What size fuse between 160A alternator and 675A batteries?

[swift1894]

Continuing my rewire. Ideal fuse size is?............?

Edited April 5, 2016 by swift1894

[Paul C]

None, you don't need a fuse between alternator and battery. There is no overcurrent possibility, because the alternator is not a load.

[frangar]

None, you don't need a fuse between alternator and battery. There is no overcurrent possibility, because the alternator is not a load.

 

Not quite true……the batteries are a power source….a short on the cable or at the alternator could prove amusing…..there are arguments for and against fitting a fuse imho!…..personally I would want to know distance between batteries and alternator….routing of cable etc….

 

Cheers

 

Gareth

[RLWP]

I wouldn't wire it that way. Run a cable from the output of the alternator to the positive terminal of the starter. That way, you have one less thick cable to bridge from the engine to the batteries

 

Beyond that, the fuse protects the cable and should have a lower rating than that. It is the size of the cable that is determined by the load, not the fuse

 

Richard

Edited April 5, 2016 by RLWP

[Robbo]

Continuing my rewire. Ideal fuse size is?............?

Fuse for the cable size at the battery end (house batteries). I would use 50mm if it's less than 2-3m away. 70mm if more. I would also run a negative cable directly to the alternator as well of the same size.

 

I wouldn't wire it that way. Run a cable from the output of the alternator to the positive terminal of the starter. That way, you have one less thick cable to bridge from the engine to the batteries

Whilst that may be okay for small alternators, a big 160amp will benefit from been directly connected. Strictly speaking for BSS purposes it should go via an isolator. Edited April 5, 2016 by Robbo

[nicknorman]

What you want to avoid is the fuse blowing from fatigue/ old age since that would probably kill the alternator . So, and bearing in mind what you are trying to protect against is a dead short circuit from a spanner across the alternator terminals etc, I would go for a fuse well abov the max load, eg 250A or more. Or route it via an isolator. Personally I prefer the former since it means the boats domestic circuits are dead with the isolator off even with the engine running.

I wouldn't wire it that way. Run a cable from the output of the alternator to the positive terminal of the starter. That way, you have one less thick cable to bridge from the engine to the batteries

 

Beyond that, the fuse protects the cable and should have a lower rating than that. It is the size of the cable that is determined by the load, not the fuse

 

Richard

No, and not really!

[swift1894]

None, you don't need a fuse between alternator and battery. There is no overcurrent possibility, because the alternator is not a load.

Hurray!

One less job.

Not quite truethe batteries are a power source.a short on the cable or at the alternator could prove amusing..there are arguments for and against fitting a fuse imho!..personally I would want to know distance between batteries and alternator.routing of cable etc.

 

Cheers

 

Gareth

Hmm. 50mm and 1 metre.

I wouldn't wire it that way. Run a cable from the output of the alternator to the positive terminal of the starter. That way, you have one less thick cable to bridge from the engine to the batteries

 

Beyond that, the fuse protects the cable and should have a lower rating than that. It is the size of the cable that is determined by the load, not the fuse

 

Richard

The house batteries are only 1 metre from the alternator. The starter is 2 metres away but 1 metre from the starter battery.

Fuse for the cable size at the battery end (house batteries). I would use 50mm if it's less than 2-3m away. 70mm if more. I would also run a negative cable directly to the alternator as well of the same size.

Whilst that may be okay for small alternators, a big 160amp will benefit from been directly connected. Strictly speaking for BSS purposes it should go via an isolator.

The cable is 50mm and pos and neg are directly wired to the domestic batteries.

What you want to avoid is the fuse blowing from fatigue/ old age since that would probably kill the alternator . So, and bearing in mind what you are trying to protect against is a dead short circuit from a spanner across the alternator terminals etc, I would go for a fuse well abov the max load, eg 250A or more. Or route it via an isolator. Personally I prefer the former since it means the boats domestic circuits are dead with the isolator off even with the engine running.

 

No, and not really!

Well I certainly don't want to kill my alternator!

There's not going to be a dead short across the alternator terminals, I'm too careful for that and they're insulated by covers anyway.

Edited April 5, 2016 by swift1894

[Paul C]

 

Not quite true……the batteries are a power source….a short on the cable or at the alternator could prove amusing…..there are arguments for and against fitting a fuse imho!…..personally I would want to know distance between batteries and alternator….routing of cable etc….

 

Cheers

 

Gareth

 

I know but.......

[by'eck]

Hurray!

One less job.

Hmm. 50mm and 1 metre.

The house batteries are only 1 metre from the alternator. The starter is 2 metres away but 1 metre from the starter battery.

The cable is 50mm and pos and neg are directly wired to the domestic batteries.

 

Well I certainly don't want to kill my alternator!

There's not going to be a dead short across the alternator terminals, I'm too careful for that and they're insulated by covers anyway.

 

So as you have probably concluded, as long as the cable run from alternator to batteries is relatively short and cable of more than adequate dimensions, you are probably better off spending time and money on cable insulation protection and mechanical support with direct runs to batteries/distrbution bus/isolator, rather than worrying about a fuse.

[swift1894]

So as you have probably concluded, as long as the cable run from alternator to batteries is relatively short and cable of more than adequate dimensions, you are probably better off spending time and money on cable insulation protection and mechanical support with direct runs to batteries/distrbution bus/isolator, rather than worrying about a fuse.

Music to my weary ears.

Now...... Onto job 2 of 12! ?

Edited April 5, 2016 by swift1894

[RLWP]

I wouldn't wire it that way. Run a cable from the output of the alternator to the positive terminal of the starter. That way, you have one less thick cable to bridge from the engine to the batteries

 

Beyond that, the fuse protects the cable and should have a lower rating than that. It is the size of the cable that is determined by the load, not the fuse

 

Richard

 

 

No, and not really!

 

Well, that's a beautiful put-down but a bit lacking in technical detail. I'm still interested, could you explain which bit is 'no', which is 'not really' and why?

 

Just in case, I really would like to know

 

Richard

Edited April 5, 2016 by RLWP

[pearley]

I spent ages on a TVR trying to find the alternator fuse that the wiring diagram showed. Eventually found a fusible link mounted on the chassis rail under the engine. Needed to Jack the car up to replace it. Not easy with a dead battery, inoperative immobiliser and in a gravel drive.

Edited April 5, 2016 by pearley

[Paul C]

Is this a single alternator or do you have 2 of them (engine/start and domestic)?

 

If its a single alternator then the best place to connect it is to the domestic battery bank. Let the current for the engine/start battery go through the split charge relay (or equivalent).

 

 

If its two alternators, then obviously each connects to its own battery bank and RLWP's suggestion for shortening/simplifying the wiring can be used if it helps your particular installation, on the engine/start side of it. (The domestic alternator would be connected directly to the domestic bank).

[RLWP]

RLWP's suggestion for shortening/simplifying the wiring can be used

 

I'm not claiming originality for this. As an example, this is how Beta ship their engines

 

Richard

[nicknorman]

Well, that's a beautiful put-down but a bit lacking in technical detail. I'm still interested, could you explain which bit is 'no', which is 'not really' and why?

 

Just in case, I really would like to know

 

Richard

 

Yes it was a bit terse but then many of your posts on here are likewise and I thought you would consider imitation to be flattery!

 

Anyway "no" is for wiring the alternator to the starter. This is how it was done in the good old days with small alternators but depending on the engine there may be:

 

A dedicated small engine alternator (which can be wired to the starter +ve) and a large domestic alternator which needs to be wired direct (or via fuse/isolator) to the domestic battery.

 

Or

 

A single large alternator, most of whose current is destined for the domestic batteries. It is better to connect a large alternator direct to the domestic battery +ve and use the split charge relay to charge the starter battery, thus minimising the current going through the relay (these relays are often only marginally rated and of course introduce unnecessary voltage drop between alternator and battery, especially at high currents.

 

OP says his alternator is 160A so it's one of the above.

 

"Not really" is for rating the fuse to the cable. Normally one rates the fuse to the circuit, ie the cable and whatever it is supplying. However IMO in the case of a large alternator cable it is pragmatic to consider what you are trying to protect against .

 

Let's say you have cable that can take 200A continuously without overheating. So you might rate the fuse at 200A. With a 160A alternator working hard that fuse is going to get quite hot. Of course it won't blow..... Well, after 10 years of getting "quite hot" every day and cooling, it might get fed up and blow just due to old age and grumpiness. One trashed alternator.

 

So let's say you rate the fuse at 500A. Shock horror, above the max rating of the cable!

 

That means the fuse doesn't get very hot and will last much longer. With a dead short say across the alternator terminal, much more than 500A will flow and the fuse will blow rapidly. Although the cable has been passing more than its design current, it only did that for a second and isn't even warm.

 

If you can envisage a scenario where the fault current is >200A but less than 500A then there is a problem, but it's hard to imagine how that could happen. And of course most alternator circuits aren't protected by a fuse at all. They may or may not have an isolator which may or may not be operated by the user if he/she knows where it is, understands the problem, what to do about it and is even present on the boat at the critical time.

Edited April 5, 2016 by nicknorman

[RLWP]

I haven't been following previous posts on this, so I'm afraid I assumed 'alternator' meant alternator i.e. Just the one. My mistake

 

To be perfectly honest, I am with by'eck on this and would omit a fuse.

 

Why would a fuse in the output cable blowing take out the alternator, Nick?

 

Richard

Edited April 5, 2016 by RLWP

[nicknorman]

I haven't been following previous posts on this, so I'm afraid I assumed 'alternator' meant alternator i.e. Just the one. My mistake

To be perfectly honest, I am with by'eck on this and would omit a fuse.

Why would a fuse in the output cable blowing take out the alternator, Nick?

Richard

No it might just be one alternator, as per my bit after the "Or". The point really is that with modern high current alternators it is better to minimise the current path between alternator and the main load which is the domestic batteries. Anything else such as engine battery charging can be Td off that.

 

The alternator's power output is determined by its rpm (not relevant in this case) and by its field (rotor) current, which is controlled by the regulator. So if the alternator is putting out say 150A at 14v that is quite a lot of power. If the output is suddenly interrupted the voltage will rise dramatically and the regulator will stop supplying field current, however the magnetic field takes a little time to collapse and in the mean time the power has to go somewhere and that is manifested by a large rise in output voltage which will exceed the voltage rating of the diodes and blow them.

 

The BSS requires the alternator circuit to either be fused or to go via an isolator (both near the battery, to protect against high currents from the battery, not the alternator). I think lots of boats have neither but if you are rewiring you might as well make it compliant. Personally ours is routed via a 500A fuse and not via an isolator because the disadvantages of the latter are that inadvertently switching off the isolator (or bad contacts etc) can damage the alternator as explained, and that even with the isolator off the boat's circuits remain powered by the alternator with the engine running making emergency isolation due to an electrical fire a bit pointless unless you are also able to stop the engine (just been on the Severn with current running at about 2.5mph!).

Edited April 5, 2016 by nicknorman

[RLWP]

Fair enough on the wiring. So far I haven't come across an installation where the alternator output wire is larger than the starter cables. That's not to say they don't exist.

 

I'm not clear why 'the voltage' rises if you disconnect the output of an alternator

 

 

Richard

[nicknorman]

Fair enough on the wiring. So far I haven't come across an installation where the alternator output wire is larger than the starter cables. That's not to say they don't exist.

 

Richard

Well they exist on our boat! Starter cable is perhaps 25 or 35mm^2. 175A domestic alternator cable is 75mm^2. Voltage drop in a charging circuit is more significant than in a starter circuit.

[nicknorman]

I'm not clear why 'the voltage' rises if you disconnect the output of an alternator

Richard

Wires in a spinning magnetic field creates power. Electrical power is volts x amps ie P = V x A. Or to rearrange, V = P/A. As I mentioned, it's not possible to stop the power generation instantly due to the magnetic field briefly being stored even after the regulator has stopped supplying field current. So if P is quite big and A is suddenly zero, V is huge. The energy (or power, if you like) cannot be destroyed it has to go somewhere.

 

Another way of looking at it (and in fact the reality is a combination of these things) is that the property of inductance can be considered to be a resistance to a change of current flow. The formula relating voltage, current, inductance and time is V= L dI/dt ie voltage across and inductance equals the inductance times the rate of change of current. So with a rapid rate of change of current (fuse blows) the voltage across an inductance will be very high. Again it is due to the energy stored in a magnetic field having to go somewhere.

 

In fact an alternator is probably not that inductive overall and the effect is I suspect mostly due to the inability of the regulator to shutdown power generation instantly.

[RLWP]

In fact an alternator is probably not that inductive overall and the effect is I suspect mostly due to the inability of the regulator to shutdown power generation instantly.

 

Would that be changed by where the field is fed from?

 

I'm sort of happy with the 'inertia' of the magnetic field

 

Richard

[dmr]

Nick, do big modern alternators fail on an open circuit?. I think some have zener type diodes as rectifiers and I have assumed that this is to handle the open circuit failure.

Am not prepared to do any experiments on my own alternator though.

 

...............Dave

[nicknorman]

Would that be changed by where the field is fed from?

 

Richard

Sorry I don't really understand the question.

 

But whilst I'm on its worth mentioning that a regulator is normally not an analogue device in that in itself it doesn't supply a variable current via for example a partially-on transistor, since that would dissipate lots of heat. Rather it is a switching circuit that applies the system voltage to the field, or not. Ie it pulses on and off at a variable mark:space ratio. The circuit includes a freewheel diode such that when the regulator disconnects, current continues to flow through the field coil via the diode (decaying, of course) and when the regulator switches back on, due to the inductance of the field coil the current increases (relatively) slowly according to dI/dt = V/L. (Not taking into account the resistance of the field coil). The net result of the switching regulator through the inductance is a fairly steady current, thank to the property of inductance.

 

But the downside is that when the regulator shuts off due to the fuse blowing, the field current only decays relatively slowly due to the freewheel diode.

 

It is like turning the tap off when the water supply is a constant displacement pump. The pressure increases wildly until something blows!

Nick, do big modern alternators fail on an open circuit?. I think some have zener type diodes as rectifiers and I have assumed that this is to handle the open circuit failure.

Am not prepared to do any experiments on my own alternator though.

 

...............Dave

Good question, you would think it might be sensible but typically the perceived wisdom is not to disconnect an alternator when its running. As a design point, why would you add cost to cater for inappropriate usage when you can just blame the customer and sell the customer a new alternator! I think only Snibbs can answer your question.

[RLWP]

Being and old fashioned soul, I'm used to a separate wire being supplied to the alternator with the 'charge' light in it. This supplies the field current and a reference voltage (as I understand it). Chopping the output and leaving the field in place 'ought' to be OK

 

Richard

[nicknorman]

Being and old fashioned soul, I'm used to a separate wire being supplied to the alternator with the 'charge' light in it. This supplies the field current and a reference voltage (as I understand it). Chopping the output and leaving the field in place 'ought' to be OK

 

Richard

The old fashioned (and not so old fashioned) seperate wire to the charge light just supplies a bit of field current to get things started. Typically the field current and reference voltage is supplied via field diodes once it's all burning and turning, although there are variations such as remote voltage sensing. So for example switching off the ignition and thus disconnecting the warning light from the supply with it running won't cause a problem or stop charging. But disconnecting the alternator's output, will cause a problem.