Alternator wiring to starter motor via an ammeter?

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fergycool

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I have a question about alternator wiring that I think I know but I need to get proper confirmation please!

My alternator has just started playing up. The excitation light will not go out (even when I rev the engine) and the battery will not charge. From all the tests I’ve done I think it’s the alternator (or regulator) that’s gone. The alternator has always proved a little “weak”. Previously it would never charge at more than 13.2v (the other engine that also does the domestic battery charges both banks at 14.2v).

However, whilst investigating this I found that the alternator has the field output wire (I would say undersized as it’s about 12mm2) direct to the dashboard for a amp meter. That’s a distance of about 3 - 4 metres. This then goes all the way back to the starter motor positive terminal, another 4 metres.

With my naive thinking that suggests two weaknesses (not the cause of the alternator issue though). Firstly the wire is undersized. I do not know the output of the alternator and I cannot identify it. I think it’s the original on the engine which is a Perkins 4.270 (1961). The attached voltage regulator is a Lucas 21TR 12v. There are no identifying numbers on the alternator itself (that I can see).

Secondly all the way to the dashboard and back again just to measure the current seems excessive and a bad design to me.

I guess I’m going to have to replace the alternator and so at the same time I’d like to put some proper wiring in place. As I say I do not know the output of the alternator but 100A replacements seem reasonably priced.

I’m thinking of using these existing wires as a shunt for a new ammeter which will be placed in a new cable (25mm2) from the field connector to the starter positive terminal (only about a metre in length).

Does that all sound sensible? Please be as critical as you like as I'm a newbie boat owner!

Thanks a bundle.

Cheers
Ferg
 
fergycool said:
The alternator has always proved a little “weak”. Previously it would never charge at more than 13.2v (the other engine that also does the domestic battery charges both banks at 14.2v).

However, whilst investigating this I found that the alternator has the field output wire (I would say undersized as it’s about 12mm2) direct to the dashboard for a amp meter. That’s a distance of about 3 - 4 metres. This then goes all the way back to the starter motor positive terminal, another 4 metres.
Click to expand...

The long run of undersized cable would have caused a voltage drop, so it's not surprising you only got 13.2v charging. The alternator wasn't "weak", the wiring was poor. If the alternator now has a fault, you should remove it and get it checked at an auto-electrics place.

As for an ammeter, do you need one? Is there one for the second engine? A better solution would be to get a cheap(ish) battery monitor (Nasa BM1, etc) and install it on the domestic battery bank - it will then monitor the total current into and out of the batteries, as well as giving you a voltage reading. You can't use the existing ammeter wiring for a shunt-based ammeter, as the wires from the shunt need to be a twisted pair.
 
fergycool said:
I have a question about alternator wiring that I think I know but I need to get proper confirmation please!

My alternator has just started playing up. The excitation light will not go out (even when I rev the engine) and the battery will not charge. From all the tests I’ve done I think it’s the alternator (or regulator) that’s gone. The alternator has always proved a little “weak”. Previously it would never charge at more than 13.2v (the other engine that also does the domestic battery charges both banks at 14.2v).

However, whilst investigating this I found that the alternator has the field output wire (I would say undersized as it’s about 12mm2) direct to the dashboard for a amp meter. That’s a distance of about 3 - 4 metres. This then goes all the way back to the starter motor positive terminal, another 4 metres.

With my naive thinking that suggests two weaknesses (not the cause of the alternator issue though). Firstly the wire is undersized. I do not know the output of the alternator and I cannot identify it. I think it’s the original on the engine which is a Perkins 4.270 (1961). The attached voltage regulator is a Lucas 21TR 12v. There are no identifying numbers on the alternator itself (that I can see).

Secondly all the way to the dashboard and back again just to measure the current seems excessive and a bad design to me.

I guess I’m going to have to replace the alternator and so at the same time I’d like to put some proper wiring in place. As I say I do not know the output of the alternator but 100A replacements seem reasonably priced.

I’m thinking of using these existing wires as a shunt for a new ammeter which will be placed in a new cable (25mm2) from the field connector to the starter positive terminal (only about a metre in length).

Does that all sound sensible? Please be as critical as you like as I'm a newbie boat owner!

Thanks a bundle.

Cheers
Ferg
Click to expand...

You mean I guess the stator output wire where you say "field" (The field winding is on the rotor fed from the regulator via the brushes)
Yes you need a shunt in the direct connection from alternator to starter motor. Light wires (fused)can then be used to connect the meter movement or display to the shunt
 
fergycool said:
I’m thinking of using these existing wires as a shunt for a new ammeter which will be placed in a new cable (25mm2) from the field connector to the starter positive terminal (only about a metre in length).
Click to expand...

Also be aware that there is no requirement to connect the alternator output to the starter motor. Just a way to save some copper wire.
 
pvb said:
The long run of undersized cable would have caused a voltage drop, so it's not surprising you only got 13.2v charging. The alternator wasn't "weak", the wiring was poor. If the alternator now has a fault, you should remove it and get it checked at an auto-electrics place.

As for an ammeter, do you need one? Is there one for the second engine? A better solution would be to get a cheap(ish) battery monitor (Nasa BM1, etc) and install it on the domestic battery bank - it will then monitor the total current into and out of the batteries, as well as giving you a voltage reading. You can't use the existing ammeter wiring for a shunt-based ammeter, as the wires from the shunt need to be a twisted pair.
Click to expand...

Thanks for your answer. You are right about the wiring. I've always considered that one to be lower charging but now I know why! I have some cheap voltmeters on each battery (2 x starter and the domestic). So in the short term, once the alternator is either replaced or fixed, I could get rid of the ammeter entirely until I got a battery monitor.

Cheers
Ferg
 
VicS said:
You mean I guess the stator output wire where you say "field" (The field winding is on the rotor fed from the regulator via the brushes)
Yes you need a shunt in the direct connection from alternator to starter motor. Light wires (fused)can then be used to connect the meter movement or display to the shunt
Click to expand...

Sorry about my poor terminology! I'm slowly learning this stuff!
 
BabaYaga said:
Also be aware that there is no requirement to connect the alternator output to the starter motor. Just a way to save some copper wire.
Click to expand...

I assume you mean instead of connecting directly to the +VE of the battery? Connecting to the starter motor terminal would make one less cable on the battery post.
 
fergycool said:
I assume you mean instead of connecting directly to the +VE of the battery? Connecting to the starter motor terminal would make one less cable on the battery post.
Click to expand...

I only imagined it might be awkward to position a shunt in the short wire between the alternator and the starter motor terminal, the shunt might want to sit on a bulkhead? You could also take the charging wire to a breaker terminal, if that is more convenient.
 
nigelmercier said:
I think an ammeter in the starter battery circuit is a waste of time. It is either going to read a large negative number, or a small positive one.
Click to expand...

You know I've never actually used it. I only assumed that it was the naivety of my "beginner boating status" and all you professional boaters used them all the time ;-)

I guess I know when it's charging and when it's low from the level of the voltage showing. Nothing other than the starter motor and a bilge pump are connected to this particular battery.

I think I'm going to make life easier for myself and make two less crimps and just use a straight cable to the starter battery from the alternator!

Thank you!
 
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