Which battery fuse ?

[bobgosling]

What size battery fuse would people recommend for a 65Ah engine start battery turning over a BD722 Beta Diesel 20hp engine ? I realise that the current can momentarily surge to quite a high value but cannot find any info on how to calculate which rating of fuse to use.

The one on the domestic side is a bit easier. In fact I'm thinking of just having a circuit breaker on that side. One of the big jobbies from Maplins , designed for mega-stereos in cars looks like it should do the job. Even has gold-plated corrosion proof contacts ! Anyone think this is a bad idea ?

 

[alb40]

Engine side of electric should never need to be fused.

Say for instance, you were caught out somewhere and needed to restart the engine to get out of danger, but the fuse was gone? Says it all really.

 

[Ships_Cat]

I also go along with no fuse and depend on good cabling practices instead. If you have an engine that is isolated from the battery negative as many modern marine engines are, then there is even less risk from shorting.

Each circuit off your service battery should be individually breakered/fused and again I go along with no fuse/breaker at the battery itself as long as good cabling practices are followed and the run from battery to the distribution breakers/fuses is not more than a couple of metres. I personally would not put the breaker in the battery compartment or too near it in any event due to the possibility of danger of sparking from a throwing breaker or when remade.

On the gold plated car stereo terminals, breakers, etc, I have found that these may have plated steel terminal screws in them (ones I have checked have had screws that are very magnetic, which does not, however, discount the possibility that they are plated ss). How long the screws, if steel, would last on a boat I would not know but they should be easy to replace with ss or brass. I have such a fuse block and a terminal block in my own boat and the screws are fine after 4 years, but the boat is a very dry one.

John

 

[Malish]

Agree with the others re not fusing the positive feed to the engine. Most problems I have encountered relate to the negative (return) side - make sure it is well made and in good condition otherwise the starter will either fail to function or the return power will find alternative routes. On the feed to the domestic I use a mega fuse (100A as I recall) from vehicle wiring supplies, self contained in its own box, never blown it yet and hope I never do!

 

[marcusgillies]

i agree with the starter battery not needing a main fuse but regarding domestic /service batteries
Its ok saying no fuse here and no fuse there but remember fuses /breakers are there to protect cable, would you rather the cable burned out or the fuse blew / breaker trip!!!

 

[CSFenwick]

Intrigued that fusing the domestics supply is recommended but not the engine start. I would have thought the purpose for doing one is the same as the other?

 

[aidancoughlan]

I looked at a Merlin electrics package at the boat show last year - I think from memory they had specified 200A fuses on both engine and domestics. Like CF1 above, I am intrigued as to why it is not a good idea ?

 

[brianrunyard]

Domstic fuse rating depends on rating of the cable, fuse should be "The weakest Link". No use having a 100 amp fuse if the cable's rated less than 100 amp, or in the case of a problem the cable will burn out (and possibly cause a fire) and the fuse will remain intact.

 

[Jean]

If fuse has blown, then probably saved a greater potential danger than lack of engine (FIRE). I have seen the results of a short on a starter circuit, it makes quite a mess. However, I can understand your concerns. Another option would be suitable fuse in circuit and an emergency use switch in parallel with the fuse. Fuse blows and can then still attempt to start engine with bypass switch closed but then aware of potential serious problem, ie, blown because of serious fault.

 

[pvb]

Use a Mega Fuse...

I'd recommend using a Mega Fuse; these are easily available in various ratings. You need to buy the matching mounting block for it too. The advantage of the Mega Fuse is that it has a good slow-blow characteristic. For your engine, the starter motor will probably use no more than 80-100A, so a 100A or 125A Mega Fuse would be OK (and these will actually withstand double their rated current for about 10 seconds before blowing). It's a good idea to keep a spare fuse on board, although in an emergency you could simply swap a cable on the fuse mounting block to take the fuse out of circuit.

 

[PBooth]

Some modern vehicles have fuses in the main battery lead, usually when the battery is located a distance from the engine. This increases the chance of a short on the main battery lead, I have seen 500 amp 'mega' fuses used (this is on a Renault Master van) the fuse should be rated at less than the current carrying capacity of the cable. On my boat, I don't have a fuse in the main battery leads (they are very short), but I have a 50 amp circuit breaker protecting the cables to each of the distribution panels and the panels each have 8 minature circuit breakers so every circuit has it's own switch and circuit breaker. Apart from the cabin lights, every electrical consumer has its own circuit, this means that a fault will only take out 1 item and also fault finding is easy.

Peter