Fuse board to Engine Ground?

[xeitosaphil]

Having just got around to attempting to fit my digital ammeter, which has been waiting for the end of the sailing season to arrive, and fitting the shunt to the 6mm2 - 10awg negative from the fuse panel to the battery in an attempt to monitor discharge rates of the domestic supplies, I have just discovered that this negative does not go to the domestic battery as I thought, but instead goes direct to the engine ground bolt?
I have already installed the shunt in the Negative wire between the fuse panel and the engine before I found this out.
Would this mean that the starting current could affect the ammeter by the fact that the ground is common to both domestic /starting batteries and the fuse panel? Also the starter motor has an earth strap which is fitted direct to the engine, so would this be part of a return path back to the Start battery?
At present the Negative from the domestic battery goes to a large Negative Distribution stud then to the engine ground bolt. The starting battery negative goes straight to the engine ground bolt.
If I was to take the fuse panel negative off the engine, and extend it to the domestic battery would this isolate the starting current from the Ammeter Gauge?
The digital /ammeter is only rated at 50amps, so wouldn't take starting current?

Needless to say I have not powered this up yet as I am concerned since I found this out?

 

[David2452]

I think you have answered the question in your post, if the small 6mm2 cable (quite small for a main distribution panel BTW) was already taking the starter current you would already be in trouble.

 

[VicS]

If i understand you correctly the stater current will not pass through the shunt. It will pass through the direct connection between the engine and the battery.
As Davidnnnn says above above the wire you have fitted the shunt in is too small to carry the starter current anyway.

Your ammeter will not measure is any other current to and from the engine. That will almost ceratinly include the charging current from the alternator which is presumably earthed to the engine block and then directly back to the battery.

It will just monitor the current used by the equipment fed from the panel ... all you apparently wanted to monitor anyay

 

[xeitosaphil]

I think you have answered the question in your post, if the small 6mm2 cable (quite small for a main distribution panel BTW) was already taking the starter current you would already be in trouble.

David thank you, I assume you mean that if I haven't got a problem now, I won't have one in the future so I don't need to move the wire off the engine then?

Just as a matter of interest, I only have electronics and lighting curcuits + cold water pump fed from the dis/ brd , biggest draw being the Auto pilot and VHF, so I thought the original supply wires would be suitable, but do you think I should upgrade the cable size then?
What would be the norm -16mm, or bigger?

 

[xeitosaphil]

If i understand you correctly the stater current will not pass through the shunt. It will pass through the direct connection between the engine and the battery.
As Davidnnnn says above above the wire you have fitted the shunt in is too small to carry the starter current anyway.

Your ammeter will not measure is any other current to and from the engine. That will almost ceratinly include the charging current from the alternator which is presumably earthed to the engine block and then directly back to the battery.

It will just monitor the current used by the equipment fed from the panel ... all you apparently wanted to monitor anyay

Vic I was only thinking of monitoring the discharge from the Domestic battery bank with this ammeter, but if I wanted to monitor the alternator charge rate as well where should I put the shunt? the actual alternator charge current is sent via the starter motor feed to the 1-2-both switch where it is down to me to decide which bank to charge.

 

[David2452]

David thank you, I assume you mean that if I haven't got a problem now, I won't have one in the future so I don't need to move the wire off the engine then?

Just as a matter of interest, I only have electronics and lighting curcuits + cold water pump fed from the dis/ brd , biggest draw being the Auto pilot and VHF, so I thought the original supply wires would be suitable, but do you think I should upgrade the cable size then?
What would be the norm -16mm, or bigger?

Yes indeed, it would have already have shown a promlem shoving starter currents down 6mm2 cable.

I would upgrade it to 16mm2 to avoid voltage drop were it my choice, but I don't know what the total draw is so can not be precise, it would of course be both feed and return cables not just the return.

 

[xeitosaphil]

Yes indeed, it would have already have shown a promlem shoving starter currents down 6mm2 cable.

I would upgrade it to 16mm2 to avoid voltage drop were it my choice, but I don't know what the total draw is so can not be precise, it would of course be both feed and return cables not just the return.

David I see vicS is now off line, is there any chance you could look at the question I asked him ( post 5 ) I would be gratefull for any comments ?

 

[VicS]

Vic I was only thinking of monitoring the discharge from the Domestic battery bank with this ammeter, but if I wanted to monitor the alternator charge rate as well where should I put the shunt? the actual alternator charge current is sent via the starter motor feed to the 1-2-both switch where it is down to me to decide which bank to charge.

I'm back again ( actually I am invisible so you should not see me in the list of currently active users even when I am)

Not quite so straight forward to measure the charge current.
If you had the ammeter shunt in the positive side you could reroute the alternator output to pass through it rather than along the main positive cable between battery and starter

But maybe you ammeter cannot be wired in the positive side ?????


If you had dedicated starter and domestic batteries you could wire the shunt in like it is done for the NASA BM1 & BM2 battery monitors.

See page 3 of the instructions at http://www.nasamarine.com/images/file/BM1+-BM2+.pdf

The snag with a 1,2,both,off battery selector switch would be if using the domestic battery for starting the starter current would go through the shunt.

EDIT Just realised that the NASA wiring diagram shows the alternator output going to the domestic battery with no means shown of charging the starter battery !!

 

[xeitosaphil]

I'm back again ( actually I am invisible so you should not see me in the list of currently active users even when I am)

Not quite so straight forward to measure the charge current.
If you had the ammeter shunt in the positive side you could reroute the alternator output to pass through it rather than along the main positive cable between battery and starter

But maybe you ammeter cannot be wired in the positive side ?????


If you had dedicated starter and domestic batteries you could wire the shunt in like it is done for the NASA BM1 & BM2 battery monitors.

See page 3 of the instructions at http://www.nasamarine.com/images/file/BM1+-BM2+.pdf

The snag with a 1,2,both,off battery selector switch would be if using the domestic battery for starting the starter current would go through the shunt.

EDIT Just realised that the NASA wiring diagram shows the alternator output going to the domestic battery with no means shown of charging the starter battery !!

Quote - But maybe you ammeter cannot be wired in the positive side ?????

Yes Vic this is correct.



Quote - The snag with a 1,2,both,off battery selector switch would be if using the domestic battery for starting the starter current would go through the shunt.[/QUOTE]

Does this mean then that when I have started the engine with the domestic battery bank in the past, that the starting current has gone through the existing 6mm2 Negative distribution fuse panel cable? ( have only done it once to my knowledge but with no visible effect?)

 

[VicS]

Does this mean then that when I have started the engine with the domestic battery bank in the past, that the starting current has gone through the existing 6mm2 Negative distribution fuse panel cable? ( have only done it once to my knowledge but with no visible effect?)

No the return path would be via the engine ground bolt and what you describe as a "large negative distribution stud" to the battery negative.
All these negative cables should be heavy battery cable.

All that should flow through the negative cable from the fuse panel is the return currents from the items supplied by the fuses.

6 mm should be large enough to carry that load safely if its no more than about 60 amps .
If the cable is long though the voltage drop might be of some concern at the higher currents


I am going off line now!

 

[vyv_cox]

My ammeter is wired exactly as you have described. Connection has taken place in stages, so I have a fairly good knowledge of how it all happened. Initially I wired it in as you have, which meant that it showed all the discharges but did not show charge from the ammeter, although it did show charge from the solar panels. My ammeter has an earth wire that originally went to the engine block (Yanmar 3GM30F) When I diverted this to also pass through the shunt I was able to measure charge as well. However, on my other boat the alternator earths via the casing to the block and I am unable to measure charge.

Your heavy starter cables and earths are wired in parallel to the domestic circuit. The earth in this case does not pass through the shunt.

 

[fisherman]

Not to do with your question, but should your starter motor neg be strapped to the engine block? Mine is insulated return. It may have been altered, particularly if the senders are single pole, I expect Vic S will know.

 

[xeitosaphil]

No the return path would be via the engine ground bolt and what you describe as a "large negative distribution stud" to the battery negative.
All these negative cables should be heavy battery cable.

All that should flow through the negative cable from the fuse panel is the return currents from the items supplied by the fuses.

6 mm should be large enough to carry that load safely if its no more than about 60 amps .
If the cable is long though the voltage drop might be of some concern at the higher currents


I am going off line now!

Sorry Vic if I am being a bit daft here, but I am a little confused ?

In reply (9) you say "The snag with a 1,2,both,off battery selector switch would be if using the domestic battery for starting the starter current would go through the shunt".


And in reply (10 ) you say " No the return path would be via the engine ground bolt and what you describe as a "large negative distribution stud" to the battery negative".

i.e. missing the shunt?


Perhaps its because it was late and we should both be in bed, but just need to know which it is, so I can get it right and I don't fry the gauge?

 

[vyv_cox]

In reply (9) you say "The snag with a 1,2,both,off battery selector switch would be if using the domestic battery for starting the starter current would go through the shunt".

My installation uses a 1, 2, both, off selector switch and I almost always start the engine using the domestic batteries. The output terminal at the switch has both the heavy lead to the starter motor and the main wire to the switch panel connected to it. The return from the shunt goes to the engine block, which in turn is connected to the negative of the batteries. The two systems are therefore connected in parallel and the starter motor current cannot pass through the shunt.

 

[xeitosaphil]

My ammeter is wired exactly as you have described. Connection has taken place in stages, so I have a fairly good knowledge of how it all happened. Initially I wired it in as you have, which meant that it showed all the discharges but did not show charge from the ammeter, although it did show charge from the solar panels. My ammeter has an earth wire that originally went to the engine block (Yanmar 3GM30F) When I diverted this to also pass through the shunt I was able to measure charge as well. However, on my other boat the alternator earths via the casing to the block and I am unable to measure charge.

Your heavy starter cables and earths are wired in parallel to the domestic circuit. The earth in this case does not pass through the shunt.

Hi Vyv,
Thanks for your input, Just thought I would mention that my 2Gm20 is not isolated earth reurn,somebody must have changed it, as all the senders on the engine are single power feeds with the earth coming from the engine, as Fisherman describes in the next post from yours.I don't know if this makes any difference to your explanation? The negative feed to the ammeter is indeed supplied from the fuse board so will actually feed through the shunt fitted between the Dis / Brd and the engine ground.So may pick up the alternator charge, but my alternator earth is the same as you said in your last sentance, earthed to the block so it may not measure the charge?

I think the only thing to do is give it a try and see what happends?

Will post the results when I have tried it

 

[VicS]

Sorry Vic if I am being a bit daft here, but I am a little confused ?

In reply (9) you say "The snag with a 1,2,both,off battery selector switch would be if using the domestic battery for starting the starter current would go through the shunt".


And in reply (10 ) you say " No the return path would be via the engine ground bolt and what you describe as a "large negative distribution stud" to the battery negative".

i.e. missing the shunt?


Perhaps its because it was late and we should both be in bed, but just need to know which it is, so I can get it right and I don't fry the gauge?

Sorry for the confusion.

In #8 ( and 9) I was suggesting a snag if the shunt is wired like the BM1 shunt and the domestic battery was selected for engine starting.

In #10 I was refering to your fear that you may have put the starter current through the 6mm² negative connection to the fuse panel. You wont have done because the negative path is as I describe.

At least that is how I undersatnd it from your descriptions.

Diagrams called for really but have to go do the Sainsbury's shopping now

Back later.


Halcyon will be along before much longer and will advise you to use an ammeter with a "Hall effect" sensor instead of a shunt

 

[vyv_cox]

If the ammeter has an earth wire to the block you can divert it through the shunt. If there is no separate earth wire then there is probably nothing you can do to read the charging current. However, my workshop manual shows all models to have a wired earth.

 

[xeitosaphil]

Sorry for the confusion.

In #8 ( and 9) I was suggesting a snag if the shunt is wired like the BM1 shunt and the domestic battery was selected for engine starting.

In #10 I was refering to your fear that you may have put the starter current through the 6mm² negative connection to the fuse panel. You wont have done because the negative path is as I describe.

At least that is how I undersatnd it from your descriptions.

Diagrams called for really but have to go do the Sainsbury's shopping now

Back later.

Halcyon will be along before much longer and will advise you to use an ammeter with a "Hall effect" sensor instead of a shunt

No problem, now I know that the starting current is not going through the 6mm2, and not going to fry the gauge, I can breath easy again. "Phew" .

The monitoring I can play with? I'm going to the boat tommorrow and will report back when I know more.

Thanks everybody for your help so far, Philip

 

[xeitosaphil]

If the ammeter has an earth wire to the block you can divert it through the shunt. If there is no separate earth wire then there is probably nothing you can do to read the charging current. However, my workshop manual shows all models to have a wired earth.

Vyv,

The digital ammeter doesn't have a seperate earth wire, but it does have a live and negative feed which comes from the Fuse/ brd , and the negative return from the F/B goes through the shunt to the engine block so it might work?

I must add that its not a BM1

Its one of these

http://www.ebay.co.uk/itm/Mini-DC-5...tem27b3910e42&clk_rvr_id=428603880080&afsrc=1

 

[halcyon]

Halcyon will be along before much longer and will advise you to use an ammeter with a "Hall effect" sensor instead of a shunt

If people like to struggle who am I to butt in.

Note the ebay web site says it needs a separate power supply, so cannot us the same earth for meter and shunt. It needs a battery or isolated power supply to power the meter.

Brian