Alternator - fuse or not.

[Graham_Wright]

With the alternator connected directly (or nearly directly - via starter motor connection) to the battery, there is little to go wrong.
However, if there is an ammeter in the circuit, there is a risk of shorts at the ammeter connection. Typically they have a metal strap, embracing (and insulated from) the contacts, which secures the ammeter to what might be a metal panel. The cable is probably sized for around 60A. A short at the ammeter would draw an excessive current from the battery and perhaps result in a melted cable and/or a fire.

Introducing a (say) 60A fuse at the battery end would protect against this but would introduce a risk of blowing the alternator diodes if ruptured during charging.

Arguably this would be preferable to setting fire to the boat.

Views?

 

[lw395]

Get an ammeter with a remote shunt and fuse the small wires to the meter.
If you must have a panel mount ammeter that belongs in a morris minor or 70s rally car, then you can fuse it, at risk of destroying the alternator if the fuse blows. In the old days, the ammeter would be in the battery lead, so if the fuse went, the alternator would still have some load from the car's lights, heater, ignition etc.
Other than that, take proper measures to ensure it can't get shorted to ground. Doubleinsulate terminals, protect the wires, insulate other metal things, especially any that are ground.

 

[Graham_Wright]

Get an ammeter with a remote shunt and fuse the small wires to the meter.
If you must have a panel mount ammeter that belongs in a morris minor or 70s rally car, then you can fuse it, at risk of destroying the alternator if the fuse blows. In the old days, the ammeter would be in the battery lead, so if the fuse went, the alternator would still have some load from the car's lights, heater, ignition etc.
Other than that, take proper measures to ensure it can't get shorted to ground. Doubleinsulate terminals, protect the wires, insulate other metal things, especially any
that are ground.

I agree with the rational but I would like the ammeter to match the other instruments on cosmetic grounds.
I have turned a plastic shroud to replace the metal strap so the risk has reduced to a short between the body and the terminals (which are double insulated).

It is an idea to have a light load on the alternator perhaps at the alternator itself to protect the diodes in the event the fuse blows. I like that. EDIT. No I don't! it would draining all the time from the battery!

I have another alternator which produces 130 amps for the domestic batteries and that will be shunt connected. However, the shunts will be the cables themselves using milliammeters. This technique I've used for monitoring the glow plug circuit.

Thanks for the idea.

 

[Deleted member 36384]

... I would like the ammeter to match the other instruments on cosmetic grounds. ...

Much the same as me but I approached it differently, on an older boat with original equipment. My boat has the ammeter disconnected and a shunt type digital meter on the electrical panel. The engine panel currently has the old ammeter still in place but disconnected, hence aesthetically all gauges are the same style with no holes in the engine panel. My plan is to fit either an exhaust temperature gauge or fuel filter vacuum gauge into the engine panel where the ammeter currently sits. The engine panel does have "Charging Current" engraved but nothing that a new label can't sort out. Lots of vintage gauge styles for Smiths and VDO on eBAY e.g. vacuum, temperature that could be used to fill space and repurposed.

Just an idea, probably safer, also better technology for current monitoring is available on the shunt systems.

 

[BabaYaga]

With the alternator connected directly (or nearly directly - via starter motor connection) to the battery, there is little to go wrong.
However, if there is an ammeter in the circuit, there is a risk of shorts at the ammeter connection.

I would argue there are good reasons to fuse such a charging cable, regardless of ammeter.
If the cable terminal would come off the alternator somehow there is a great risk it could land on the engine block.

 

[pvb]

With the alternator connected directly (or nearly directly - via starter motor connection) to the battery, there is little to go wrong.
However, if there is an ammeter in the circuit, there is a risk of shorts at the ammeter connection. Typically they have a metal strap, embracing (and insulated from) the contacts, which secures the ammeter to what might be a metal panel. The cable is probably sized for around 60A. A short at the ammeter would draw an excessive current from the battery and perhaps result in a melted cable and/or a fire.

Introducing a (say) 60A fuse at the battery end would protect against this but would introduce a risk of blowing the alternator diodes if ruptured during charging.

Arguably this would be preferable to setting fire to the boat.

Views?

Fuses are supposed to stop fires, by disconnecting the cable if current becomes too high for too long. In the scenario you describe, a fuse would be a worthwhile addition; in the highly unlikely event of the ammeter shorting, a fuse would stop a fire. The alternator might still be damaged, but it would have been anyway.

lw395's advice to get an ammeter which has a remote shunt is good, except he then suggested you need a fuse in the small wires to the meter, which is totally wrong as they only handle a few millivolts.

Even with a remote shunt ammeter, I'd still suggest fusing the main cable. Some years ago, I replaced an alternator and foolishly got the connections muddled. There was a gentle "pffft" sound as the MegaFuse I'd installed blew. I was grateful it worked so well.

 

[Graham_Wright]

Get an ammeter with a remote shunt and fuse the small wires to the meter.
If you must have a panel mount ammeter that belongs in a morris minor or 70s rally car, then you can fuse it, at risk of destroying the alternator if the fuse blows. In the old days, the ammeter would be in the battery lead, so if the fuse went, the alternator would still have some load from the car's lights, heater, ignition etc.
Other than that, take proper measures to ensure it can't get shorted to ground. Doubleinsulate terminals, protect the wires, insulate other metal things, especially any that are ground.

But only if they happened to be switched on.

 

[lw395]

...
lw395's advice to get an ammeter which has a remote shunt is good, except he then suggested you need a fuse in the small wires to the meter, which is totally wrong as they only handle a few millivolts.

.....

If the shunt is in the +12V, the small wires want fusing.
If the shunt is in the ground side, you might convince your self that they are protected by everything else being fused....

 

[lw395]

But only if they happened to be switched on.

Indeed.
This may be why very few cars before about 2005 have fuses in the alternator-battery line.
One might note that very few cars have ammeters either, but many do these days, it's just that they report to the ecu not the driver!

 

[Lon nan Gruagach]

I agree with the rational but I would like the ammeter to match the other instruments on cosmetic grounds.
I have turned a plastic shroud to replace the metal strap so the risk has reduced to a short between the body and the terminals (which are double insulated).

It is an idea to have a light load on the alternator perhaps at the alternator itself to protect the diodes in the event the fuse blows. I like that. EDIT. No I don't! it would draining all the time from the battery!

I have another alternator which produces 130 amps for the domestic batteries and that will be shunt connected. However, the shunts will be the cables themselves using milliammeters. This technique I've used for monitoring the glow plug circuit.

Thanks for the idea.

Open the meter, take the shunt out, wire the shunt in the alternator output cable.
Run small wires to the remnants of the meter and recalibrate it. Might need to shave the shunt's width a bit to up the volts to allow for extra length from shunt to meter
Do fuse the teensy weensy wires that are now connected directly to alternator output, they dont handle 60A too well

 

[Graham_Wright]

Open the meter, take the shunt out, wire the shunt in the alternator output cable.
Run small wires to the remnants of the meter and recalibrate it. Might need to shave the shunt's width a bit to up the volts to allow for extra length from shunt to meter
Do fuse the teensy weensy wires that are now connected directly to alternator output, they dont handle 60A too well

What shunt?!

Tis a car ammeter (Morris Minor style!).

I have found, with a sufficiently sensitive ammeter, you can use the connecting wires that need to be monitored as the shunt. E.g. my glow plug leads are connected from the source and last glow plug to a milliammeter. Fsd reading = 4 glow plugs drawing current.

 

[Lon nan Gruagach]

What shunt?!

Tis a car ammeter (Morris Minor style!).

I have found, with a sufficiently sensitive ammeter, you can use the connecting wires that need to be monitored as the shunt. E.g. my glow plug leads are connected from the source and last glow plug to a milliammeter. Fsd reading = 4 glow plugs drawing current.

Am I right in assuming that the alternator current flows through the ammeter?
If so then there is a shunt inside, the coil windings arent going to be 60A, there will be a coil of fine wire (almost certainly wound to produce FSD at 1mA) and a stonking thick resistor in parallel, looking like a bus bar across the terminals... called a shunt.

Virtually all analogue meters have a 1mA FSD movement as their core with appropriate parallel or series resistors to end up with the desired metering.

 

[Graham_Wright]

Am I right in assuming that the alternator current flows through the ammeter?
If so then there is a shunt inside, the coil windings arent going to be 60A, there will be a coil of fine wire (almost certainly wound to produce FSD at 1mA) and a stonking thick resistor in parallel, looking like a bus bar across the terminals... called a shunt.

Virtually all analogue meters have a 1mA FSD movement as their core with appropriate parallel or series resistors to end up with the desired metering.

I'll have a look but I strongly doubt it! I used to work for Smiths' Industries. If I'm wrong, I'll be delighted.

 

[Graham_Wright]

I'll have a look but I strongly doubt it! I used to work for Smiths' Industries. If I'm wrong, I'll be delighted.

I'm delighted. I was wrong, you were right and the future installation will be easier.

I might have been thinking of thermal meters but they have a very slow reaction time.

May I plead old age?

 

[Lon nan Gruagach]

I'm delighted. I was wrong, you were right and the future installation will be easier.

I might have been thinking of thermal meters but they have a very slow reaction time.

May I plead old age?

No, you may not plead Old Age.
I was beginning to expect an education from you after seeing this:

 

[matthewriches]

Proper job lol

No, you may not plead Old Age.
I was beginning to expect an education from you after seeing this:

 

[lw395]

No, you may not plead Old Age.
I was beginning to expect an education from you after seeing this:

The way it's being held suggests putting a bit of spin on it as it's hurled towards the skip.

 

[VicS]

This car ammeter has no electrical connections at all Therefore no risk of shorts to a metal panel

 

[Graham_Wright]

Presumably inductive but with DC? DC current meters tend to be clever. Further info please.

 

[Graham_Wright]

No, you may not plead Old Age.
I was beginning to expect an education from you after seeing this:

Yes - that rings a bell. But the one I opened definitely had a shunt and a moving coil.
I shall do a test on volts drop across the charging cable but I would expect to able to connect between the ends (with a lightly fused cable) and tune with a shunt across the meter.