use it at sea, so I would be very reluctant to encourage you to use automotive units with blade connections because of the corrosive tendencies of salt water.

There is also the matter of radio aerials to consider. As far as VHF etc. you must ask someone else, but a lot of boats use as "car" radio and aerial for entertainment. A car aerial and radio needs a good "earth" connection to minimise interference. I would advise trying to insulate the base of the aerial and connecting it to the set with a length of large (say 44/0.30mm) cable in lieu of the "body"earth line.

In the UK inland market, it is not unusual to find that money has been saved by using a single wire for items remote from the controls, this would require a negative to hull connection, but is bad practice, and only escapes scrutiny because usually the items wired in this way are only used intermittently, for short periods of time. I trust no-one has tried to save costs on your masthead light in this way!.

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The 11ac (and 10ac) used an external voltage regulator and warning lamp control system. As far as I know all later types do not, so I suggest it would be far easier to rewire the charging and control circuit. You should also consider the probable higher output of the new machine and thus the need for larger wires.

Basically you have three terminals on the new machine plus a possible earth connection (without the number or a picture of the back, I can not be sure).

The largest terminal (possibly labelled B+) should be connected to the battery/blocking diode/split charge relay with a length of 120/0.30mm cable. This is 60 amp cable.

A wire of about 14/0.30mm (not critical) should run from the ignition switch to one side of the ignition warning lamp, and then from the other side of the lamp to the alternators warning lamp terminal. On a Lucas machine this will be the small blade close to the large + connector above.

That's it - add a 120/0.30 earth wire if required.

If you have a split charge relay its coil is fed from either the small alternator terminal above, or a phase tap terminal. This is another small blade (on Lucas) usually remote from any other terminals. The phase tap is often not fitted on Lucas automotive alternators.

If you have a revcounter on a diesel, this could also be "pulsed" from the phase tap terminal. If this is the case, and you do not have one you need to get an autoelectrician to solder a tap to one of the internal stator wires (one of the three solid wires coming out of the body to the diodes) I suspect that the D+ terminal (if it is small) may be a phase tap.

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These batteries are typical of nearly all the cases I have seen in recent years, the "filler" caps are the six circles with shallow cross groves on them. The groves are designed to take a coin to unscrew them.

Look down the hole and you will usually see some type of level indicator. This could be a plastic bar across the centre of the hole, or a plastic "ledge" across half the hole. You should also be able to see the separators (most people call them the plates, but the plates are several mm below the top of the separators).

Some batteries have a level mark on the outside of a translucent case - not much use in the typical narrow boat :-)

Top up to either the level indicator/mark or to about 3mm above the separators. If you can see the separators, the 3mm rule is usually easier.

Do not overfill if you can avoid it, because it makes it harder for the venting system to separate the acid from the gasses whilst still in the battery. Please use the proper de-mineralised or distilled water obtained from a garage or car accessory shop (unless you can get it from work). Fridge water, and other "substitutes" will contain impurities that are likely to shorten the life of your battery.

As a note of interest, some large transport fleets have used tap water, but this is because either they have had it analysed and proven as safe, or more likely, because they know that their batteries are destroyed by their staff/operations before the water can damage them.

Some level indicators prevent an automatic topping up bottle touching the separators, so you end up overfilling the cell.

If you have to frequently top the batteries up you either have a faulty alternator controller/voltage regulator (in which case the batteries are likely to be covered in acid drops or white splashes with the terminals covered in a whitish "fur coat") or one or more cells are beginning to fail - so start saving.

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I shall assume this is a UK Inland based craft and talk about battery master switches. If this is a coastal boat it might not have them.

First, I have never liked the use of voltmeters to indicate state of battery charge, especially in marine use. Something called surface charge in the battery gives a false high reading unless certain precautions are taken, and this can be difficult with marine circuits. Assuming wet cell, lead acid batteries, the best way to assess state of charge AND battery condition is by Hydrometer reading (thus I would never use maintenance free batteries). Make sure the hydrometer can give a numerical reading, and take a reading from each cell. The coloured bands on the hydrometer float (from you local auto shop) will tell you how charged it is, and when reasonably fully charged, a difference of more than 0.05 between cells indicates a faulty battery.

An ammeter can only indicate by inference - The lower the reading, the more fully charged the battery is - unless something in the circuit has a high resistance etc.

You alternator is either a Lucas 10AC or 11AC. I would advise a 50 amp ammeter. Your question about 30 - 0 - 30 ammeters causes me concern. These are normally cheap, automotive, moving iron types, which require heavy gauge wire (120/0.30mm) to be run between alternator, ammeter, and master switch (for example). This is not good practice because it introduces unwanted resistance into the charging circuit. I would advise a good quality, shunted ammeter, which has a shunt (a special piece of brass strip) in the charging lead, and the ammeter is connected by thin wires to either end of the shunt. Whilst you are about it, I would advise getting a higher reading one to allow for possible future alternator change.

Now you have to decide if you want to monitor alternator output, or input & output from the battery. The former is easy, the latter is more difficult.

If you just want to monitor alternator output (as mine does). You will find one or two thick wires on the back of the alternator. One would be the "earth" wire if fitted (I trust this would a negative earth alternator, but they were available in positive earth, in which case simple change - for +). The earth wire (if fitted) should go to the engine or back to the battery negative. The other wire (possibly brown) is the one to worry about. The other end of this wire will be at the master switch or battery +. Fit the shunt into this wire, or if you must use a moving iron meter, disconnect it at both ends and replace it with the wire running to/from the ammeter.

If you want to measure the current into/out of the battery you need to disconnect all the wires from the switched side of the master switch, apart from the large, starter wire. If no master switch, disconnect all the thinner wires from the battery + terminal. Make up a good "insulated junction bolt" and join all these wires to it - again, make sure the main starter cable is not involved in this. Reconnect these wires via the shunt (or use the moving iron ammeter wires to reconnect).

Is there any other way i can link into a circuit before I install a complete new circuit (I can do this as I have a spare power supply and fuse available). Also could you advise as to what amp I should fuse at. I would value your help on this matter.

I am not surprised that you keep blowing the lighting fuse - especially if you have one of these fancy, exposed, fuse panels.

First look on your shower pump to find out what its current or power rating is. Then look at www.reading-college.ac.uk/marine , electrical section of course notes to find out about cable sizes and how to convert watts to amps.

Now look at the size of both the pos. and neg. lighting cables - count strands if you have to. Calculate the current drawn by all the lights on that circuit and see how much (if any) spare current capacity you have in the wires - also calculate the volt drop through the wires with the lights and pump working. (on website)

If you find you have less than (say) 0.5 voltdrop and there is more than enough cable capacity, simply install a higher rated fuse to suit the current draw.

If you do not have enough capacity in the cables, or you would have too much voltage drop, you will need to run a new supply. I would vastly overspecify the cable, so further additions can be made more easily.

I have no definitive calculation, but I would "over fuse" a shower pump by between 2 & 5 amps, so a 8 amp shower pump would get a 10 amp fuse. The only thing to watch with pumps, is that if they jamb, the motor draws a much higher current, so its probably better to err on the lower side, so the fuse blows easily and protects the stalled motor.

What I would like to know is if I fit the above battery management system, I am told that it will cook the batteries and they will eventually fail. Is this good advice ???? The one I am thinking of fitting is the new digital model which has a temp sensor fitted.

Alternators (well the modified automotive ones that we use) are self regulating for current. This means that as the current rises the potential maximum voltage falls, until, somewhere close to maximum output, the voltage available will no longer be enough to "push more amps" through the system. This means that your alternator may well run hotter that without a controller, but it can not damage itself.

The next thing is the batteries. Without the controller, you will only charge your batteries to about 80% of full charge, and unless you also have an external means of charging your batteries, the 20% of uncharged capacity will sulphate by next spring. This means you loose 20% of you capacity this winter, another 20% next winter, and so on.

It is certainly true that there is a charging rate above which there will be damage to batteries. This is also related to temperature and the higher the charge rate, the more likelihood of temperature damage.

For years the accepted bench charging rate of a battery was 10% of its capacity (so yours would indicate 27a if you mean three domestic batteries or 18a if you mean two domestic and 1 engine battery maximum charge rate). However I have seen a website that quotes 20% of capacity, and I know that all of our charging systems regularly exceed the 10% figure without obvious damage.

Providing the batteries are in good condition, and providing the controller measures battery temperature (like the Adverc) my feeling is that there will be very little, if any, damage. I also suspect that any damage that does take place will be minimal when compared with the sulphation problem.

The next point is that your alternators are almost certainly "machine sensing". This means that the voltage regulator takes the voltage measurement from the alternator. For a variety of reasons it makes better sense for a boat's system to measure the voltage at the battery. This helps to ensure a full charge. Any half decent controller will convert the alternator to battery sensing, although you may also have to buy a new regulator/brush pack for your alternator - or modify it yourself.

All I can say is that I have a controller fitted to my own boat, an Adverc, and now, after three years, I can detect no damage battery and no overcharging. I am more than happy with it.

I have some concerns over a number of these systems and believe that the owner should not be able to tweek the charging voltage and should not be able to retrigger its charging sequence. I am also of the opinion that the memory devices used in digital devices are lifed, both in absolute terms and in the number of rewrites they can accept. I know that I have a transistor radio that is well over 30 years old that still works. Do not believe the comparison table that was published in the early adds for the product you mention. My system did malfunction due to a less than perfect job on my behalf, and the Adverc informed me of a fault immediately. My choice of Adverc was informed by an article, several years ago, in the magazine published by the Institute of Road Transport Engineers, that compared a number of different makes - ambulances, trucks with tail lifts, mobile showrooms etc have very similar battery requirements to ours.

All I can say is that I have an advanced alternator controller and I certainly would not have if I found any hint of battery damage. I have some niggling concerns over your choice of unit.

I also wonder why you are thinking of doing this - if it is flat batteries, look at www.reading-college.ac.uk/marine  under the electrical section of course notes. This will allow you to check the basics before spending money.

Also see continuation below.

The Stirling's job is two fold.

1. It measures the charging voltage at the battery (rather than at the alternator output), so any voltage lost in the wiring is compensated for. This is absolutely vital if you decide to use split charging (blocking) diodes, but only very highly desirable if you split the charge by some other method.

2. By various means and strategies it increases the charging voltage to ensure your batteries become fully charged, but are not damaged.

The way to wire the Stirling, and how to modify your alternator SHOULD be explained to you, both in the instructions, and by the vendor. You may well have to buy a modified alternator voltage regulator so the Stirling can also be connected to the alternator rotor circuit. This is true of any effective advanced controller - my alternator required an extra £32 for the modified regulator (and I was an autoelectrician).

Now charge splitting.

Connect both battery bank negatives together, using cable no thinner than your main charging lead. The engine battery will be
"earthed" somewhere on the engine, so this also provides a charging "earth" for the domestic batteries.

You now "Y" the main charging lead, by using two similar sized cables, and take one to the domestic master switch (or dom.
battery positive) and one to the engine master switch (or eng. battery positive). This Y junction is where you have to fit your charge splitter.

Leaving the hypothetical Y junction in place at the moment. The alternator output will travel up the wire, to the Y and then try to travel up both branches.

The engine battery will be usually be nearly fully charged, so the voltage "pushing back down" the wire will be about 13.6v, the
alternator output will be about 14v, so there is only 0.4v a difference to push current through the engine battery. This will not push much charging current through the battery.

The domestic battery is usually much more discharged, so its voltage will be about 12.3, giving a charging voltage (14 - 12.3) of about 1.7v. This pushes far more current through the flatter batteries.

As each battery bank's voltage rises, the charging current falls to suit each banks state of charge. As long as the voltage regulator (alternator's own or advanced one) holds the alternator's output voltage at a safe level each battery will "take" the current it needs and not be overcharged and damaged.

Each battery "takes" what it needs without any extra components. The extra bits are to stop one battery bank discharging itself into the other one.

Now on how to split the charge.

1. Fit a big red switch in the Y - the switch will/usually does have four positions - bank 1, bank 2, both, off. I do not like this because it has to be manually switched to both whilst running, and turned to another position when stationary - you can bet someone will forget, one day, and you will end up with flat batteries.

A cheaper example of this method is to leave the branch running to the engine battery intact, and put and ordinary battery master switch between that junction and the domestic battery cable. As long as you turn this switch on when running, and off when stopped, you will get effective charge splitting.

2. Fit a blocking diode pack in the Y. The terminals on the pack should be marked so it is easy to connect.

Just make sure that you read the full technical specs. BEFORE purchase, because many quoted capacities require a maximum temperature which is often exceeded in an engine room, AND and air flow or a minimum speed - often not achievable. If you can not provide this environment, you have to buy a much larger capacity unit (at a larger price) and de-rate it to suit your conditions.

3. Fit a split charging relay. The engine side of the Y remains connected as in the simple switch example above. The domestic battery side of the Y is connected to the junction via a high capacity relay by the relay's large or screw terminals. The smaller (usually blade terminals) one is connected to the alternator's warning lamp connection and the other to negative. When the alternator starts to charge the relay contacts will close and allow both batteries to be charged, when the alternator stops charging, the relay opens to prevent the engine battery discharging into the domestic batteries.

Just make sure the relay you are sold can handle the maximum alternator output. This means that it should have two SCREW terminals - not two large blades.

Personally I would go down the relay route (look at the Vehicle Wiring Products of Manchester) catalogue - they do have a website.

There are diagrams for split charging under course notes on www.reading-college.ac.uk/marine .

 I'm still not entirely clear as to how to proceed with my installation.

Being new to narrowboating, I wasn't aware how regularly I need to run the engine to recharge the batteries on my boat - subsequently they are now too low to be recharged from the alternator. Following advice from a fellow boater, I have disconnected the batteries and brought them home for recharging. This is where I get muddled.

I explained to staff at the local Maplin shop that I need to recharge 6 x 6v batteries (parallel connected to make 3 x 12v) for a boat and asked what kind of charger I need for this. They suggested a Pro User Portable Power Station PS1800 which I duly bought. Among other things, it seems to be geared towards starting car batteries with a high boost of power and I am now worried that this might damage my batteries and that I should be using more of a trickle charge.

Are you able to give me any advice on whether I have bought the wrong  thing and what I should be doing??

First of all (without seeing the boat and doing some tests) I would not be so quick to say the batteries are so flat that the alternator will not charge them. If they are in good condition, once the engine is started, they should charge from the alternator.

I suspect they are either badly sulphated or they have one or more dead cells.

Sulphation - when a battery is left in a fully or partially discharged state a chemical change takes place in the cell material and forms a hard coat of lead sulphate. This effectively prevents the batteries charging. As a normal alternator will only charge the batteries to about 80% of fully charged you should expect to loose about 20% of the remaining cell capacity each year. (Hence advanced alternator regulators to try to get them closer to 100% charged).

A modern, microprocessor controlled, professional charger (at £200 trade) is said to be able to recover a sulphated battery. It is also said that a series of long charge/discharge cycles may do the same (Charge and then discharge with a bulb, and immediately re-charge).

Faulty cells - over time, the material on the battery plates starts to fall off and build up in the bottom of the cell. The plates also do "a little dance" when subject to heavy discharges or charges, this can rub through the separators. In both cases a full or partial short circuit is produced which prevents the batteries being charged.

To check for this, top up each cell to either the mark on the case or about 3mm above the plate things you can see when looking into the cell filler. Put on charge for at least 4 hours to ensure the acid is mixed with the distilled/de-mineralised water and then take hydrometer readings (use one with a float that gives numbers, not floating balls). The hydrometer will tell you how charged/discharged the cells are, but if the numerical readings differ more than 0.050 between cells, the battery is probably beyond use.

Take care with the acid, do not flick it into your eyes, wash it of skin asap, and note that it burns holes in cloths when you next wash them. Wash the hydrometer after use.

Conventional wisdom says the charge rate should be no more than 1/10th of the batteries amp hour capacity (in your case, the capacity of a single battery of each pair). You can use a lower charge rate, but it will take longer to fully charge the battery (pair). If you use a higher rate you run the risk of overheating the cells and damaging them. At a rough guess I suspect each of your 12 volt pairs would have the capacity of about 120 - 150 amp hours - look on the cases, and if they are all the same physical size you can assume they all have the same capacity - hopeful you will find a label somewhere. If they are of different sizes, keep each battery in the pair the same size.

The theory is that you note the relative density readings every hour and stop charging when they fail to rise over two consecutive hours (sulphation & faulty cells excepted). You should also correct the hydrometer reading for temperature, but I would advise you to ignore that.

I think you have bought the wrong thing - I understood, as you do, they are for STARTING a car so its alternator can charge the battery - not "bench charging" batteries.

The purists would say you need a three stage, microprocessor controlled charger, but look at the cost. Because of the capacity of most boat batteries you certainly need one that will deliver a much higher charge rate than those easily found, and as I am too busy to mess about with relative density readings every hour, you also need a charger that tells you when the batteries are "done".

I bought a unit from Halfords (I think it cost about £45) that gave an 8 amp charge rate and stated it was suitable for large diesel cars and vans (I use 110 amp hour batteries - so I can bench charge at
up to 11 amps). When the batteries are fully charged (so the instructions say!) it switches to a float or maintenance charge which will not harm the battery, but will keep it from self discharging. It shows this by a light changing from red to green. I just top up the cells if required, connect the charger (cell lids unscrewed, but in place) and leave it until I notice the light is green.

On the news group uk.rec.waterways, a number of people have found similar models that are cheaper (one came from Argos I think).

Once the batteries are charged, you can connect the charger to the full bank just to keep them topped up, but to try to do it from dead flat would take a very long time and make finding faulty cells etc more difficult through simple numbers.