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Subject: Split charge relay

I have another problem with battery charging.

I run a standard 12v/55amp alternator. I use a starter battery 90 amp and use 2 x 110 ah domestic batteries. The alternator is connected to the start and is linked to the domestics via a simple manual switch which I turn on when running the engine and turn it off when using the domestics as not to drain the start batteries.

Before this there was a black relay box type thing that was connected to the start battery live terminal. This connection then ran to the black box via a small bolt on terminal and back out of the black box via another small bolt on terminal on to the domestic live terminal both cables 5mm

Also on the black box were 2 smaller spade connections this is where I get lost where are these other 2 cables run to, my friends boat has the same type of unit and he does not know what it does either. Have you seen this type of unit and where do the smaller 2 spade connections go to? Some elecys have said alternator some just walk off.

If you have any idea as to the purpose of this relay or charge unit and the connections I would be grateful.

Yours thankfully Steve



Dear Steve

I am about 98% sure that your relay thing is a split charge relay, so why you also need the big red switch I fail to understand. A couple of battery isolator switches would have done. It might have been fitted so you could use the domestic batteries to start the engine if the engine battery went flat, I would need to inspect the wiring to be sure.

One of the thin wires goes to battery negative- its where the other one comes from that's the problem - it all depends upon your alternator.

It could come from:-

the 61, L or AUX terminal if your alternator has such a thing.

a rising oil pressure switch on the engine

the "ignition on" terminal of the ignition switch

possibly the alternator side of the ignition warning lamp.

The way it works is that when the alternator has no output (engine stationary) there is no current flow down the thin wires, so the relay contacts stay apart isolating the domestic and engine battery.

When the engine is running and the alternator is charging the relay is energised, connecting the engine AND domestic battery together. The alternator output has a higher voltage than either battery so no charge is exchanged between batteries, but the alternator output is automatically proportioned so the flattest battery gets the larger portion of charge.

You will see that connecting it to the ignition switch and leaving the switch on with the engine stationary would result in both batteries being connected together and discharging together.

Diagrams in the charging section of the electrical notes on this website.

Tony Brooks



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Subject: Ammeter type.


Having read your comments about using the right type of ammeter to measure the alternator output I would like your opinion regarding a Durite ammeter I have bought for that purpose. In the installation instructions it requires that the cable being monitored be cut and the two ends connected to the ammeter.

Is this the wrong type of ammeter, it seems to be a very common/popular one, or is it satisfactory?

Best regards, and many thanks,



Dear Geoff

In my view what you have bought is not much more than a toy for the boy racer market. I am sure that it is a moving iron type with a short scale.

In my experience these meters can cause the alternator to pulse on and off load, but not in all cases. It's probably something to do with resistance. Also to get the meter where you can read it you usually have to extend the main charging cable. They are also not very accurate because of the short scale.

The ideal ones use an external shunt - you still cut the main cable, but you then insert a special very low value resistor called a shunt. The meter is also connected to the shunt by thin cable and the cable can be as long as you like because it passes not much more than zero current.

Unfortunately the difference in cost is very large - your 20 against 80 to 100 unless you can find someone breaking ex MOD trucks and extract both the ammeter and shunt.

Your meter will work and as long as you use thick enough cables (calcs required on this website in Electrical Notes) and the alternator does not pulse it will give some indication of charge. You should certainly be able to detect a failed alternator diode or imply failing batteries. It is just not ideal in my view.

Please take great care in fitting your terminals. If you create even a small resistance your battery charging will be compromised and a short circuit could well set fire to your boat.

Tony Brooks



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Subject: Monitoring Battery Condition & Inverter

I have purchased a 32 ft 6 year old n.b.

It has a 60 amp alternator and 4 x 110 ah batteries. 1 x starter batt. 1 x Domestic batt. 2 x batt. connected to a Skytronic 600 watt inverter. The alternator, domestic battery and the 2 inverter batteries are connected to a 3 position manual battery switch.

Position 1 is "Off" isolating the domestic battery from the inverter batteries and the alternator.

Position 2 switches the inverter batteries into the charging system.

Position 3 appears to allow all 3 batteries to be switched together for charging. In this position, when the engine is not running the circuit information I have would seem to suggest that if the switch is left in Position 3 the domestic battery is connected in parallel with the 2 inverter batteries giving an additional 110 ah supply.

The normal method of running is: Engine On: Master switch to position 3. (Inverter ON). Engine Off: Master switch to Position 1 Off. (Inverter ON).

Question 1.

On the domestic side there is a battery condition meters but nothing to indicate the condition of the inverter batteries. What can I/should I fit to give better battery information. Is it simply a matter of connecting another battery condition meter across the dc input terminals at the inverter?

Question 2.

Could you express an opinion as to whether my current method of control of batteries/charging via the battery master switch is satisfactory or needs to be upgraded? We have little in the way of 12 volt DC equipment, a water pump and a shower pump. On the AC 240 volts side we have a couple of fluorescent lights, a TV and a fridge stated as consuming 100 watts.

David K

Answer 1

If this meter is about 2" in diameter and has bright colours on its face it is probably a highly inaccurate piece of kit that is sold to boy racers to make their cars look "good".

If you look in the electrical notes on
you will find a section on meters and also charge splitting. There is a diagram that shows how you can use a two-way switch (in your case you may want to use a three way switch) so one voltmeter can be switched to give a reading from each battery in turn. This way you can afford to buy a better quality meter that may give you some reliable information.

As batteries discharge you need to measure down to a difference of 0.1 volt to assess state of charge (readings on website), so a good meter is required. It would also be a good idea if you could assess voltage fluctuations because as an alternator's brushes wear often the first thing you find is a fluctuating voltage. Digital meters sample the voltage at discrete points in time and then display the result so although they are far easier to read down to 0.1v they may also miss the voltage fluctuations. In my view a "needle" type meter is best. I also have fitted a shunt type ammeter in the alternators output line. By getting to know how both meters react from day to day one can soon identify potential problems before they result in some kind of failure.

If your battery condition meter is a digital type that shows how much "charge" is available to you then you need to address the manufacturers because if it is doing anything other than converting voltage to a pseudo state of charge reading switching it to another battery may well totally confuse it . However some of this type are very sophisticated and may cope.

Answer 2

The website makes my opinion on these "big red switches" quiet clear. They all too easily allow one human slip to result in all battery banks being flattened. Also if a fault causes the switch to break before it makes during switching (unless you stop your engine whilst switching) you are likely to end up requiring extensive work on the electronics inside your alternator. However this type of system is in common use on yachts. Remember that a failure of this single switch could leave you without any electricity for any systems.

Before contemplating anything else I must advise you to do a power audit as described on the website. This will give you a good basis upon which to base any other decisions.

If this were my boat (given the loads you mention) I would change the two mains fluorescents for good quality (tri-lights) 12 volt ones. So that you keep the lighting if the inverter fails.

I would then join the inverter and domestic batteries to form one bank (remember, a good voltmeter will allow you to monitor how discharged they are becoming). I would throw the big red switch away and substitute two master switches (one engine battery and one domestic bank), both in the positive leads.

I would then fit a split charge relay to charge the domestic bank - again all instructions in the notes (or come on a Boater's Electrical Course).

A modern inverter plus A rated domestic fridge is now as electrically efficient as a 12v fridge, so that combination can stay, however you must be aware that a fridge draws a great deal, of current when the motor starts and I would have doubts about the long term life of a 600 watt unit running a fridge, but only time will tell - if you have to replace it in the future make sure that whatever you buy has a sufficiently high surge rating.

If the TV is working OK (no funny lines or colours on the screen caused by a modified sine wave supply) then that's OK as well because it is not normally in use all day.

One thing to be aware of is that modern electronic equipment may not get on with a modified sine wave supply (if that is what your inverter delivers) so take care if you are thinking of adding more mains equipment.

Also always do a power audit before buying more electrical equipment unless it draws less than 1 amp at 12 volts.

I hope this helps

Tony B


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Subject: Wiring the charging system & Mains earthing

Sometimes when I get many questions in one email it is easier to give separate answers within the text of the email. This is one of them.

Dear Paul

Answers and comments in your text

I think you need to study the electrical notes on These will answer some of your questions

Hi Tony

I am wiring my own narrowboat, 35ft and had a few questions that I am not 100 percent sure of.

My background is electronics but not boat wiring and there are a few things that I am not clear about.

Firstly I am using a Volvo Penta engine, which as been made to fit in my boat, the alternator is new but from a car not full marine or Penta, from memory it is rated at approx. 55 amps full output. I am using a battery switch initially. But intend in the future to move to an alternator charge manager and fit battery bank isolator diodes.

1. Most "marine" alternators are in fact automotive ones with nut terminals added. In most cases the case is still at negative potential despite there being a "neg" terminal. Just make sure that if you have a "two Lucar Blade" output terminal set-up that you split your main charging wire so it connects via two, rather than one terminal.

2. Fit a split diode system if you must, but I, and other independent experts, would advise that you use a split charge relay.

Firstly I assume I take the case of the alternator along with the engine to a single tie point on the boat metalwork this would of course mean that the negative of the starter and leisure bank would also tied to the boat metalwork meaning that the whole boat would be battery negative? Or should I try to isolate this from the Boat metalwork.

Normal practice is to take the main battery negative to the engine block and leave it at that. Instrument negatives and domestic circuit negatives usually are wired from the battery terminals, but you could take them from the engine block - it can get a bit of a spiders web though.

When plugging into earth shoreline does the earth of the socket also tie to the metalwork of the boat?

DISCLAIMER - I am no mains expert. Double check everything I say.

Only do that if you will be moored miles away from any other boats with shorelines on (see notes).

Ideally use an isolating transformer (soft start) or if that is too expensive use a galvanic isolator (zinc saver) between the shoreline inlet and the boat's wiring. Then connect the boat side of that to the hull (BMEA say the same point as the 12v negative, but if you did and the attachment point fell off the 12v would become live at mains potential under fault conditions. So I would drill and tap either an engine bed or a steel bulkhead and connect the mains earth to that.

You must then fit an RCD so any short to earth fault trips it.

And can I protect these through earth leakage breakers.(I asume this is some type of RCD)

To make things even more fun the boat is wired in 240 volts and I assume that if live or neutral of the inverter touches the boat metalwork, it would be nice if the earth leakage breaker could kick in any thoughts.

See above. If a short to earth developed and you did nothing to protect from the consequences anyone stepping on or off the boat and any swimmers could receive a fatal shock unless a complete earth circuit existed between hull and company earth. An isolation transformer effectively makes the mains on your boat a seperate entity to the shoreline mains, so only people on the boat would be at risk

Please do not fail to bond the earth (on any system) to the steel hull and you MUST use a RCD as close to the transformer/isolator/entry point as possible to protect against shorts to earth.

Fuses or circuit breakers are then used to protect the cables.

Hope this helps

Tony Brooks


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