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Engine and service battery linking

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    I believe most Nauticats have an electrical relay which is fed from the engine alternator. Its function is to link together the starter battery with the service batteries when the engine is running so that the alternator charges all the batteries in parallel.
    When the engine is stopped the feed to the relay disappears and the battery link goes open circuit.

    I am thinking that in the event of a battery failure then a simple separate 12V feed wire to the relay would enable the batteries to be linked without the engine running. The main circumstance for this would when the starter battery fails so that the service batteries could be used to start the engine.

    I assume this could be done with a small gauge wire, including an inline switch, from a suitable service battery feed to the relay.
    I should say at this point I’m not even sure where the relay is located on my boat.

    I wondered if anyone has done this and if so can they share their knowledge about it?

    Note: I have also posted this on the international Nauticat Yahoo group


    Steve McCarthy


    David Babsky

    I haven’t “hot-wired” that relay to switch in the “house” batteries (..and some of the charge from those would then go to charging up the engine-start battery, thus dropping, I think, the overall voltage before you even turned the starter key!)

    What I HAVE done is to put in another switch – obtainable from almost any chandlers – which can switch between the engine-start battery (just a single one on our boat) and the “house” batts (three in parallel on our boat) so that EITHER of those can be used to start the engine. This means that if the starter-batt is flat (terrible situation, as completely flat lead-acid batts never, apparently, and in my experience, never recover their original capacity or “staying power”) then the house batts can be switched in, and there’s no consequent voltage drop as a result of their trying to boost the flat starter battery.

    As soon as the engine starts – which it does, with the power of three “house” batts – and the relay switches on, then the engine-start batt gets quickly brought back up to power, but without draining the “house” batts, except during that engine start.


    Presumably the cable and the switch have to be able to take the current load of engine starting, so up to say 100 amp.

    I now realise that what I was suggesting is completely impractical because the existing link cable and relay contact would not be able to take that sort of load.

    I guess your option is the only workable one.



    victor crowhurst

    I carry a long pair of jump-leads on-board my 37, so I could bridge from any of my domestic batteries. I’ve burnt out three of those relays by letting the domestic batteries get low. In this situation, on starting up, very large currents flow across that relay, 30-70A which despite its spec, it can’t handle and the plastic base melts and the contacts separate. I now constantly keep the MasterVolt trickle charger on.

    David Babsky

    Yes, the changeover switch – this is the one I’ve installed: and it can cope with 300 amps – needs to be chunky enough to carry an engine-cranking (starting) current. And I put really heavy cabling between the batteries and switch, etc. (Cabling, as far as I remember, from the battery company which gives the Nauticat Association a good discount on boat batts; Barden batteries.)

    Nice to see a comment from Dr Vic ..all best wishes to you and “Nautinto”, Vic!


    I hadn’t thought about the low charge situation of the domestic batteries potentially causing problems as well.
    Regarding the switch I can see there might be various ways a switch could be used. A simple way would be a simple changeover switch in the feed to the starter motor which selected either the starter battery supply or the domestic battery supply.
    David presumably your switch, being 4 way, is a rather more sophisticated circuit arrangement?

    David Babsky

    Stephen, you wrote “David presumably your switch, being 4 way, is a rather more sophisticated circuit arrangement?” ..No; it’s extremely simple. (See that web page )

    It has positions 1 and 2, with terminal 1 connected to the starter batt positive and terminal 2 connected to the “house” batts positive.

    There is a 3rd terminal connected to the starter system positive.

    In position 1, the normal starter batt starts the engine it usually does. In position 2 the “house” batts start the motor. In position “Both” the terminals 1 & 2 are connected together and BOTH start the motor (if necessary). When the motor starts, because its relay connects both sets of batteries, that’s exactly the same as the “Both” position on this selector switch. That’s it. Simple.

    The negative cable isn’t attached to this switch ..the neg just runs as normal from both batts to the engine casing / starter motor casing and the rest of the negative circuitry.


    Thanks David.
    I understand it.
    I guess the difficulty is finding the existing cables and working out where to fit a switch which is easily accessible, no mean feat in a 321.
    I will go and look tomorrow when I have the boat lifted out.

    I started on this because I have an 8 year old starter battery which as far as I can see has a very easy life. The number of engine starts per year is very low (relative to say a car) and then it spends all its time being charged either by the alternator, or when berthed, by a Mastervolt charger.

    David Babsky


    I can’t remember the layout of a 321, but in our (once Dr Vic’s!) N33 I put the changeover switch in the little floor-level cupboard under the left-hand seat in the wheelhouse; where the Ebersp├Ącher heater and incoming mains switch sit. (I think the autopilot’s in there, too.)

    Eight-year-old battery? If it’s been run flat in those 8 years it’s unlikely to work at peak performance now. And being constantly charged? ..You do check the liquid level inside the battery from time to time, don’t you? Even at constant trickle charging, some of the water’s bound to disappear after a while, and that’ll reduce the batt’s capability.

    (I’m off to our boat now, to check the batts, etc, so may not be able to reply again till Friday..)


    Just back from the boat.
    Access to the cabling behind the master battery switch and feed to the starter motor on a 321 is virtually impossible without dismantling parts of the boat under the nav station.
    As a result trying to put in a big switch would not be an option for myself.
    So I’ll just to have to stick with a set off car jump leads if the starter battery fails.
    The only issue with that is the need to lift the saloon floor and then take of the lid of the battery box – not something I would relish in even a moderate sea.

    Thanks for all the advice.

    David Babsky

    Stephen, “..So I’ll just to have to stick with a set off car jump leads if the starter battery fails. The only issue with that is the need to lift the saloon floor and then take of the lid of the battery box – not something I would relish in even a moderate sea.”
    So why not install a set of those car jump leads – screw them to the battery terminals properly, not just with croc clips! – and then route them to any convenient place where you could fix a heavy-duty changeover (or rather “interconnect”) switch? ..You’d only need to use positive cables, as you could connect the negative terminals – of both batteries – together permanently.
    Choose any convenient place for the heavy-duty “interconnect” switch, and then if the “starter” battery’s flat, just twist the switch to connect the house batts to it, and then switch back to “not connected” when the engine fires.
    That’d surely be far better than lifting “..the saloon floor and then take of the lid of the battery box – not something I would relish in even a moderate sea.”

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