Battery main fuse rating?

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KenMcCulloch

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I'm planning to install heavy-duty fuses at my batteries as part of this winter's refit. The heaviest normal load is the engine starter motor, rated at 1kW. Will a 100A fuse be adequate or is there a risk of the fuse blowing under the starter load?
 
100 amp will definitely blow on a starter circuit, unless you're starting a sewing machine!
I have never seen a fuse on the starter main cable, possibly because every extra connection or terminal is another potential bad connection, but usually the fuse rating on a circuit is determined by the cable size.
I would guess you will need at least 500 amp for a starter on a small (25HP) diesel to allow for cold weather cranking.
 
I think you need more like 400+ amps. Your starting battery will be rated for a certain number cold cranking amps, and your engine will have a recomended cranking amps which hopefully the battery can provide. The cables to/from the starter should be rated for this many amps and the fuse should be perhaps 50-100 amps more, to protect the cable. These are big chunky expensive fuses - I've just fitted some - hope they never blow!

More precise technical responses likely to follow mine. There are some very knowledgeable people on the forum.
 
I use a 250A MegaFuse on the dedicated starter battery for my Volvo 2003. Remember that the fuse is there to protect the cable - and should be rated accordingly.
 
I don't think it's really practical to fuse a starter. What eventuality are you trying to protect your boat from?
The only instance where a fuse would be desirable is if the starter cable shorted to the engine or -ve. Better to concentrate on preventing this by checking the cable is soundly mounted, not chafing or being chafed etc. Also worth checking that the flexing of the cable due to engine vibration isn't stressing the connections.
I would consider fusing everything else though, possibly in groups, so that shorting a cabin light cable cannot take out the depth sounder or something!
 
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I don't think it's really practical to fuse a starter. What eventuality are you trying to protect your boat from?
The only instance where a fuse would be desirable is if the starter cable shorted to the engine or -ve.

[/ QUOTE ]That's exactly what the fuse is protecting you from! A short on a starter battery cable is very likely to start a fire.
 
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I don't think it's really practical to fuse a starter. What eventuality are you trying to protect your boat from?
The only instance where a fuse would be desirable is if the starter cable shorted to the engine or -ve.

[/ QUOTE ]That's exactly what the fuse is protecting you from! A short on a starter battery cable is very likely to start a fire.

[/ QUOTE ]
This is exactly my concern, following a post a few weeks ago describing a serious fire caused by battery cables shorting. I am the current custodian of a quasi-historic vessel and would be mortified if she went the way of the Cutty Sark! Yes, all my domestic circuits are independently fused but that's not the point here.
 
I've actually metered the starter current on a Yanmar 2GM20 at 200A max which I think is reasonably representative of small diesel engines. I would fuse at about 250A but the key point is that you must use cables that are thick enough to carry more than 250A at 12V otherwise the fuse won't blow in the event of a short. I would suggest 35mm sq plus cable depending on length and make sure the ends are crimped with a proper tool and sleeved to help prevent shorts and fatigue.
 
My point was that shorting the starter cable is preventable in a seaworthy installation.
A 250 Amp fuse is of limited value in fire prevention, as your cable, connections and battery are unlikely to be capable of surviving this current for very long. (It takes several times the nominal current to blow a fuse quickly, the factors depend on the type of fuse.) The only effective protection is proper installation and regular inspection.
Also having a fuse introduces more connections, each of which introduces a possible failure point both mechanical and electrical.
It's perhaps better to look at keeping the unfused wiring as short and simple as possible, perhaps by having a starter battery nearer the engine.
 
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I've actually metered the starter current on a Yanmar 2GM20 at 200A max which I think is reasonably representative of small diesel engines.

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To measure the starting current accurately is very difficult. Meters have a reasonable time delay and averaging built in.
If you measure the starting current with a high speed oscilloscope you find the initial inrush current is very high but this lasts only for ms.
Fuses are thermal devices and therefore a 250A fuse will not necessarily blow with a much higher current if the duration is short, but the results are unpredictable. Measuring a starting current on a meter of 200A does not mean a 250A fuse will or will not blow.
I know that's not much help.
The whole issue of fusing the starting circuit is a complex one with fierce proponents on both sides.
My starting circuit is not fused and I must admit that makes be nervous. My main reason for not fitting a fuse is that the starting current can vary considerably, particularly the short term inrush current. You could fit a fuse and have it work perfectly for 6 months, but then one night its cold the boat starts dragging to the rocks just behind. You hit the starter button, but the engine is colder than normal and the fuse is older (Its probably had multiple starts close to its capacity). The fuse blows. Can you diagnose the problem and change the fuse in time?
 
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My starting circuit is not fused and I must admit that makes be nervous. My main reason for not fitting a fuse is that the starting current can vary considerably, particularly the short term inrush current. You could fit a fuse and have it work perfectly for 6 months, but then one night its cold the boat starts dragging to the rocks just behind. You hit the starter button, but the engine is colder than normal and the fuse is older (Its probably had multiple starts close to its capacity). The fuse blows. Can you diagnose the problem and change the fuse in time?

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One way around this (assuming 2 batteries and a simple 1-2-Both switch) would be to fit 2 fuses, one for each battery. This would allow the fuse to be physically very close to the battery + terminal and on the cold, dark winter night when the engine doesn't start you just switch to the domestic battery (probably the first thing you'd try if you turn the key and nothing happens) and worry about diagnosing and changing the fuse once you're away from those rocks...
 
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One way around this (assuming 2 batteries and a simple 1-2-Both switch) would be to fit 2 fuses, one for each battery. This would allow the fuse to be physically very close to the battery + terminal and on the cold, dark winter night when the engine doesn't start you just switch to the domestic battery

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Good thought.
The biggest concern apart from trying to keep things as simple as possible, is what rating do you make the second fuse. If the first one has blown because the engine is cold and needs more starting current than normal, then the second will blow also.
I can see all sides to the argument but there are really no easy answers
 
I have fitted a 750 Amp fuse into each of my 3 battery banks. The size is mainly due to the big bow thruster I have which is 12Volt and is feed by 40 mm x 20mm aluminium buss bars from my main battery banks
 
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Fuses are thermal devices and therefore a 250A fuse will not necessarily blow with a much higher current if the duration is short, but the results are unpredictable. Measuring a starting current on a meter of 200A does not mean a 250A fuse will or will not blow.

[/ QUOTE ]I use and recommend MegaFuses. These are very well-engineered automotive fuses. They will tolerate currents of 135% of their rating for at least 2 minutes, so are very resistant to nuisance blows. However, if there's a short circuit in the starter cable, the current which will flow is simply huge - probably around 1000A with many batteries. In this case, the MegaFuse will blow in a maximum time of 2 or 3 seconds, and probably much quicker than that. I've only shorted a battery once, when replacing my alternator, and the MegaFuse blew instantly.
 
We've all made stupid mistakes, some of enough are brave enough to admit them on the forum, but fitting a fuse is no substitute for basic precautions like disconnecting the battery before the alternator. The alternator wiring could very easily have caught fire first, or the battery exploded.
Also consider whether the fuse should be located in the space that the battery vents into...
BTW, if you look around www.rswww.com, you can get datasheets showing blow time/current. To blow in 2 seconds requires 4xInominal so we are talking 1000A for a 250 A fuse. But a 500A fuse might be more appropriate for a 200A start up surge on the motor. It depends on how resistant to false blowing you want the system to be.
I'm not against the fuse as such but I would be wary of the 'don't worry it's fused' sentiment, its a bit like those people who think they are immortal on a pushbike because they have a polystyrene hat on.
Do any cars or trucks have starter motor fuses? Somebody told me that the italian market version of the hillman imp had no fuses at all!
 
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We've all made stupid mistakes, some of enough are brave enough to admit them on the forum, but fitting a fuse is no substitute for basic precautions like disconnecting the battery before the alternator.

[/ QUOTE ]Quite right and, yes, I'm brave enough to admit it! /forums/images/graemlins/grin.gif

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Do any cars or trucks have starter motor fuses?

[/ QUOTE ]MegaFuses are OEM fit on a number of Ford, GM and Chrysler vehicles.
 
The possible reasons you now get fuses on the main battery leads of modern cars are 1. the wire size has been reduced to the absolute minimum to cut costs, and 2. in the event of a crash, bodywork cutting into cables.
 
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I use and recommend MegaFuses. These are very well-engineered automotive fuses. They will tolerate currents of 135% of their rating for at least 2 minutes, so are very resistant to nuisance blows.

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All fuses exhibit this sort of behavior, but the inrush currents can be much higher 135%. I didn’t make the order of magnitude clear in my post
20505.jpg

The vertical scale is 200A per division. It can be seen that the steady state starting current is about 200A. This is what would be measured by a conventional meter, but note how the current goes over 1000A (Yes a thousand amps) for a very short time. This inrush current is what makes fusing difficult. This doesn’t mean you need 1000A fuse, but will a 250A fuse blow during a normal start, maybe sometimes maybe not.
What about using a very high amperage fuse say 750A as suggested.
Could the battery cables handle 750A even for a short time?
An 85mmsquard cable (that is very very thick) is typical rated at less than 400 A and that is assuming the cable is not in contact with anything.

Lets look at typical short circuit:
The + battery cable falls off and grounds against the engine. How much current will flow? We need the internal resistance of the battery the resistance in the battery cables and all the connection switches etc and most importantly the resistance of the connection resting against a dirty and painted engine block. Will this generate 750A amps I doubt it (and as has been pointed out we need more than 750A to blow a 750A fuse)
Is it a problem Yes the current while not 750A is enough to start a fire.

So is 750A fuse silly no there is really no satisfactory solution.
 
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....there is really no satisfactory solution.

[/ QUOTE ]I find it quite satisfactory to have a 250A MegaFuse. It's been satisfactory for about 10 years. I expect it to be satisfactory for many years to come. And, the one time I stupidly shorted the alternator connection, the fuse blew in a very satisfactory manner! /forums/images/graemlins/smile.gif
 
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It's been satisfactory for about 10 years. I expect it to be satisfactory for many years to come.

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Its good to hear a relatively low rated fuse is working well for you. This provides the best protection from a short circuit and after 10 years you must have justifiably confidence that it will not blow at an inappropriate time during a normal start.
With this sort of trouble free protection I can see you are one of the fierce proponents in favor of fusing the start circuit.
Could you tell me what size is your engine?
 
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