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alternator to battery fuse requirement ??


sooty

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Very unwise in my view. Fuses fail through thermal cycling and fatigue and if it does there is every likelihood the alternator will also fail. In my view far more likely that a short circuit on a well maintained boat.

 

The major fuse between battery and domestic distribution board is a different matter.

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Thank you  for your quick replies

Yes Tony I see your your point of the danger of running the alternator into an  open circuit should the fuse fail.

Yes Loddon the isolation switch dillema I think still remains, Am I right in thinking BSS requires the alternator charging cable to connect to the cabin side of the isolation switch ?

So if the cabin battery switch is OFF and the engine is started , there is a possibility the alternator can put its full charging volts down the 12 volt cabin supply !!

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1 hour ago, sooty said:

Am I right in thinking BSS requires the alternator charging cable to connect to the cabin side of the isolation switch ?

Yes. 

1 hour ago, sooty said:

So if the cabin battery switch is OFF and the engine is started , there is a possibility the alternator can put its full charging volts down the 12 volt cabin supply !!

Yes. 

 

Daft, innit?

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Before I retired I was called out to a TVR  Tuscan that wasn't charging. The battery was flat so couldn't move it from the gravel drive it had sunk into. There was no 12 volt to the alternator output. Traced it to a  fuse mounted on the chassis rail under the engine. Hadn't blown, just corroded away. After the struggle of jacking the car up so as to get at it, I just linked the 2 cables together. 

 

As someone else said, stupid idea. 

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5 hours ago, sooty said:

So if the cabin battery switch is OFF and the engine is started , there is a possibility the alternator can put its full charging volts down the 12 volt cabin supply !!

Well if the cabin battery switch is off and the engine is then started, probably the alternator won't be excited (ie won't start to produce power) although it depends on the type of alternator. But even if it does, it should regulate at 12v and not have a problem.

What causes a problem is when the alternator is working hard and suddenly is disconnected from the battery. It takes a finite time for the stator's magnetic field to collapse and in the interim the alternator voltage will rise very high and possibly blow its internal diodes.

Obviously this latter scenario is what can happen if a fuse between alternator and battery blows. Hence don't fit one!

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I thought quite hard about this, well as hard as I can manage, and I did fit a fuse right next to the battery box so effectively fusing the alternator. I fitted a big fuse so there is reduced chance of it wearing out and it should only blow if there is a big accident. We have a "proper" engine room and the domestic alternator does protrude slightly into the passage where we walk past the engine. The wiring is secured and has a rubber boot over the connecting nut but I can see a remote possibility of a shorting accident. Still not 100% sure that I made the right decision!

 

.............Dave 

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9 hours ago, Flyboy said:

Just like every car is wired.

Except the ones with alternator fuses and there's lots.

 

6 hours ago, Mike the Boilerman said:

I'm curious as to what the potential fuse is intended to protect against. 

 

If the alternator cables are capable of carrying the full output current of the alternator, what is the point? 

 

 

It's intended to protect against a short circuit of full battery current resulting in a fire.

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8 hours ago, Mike the Boilerman said:

I'm curious as to what the potential fuse is intended to protect against. 

 

If the alternator cables are capable of carrying the full output current of the alternator, what is the point? 

 

 

The point is that the battery is a massive store of energy that can be released quickly (1000s amps) and that would spoil your day. However presuming the wiring is adequately installed, it’s a pretty unlikely scenario.

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Also, where does the fuse go?

Fuses are to protect cables against overload as  in the event of a short to earth.  Fused At  the alternator end the cable is not protected against chafing and back flow of full battery current.  At the battery end the cable between the alternator and the fuse is not protected at all and so has to be large enough to continuously carry the alternators maximum output current into a short circuit, which will be greater than the rated current.  The alternator is most likely to overheat in this situation and may take fire, especially if it is the usual fluffy oily mess inside.

N

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16 minutes ago, BEngo said:

Also, where does the fuse go?

Fuses are to protect cables against overload as  in the event of a short to earth.  Fused At  the alternator end the cable is not protected against chafing and back flow of full battery current.  At the battery end the cable between the alternator and the fuse is not protected at all and so has to be large enough to continuously carry the alternators maximum output current into a short circuit, which will be greater than the rated current.  The alternator is most likely to overheat in this situation and may take fire, especially if it is the usual fluffy oily mess inside.

N

Definitely at the battery end, since this is a source of near unlimited current. The alternator max current is finite and should be wIthin the cable thermal rating - 12v cabling is usually selected for voltage drop and is thus operating way below it’s thermal limit. Anyway I suspect an alternator feeding a short would produce less current because it wouldn’t have much voltage to feed the rotor (field current). And the problem can quickly be fixed by stopping or slowing the engine.

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12 minutes ago, Sir Nibble said:

The real purpose is in the event of a collision driving metalwork into the cabling. 

... although road vehicles, which are likely to be subject to much more severe collisions, don’t have fuses between battery and alternator.

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I wire the alternator to the starter motor so that the engine battery  isolator does for the alternator too.

 

Definitely no fuse. Never no way not ever.

 

Yes I know that wiring the alternator to the cabin batteries get them charged soonest but I would rather have an engine battery fully charged first so that at worst case I can still start the engine. It take very little time for a 90A alternator to fully charge an 80AH starter battery, and the split relay connects the cabin batteries anyway.

It also means a shorter connection from the alternator to the heavy starter wiring back to the battery, less volts drop which can be considerable with high output alternators.

As I never run either battery too far down, there is little danger of the split charge relay not being able to handle the equalizing current if one battery is lower than the other.

I've wired boats like this for 49 years without any problems so I'll stay as I am thank you.

Edited by Boater Sam
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13 minutes ago, Boater Sam said:

I wire the alternator to the starter motor so that the engine battery  isolator does for the alternator too.

 

Definitely no fuse. Never no way not ever.

 

Yes I know that wiring the alternator to the cabin batteries get them charged soonest but I would rather have an engine battery fully charged first so that at worst case I can still start the engine. It take very little time for a 90A alternator to fully charge an 80AH starter battery, and the split relay connects the cabin batteries anyway.

It also means a shorter connection from the alternator to the heavy starter wiring back to the battery, less volts drop which can be considerable with high output alternators.

As I never run either battery too far down, there is little danger of the split charge relay not being able to handle the equalizing current if one battery is lower than the other.

I've wired boats like this for 49 years without any problems so I'll stay as I am thank you.

I agree especially if the engine is indirect injection where the heaters might be used for longish periods caning the start battery like BMC's, tend to need, but with easy starting direct injection engines I prefer it taken direct to the cabin battery.

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