Alternator connection

pvb said:
I understand that you don't understand it, and I've tried to explain it, but obviously failed. Maybe someone else can convince you.
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I understand perfectly, you just don't see the problem if the shunt is bypassed.

Look at Vic's diagram. A grounded alternator adds a short from the starter battery negative, via the starter, via the engine, via the alternator, to the domestic battery negative.

[Later] This of course assumes that the starter battery negative is connected (as normal) to the shunted side of the domestic battery negative.
 
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Sarnia said:
I have been considering changing my alternator to a higher output, have discussed this in a previous post, today, back on the boat, I took it off and it is higher than i thought, is 65 amp, not 35 as anticipated, a Lucas NAB305, been on the boat over 10 years now.

The question i have is that when running the engine at 1500, the engine gauge shows 30 to 40 amps, whilst mt BM1 battery indicator only shows 10 to start then drops to 6 or 7, the feed from the alternator diaspears in a loom somewhere, I sus[ect to my BATT1, BATT2, BOTH switch.
Checking my BMI instructions it says all feeds must go to the shunt, as my solar panels do, is this the reason i am not getting an accurate reading, or am I?

I could of course with some work trace the alternator feed, any help or advice welcome , thank you.
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Have you thought of fitting a Sterling charging system to the alternator? Had one on my boat and the batteries were always charged up well.
 
nigelmercier said:
I understand perfectly, you just don't see the problem if the shunt is bypassed.

Look at Vic's diagram. A grounded alternator adds a short from the starter battery negative, via the starter, via the engine, via the alternator, to the domestic battery negative.
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Read post #37 again. The battery monitor measures the current going in to or out of the domestic battery. As long as the shunt is the only thing connected to the negative terminal of the domestic battery, it will measure accurately.
 
nigelmercier said:
I understand perfectly, you just don't see the problem if the shunt is bypassed.

Look at Vic's diagram. A grounded alternator adds a short from the starter battery negative, via the starter, via the engine, via the alternator, to the domestic battery negative.
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No . All the negative connections must be made to the negative busbar. ( perhaps the engine negative might be connected directly to the starter battery negative but that makes no difference to the arguments)

How the negative is connected to the engine will vary depending upon whether or not, and to what extent, the block is used as a negative return. There may be an earthing solenoid for the starter current or just an earthing relay for the glowplug current

Whatever the setup for the engine negative there should be no connection to the house battery negative apart from the shunt. All current to or from the house battery must flow through the shunt.
 
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pvb said:
... As long as the shunt is the only thing connected to the negative terminal of the domestic battery, it will measure accurately.
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Agreed.

Unless there is a second path as I describe in #42 above (revised), in which case the monitor will read lower than it should. Trust me, I've been there. Before I understood all this, I fitted a grounded alternator. It caused no end of problems.
 
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nigelmercier said:
Agreed.

Unless there is a second path as I describe in #42 above (revised), in which case the monitor will read lower than it should. Trust me, I've been there. Before I understood all this, I fitted a grounded alternator. It caused no end of problems.
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So what you're really saying is that, if the battery monitor's shunt is incorrectly wired, the battery monitor won't be accurate. Surely that's obvious to people?
 
nigelmercier said:
What I am saying is that a grounded alternator can cause the shunt to be bypassed, without the route being obvious.
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You obviously have something in the back of your mind but if there are no connections to the house battery, other than the shunt, nothing can by pass it. Thats the point of the warnings on my diagram.

I d have thought a grounded alternator was not uncommon.
 
VicS said:
... if there are no connections to the house battery, other than the shunt, nothing can by pass it. Thats the point of the warnings on my diagram...
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I agree that this is the most important point to make, I found various wires connected directly to the negative.

I now consider the shunt and the battery to be an indivisible unit, it makes following diagrams much easier.
 
VicS said:
Sorry not had the opportunity to look into this. It's changed since I last used the attachment method of posting pictures





Not sure what connections on the starter motor and solenoid you are looking at.

One expects to see a small terminal on the solenoid, which is the connection to the coil and two large connections which are the connections to the switched contacts.
There will be only a low resistance between the small terminal and the negative or ground.
One of the large terminals will be connected directly into the starter motor. There will be negligible resistance between this and ground.
The other large terminal will be the main battery positive connection. There should be no continuity between this and ground.

There may be a negative connection. If there is there may or may not be continuity to the starter motor body.

It would be helpful to know what the engine is. Make and model.


As in my diagram. The "all negative connections" includes the negative connection(s) to the engine
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My set up as you say, large feed to solenoid, large to starter motor, my alternator is eathed to my engine, it is a Perkins 4108
 
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VicS said:
No . All the negative connections must be made to the negative busbar. ( perhaps the engine negative might be connected directly to the starter battery negative but that makes no difference to the arguments)

How the negative is connected to the engine will vary depending upon whether or not, and to what extent, the block is used as a negative return. There may be an earthing solenoid for the starter current or just an earthing relay for the glowplug current

Whatever the setup for the engine negative there should be no connection to the house battery negative apart from the shunt. All current to or from the house battery must flow through the shunt.
Click to expand...

As my alternator feed goes to a small busbar that feeds the BATT1, 2 and OFF switch it goes through the shunt to the neg, the only thing connected to it
 
VicS said:
You obviously have something in the back of your mind but if there are no connections to the house battery, other than the shunt, nothing can by pass it. Thats the point of the warnings on my diagram.

I d have thought a grounded alternator was not uncommon.
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My suppliers of the new alternator, who have been in business some 40 years both marine and otherwise agree, grounded is common on boats.
 
Sarnia said:
The new Prestolite 90 amp will be grounded also as the existing one is now.
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Apologies, I may have got the wrong end of the stick. I was under the impression that you had replaced the alternator with a grounded one, and since then you were getting false readings, hence my overzealous insistence.

I think I got mixed up with a similar thread. Sorry again.
 
Sarnia said:
As my alternator feed goes to a small busbar that feeds the BATT1, 2 and OFF switch it goes through the shunt to the neg, the only thing connected to it
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So we come back to your original question as to why the engine ammeter and the BM1 battery monitor do not agree.

We seem to be fairly confident that the battery monitor is correctly wired to show the charge or discharge of the house battery.

What we do not know at present is how the positive circuits are wired to the selector switch and the batteries. How the domestics are fed or how the engine panel ammeter is connected.

Until we know all this we cannot begin to explain why the ammeter and battery monitor do not show the same reading.
 
nigelmercier said:
Apologies, I may have got the wrong end of the stick. I was under the impression that you had replaced the alternator with a grounded one, and since then you were getting false readings, hence my overzealous insistence.

I think I got mixed up with a similar thread. Sorry again.
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Happy to read that, no apologies required, not to me anyway, this has been a learning process for me, and , I am sure like many others here, look forward to your consistent informed contributions in many threads.
 
VicS said:
So we come back to your original question as to why the engine ammeter and the BM1 battery monitor do not agree.

We seem to be fairly confident that the battery monitor is correctly wired to show the charge or discharge of the house battery.

What we do not know at present is how the positive circuits are wired to the selector switch and the batteries. How the domestics are fed or how the engine panel ammeter is connected.

Until we know all this we cannot begin to explain why the ammeter and battery monitor do not show the same reading.
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I have checked and rechecked BM-! connections, they are as your diagram, and the NASA one supplied to me, NO connections are made to where the shunt meets the domestic negative on either side of it other than the white and black on the battery side, and the yellow on the side farthest from the battery.
Inputs from my solar panels are + to the battery + and - to the negative on the shunt, again farthest from the battery. I repeat NO connections where shown not to be.
As to the positive circuits to the selector switch, both BATT1 & 2 & BOTH are fed from a small busbar fed directly from the alternator output. Simple selection of the swiches send current either to BATT1, ie domestic, or BATT2 to starter motor, BOTH of course feeds both.
Similarly when selected they put whichever on line for supplies to boat services. I never use BATT2 for that, only for starting the engine, pretty regimented about that. I have on the odd occasion used BOTH to start but a very rare occurance.
As to the differences in the readings between engine panel ammeter and the BM-1 I would tend to believe the latter as the engine one has been on the panel for 10 years as I know of, and probably much longer with the PO. The only instrument I have changed on the panel is the temperature gauge which needed a new sensor so changed both. I have noticed, and this may be significant, when I start the engine just with BATT 2 as normal, the charge shown on the engine meter is very little, as the engine battery is pretty much always well charged, which also matches the BM-1 reading to, when I then select BATT1 and deselect BATT2 the engine meter reads much higher as does the BM-1, but the difference seems greater.
I think i have done all i can with wiring checks unless there is a further proposal, I expect anyday the new alternator, and when fitted will see the effect it has.
Would it be a good idea to make changes to the engine ammeter to correspond.
 
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