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Hello Dear boating friends,

Well I am in the middle of a re-fit of my engine bay.  Part of that involves all new batteries / cables / isolators and shunt.  I decided to use bus bars this time to make things neater.  The previous setup also had a shunt on the negative side with ALL appliances on the load side of the shunt.  Meant it was nearly at the end of the threads and looked messy, although it did work quite well.

So now I have bus bars, that has introduced a complication which I just cannot get my head around.  

Realised wherever I put the shunt, either behind or in front of the neg bus bar there are scenarios where it won’t be recording the total number of amps of all systems. This breaks Peukert's equasion which of course depends on knowing complete amps in / amps out.

This article kind of explains it: https://www.coastalclimatecontrol.com/index.php/blog/139-battery-monitor-doesn-t-show-solar,-wind,-or-hydro-output.html and suggests two shunts.  I have the Victron 702 which is a single shunt solution.

If I put the shunt behind a bus bar straight after the battery and then the MPPT and Alternator could be feeding the inverter and circumventing the shunt. Fine if they are charging batts, shunt will read, but if they are feeding inverter then won’t go via shunt.

If you put the shunt after the bus bar, the you get the opposite problem. The MPPT or Alternator could be charging the batteries and this would circumvent the shunt. If the inverter were to be drawing the it would go via the shunt. Both situations totally negate the point of a shunt.

Any tips or thoughts very much valued

 

 

 

Engine Bay v1.8.jpg

Edited by Keajre
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27 minutes ago, Keajre said:

If I put the shunt behind a bus bar straight after the battery and then the MPPT and Alternator could be feeding the inverter and circumventing the shunt. Fine if they are charging batts, shunt will read, but if they are feeding inverter then won’t go via shunt.

Depends if you are just trying to monitor the batteries, or measure total power generated.

If your alternator powers your inverter, do you care?

Edited by TheBiscuits
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The way I have always done it is to connect the shunt to the negative of the batteries and then the bus bar so as you say you dont capture stuff from the MPPT to the 12V...but as buscuits says...does it matter. What is the shunt for? It is to help the battery monitor monitor the batteries. Therefore it is only interested in what goes in or what goes out - which is what you measure with the shunt on the negative.

Ok, it doesnt line up with your power audit as less goes in and less goes out if you have an MPPT or are connected to shore line (and inverter), but you live with that. In practice I look at amp hrs used overnight, ie actually what comes out of the batteries. I am not really that interested in what comes out of solar in the day to run a pump without it going into the batteries.

edit. therefore I wouldnt wire it up as per your diagram

Edited by Dr Bob
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Just now, TheBiscuits said:

Depends if you are just trying to monitor the batteries, or measure total power generated.

If your alternator powers your inverter, do you care?

My objective is ultimately monitor batteries.  The problem here is that the alternator (or MPPT) could be charging the batteries and the Shunt wont know.  So it will falsely report the number of amps drawn.  Unless I put the shunt after the batteries and before the shunt.  Then I suppose anything going into the batteries is recorded, and anything going out of the batteries is recorded. Just if the alternator or MPPT is powering the inverter, I wont see that . . .but I guess it doesn’t matter.  

Hmm good point, thanks Biscuits

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A shunt just measure current. So it depends on which current you want to measure. Most shunts are used as part of a battery monitor - to keep track of charge going in and out of the battery so you can know the state of charge, when to stop charging etc. In which case, put the shunt between the battery negative and all other connections. Yes this means that, for example, solar can directly power an inverter, but so what?!

Most solar controllers will tell you the output current if you want to know that.

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1 minute ago, Dr Bob said:

The way I have always done it is to connect the shunt to the negative of the batteries and then the bus bar so as you say you dont capture stuff from the MPPT to the 12V...but as buscuits says...does it matter. What is the shunt for? It is to help the battery monitor monitor the batteries. Therefore it is only interested in what goes in or what goes out - which is what you measure with the shunt on the negative.

Ok, it doesnt line up with your power audit as less goes in and less goes out if you have an MPPT or are connected to shore line (and inverter), but you live with that. In practice I look at amp hrs used overnight, ie actually what comes out of the batteries. I am not really that interested in what comes out of solar in the day to run a pump without it going into the batteries.

edit. therefore I wouldnt wire it up as per your diagram

Thanks, yes, case closed, thanks guys, just needed to air that one.  Doesn’t really matter about the bits I cant record, what’s important is amps in and amps out of the batts.  Cheers

2 minutes ago, nicknorman said:

A shunt just measure current. So it depends on which current you want to measure. Most shunts are used as part of a battery monitor - to keep track of charge going in and out of the battery so you can know the state of charge, when to stop charging etc. In which case, put the shunt between the battery negative and all other connections. Yes this means that, for example, solar can directly power an inverter, but so what?!

Most solar controllers will tell you the output current if you want to know that.

Yup, I agree, I see the point now, everyone makes same point and the penny has dropped :-). Many thanks 

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1 minute ago, Richard10002 said:

Surely any current from the MPPT or alternator goes along the negative cable to the busbar, through the shunt, into the batteries, then out of the positive side of the batteries to power stuff.... thus the shunt and the monitor sees everything.

Yes it will. BUT, if the alternator (or MPPT) is running and putting amps in, AND the inverter is on (or 12v loads in the boat) then the MPPT or Alternator will actually power the inverter directly, no via shunt.  Which . . . Is ok

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4 minutes ago, Richard10002 said:

Surely any current from the MPPT or alternator goes along the negative cable to the busbar, through the shunt, into the batteries, then out of the positive side of the batteries to power stuff.... thus the shunt and the monitor sees everything.

No.

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Here is a corrected diagram, with shunt on battery side ‘before’ the shunt.    Question, do I really need a fuse on the Neg cable going from the Bus Bar to the inverter?  I was going to put a 400A fuse there.  But I wondered if it will pronounce voltage drop and likely the inverter (Victron Multi-Plus) has its own internal fuses.

Engine Bay v1.9.jpg

1 minute ago, nicknorman said:

No.

A much more succinct reply, yes the answer is no

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1 minute ago, Keajre said:

Here is a corrected diagram, with shunt on battery side ‘before’ the shunt.    Question, do I really need a fuse on the Neg cable going from the Bus Bar to the inverter?  I was going to put a 400A fuse there.  But I wondered if it will pronounce voltage drop and likely the inverter (Victron Multi-Plus) has its own internal fuses.

Engine Bay v1.9.jpg

Shunt before shunt, or, shunt before busbar ?

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You say neg cable, not sure if you mean that? Normally one would put a fuse in the positive cable going to the inverter, near the isolator. This is to protect against cable damage or disconnection that could see a very fat 12v wire shorting to hull, a fairly catastrophic situation. The fuse in the inverter only protects the inverter, not the cable.

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3 minutes ago, nicknorman said:

You say neg cable, not sure if you mean that? Normally one would put a fuse in the positive cable going to the inverter, near the isolator. This is to protect against cable damage or disconnection that could see a very fat 12v wire shorting to hull, a fairly catastrophic situation. The fuse in the inverter only protects the inverter, not the cable.

Yes I meant positive thanks for correction.  So a good practise which is recommended?  Will it cause a significant voltage drop?  I have a 400A fuse, but if you look at the surface area of the fuse (albeit its large) to the surface area of the 70mm2 cable, much smaller.  Surely it would cause voltage drop, the very thing I want to avoid.

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Is that a realistic possibility?  So thinking it through, you are protecting against the positive cable (say the 70mm2 cable) touching the hull which is the earth also.  So then hundreds of amps would turn the cable into a bar heater and then possibly a fire depending on what it came into contact with.  Hmm actually as I type it out, maybe it is important.   The only time that could really happen would be if someone was doing work and forgot to turn off the isolator.  Seems unlikely, but then again if it happened, pretty bad.

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3 minutes ago, Keajre said:

Yes I meant positive thanks for correction.  So a good practise which is recommended?  Will it cause a significant voltage drop?  I have a 400A fuse, but if you look at the surface area of the fuse (albeit its large) to the surface area of the 70mm2 cable, much smaller.  Surely it would cause voltage drop, the very thing I want to avoid.

The fuse is thin, but only over a very small length. So on a per-length basis, the voltage drop is significant. But since the length is only perhaps 10mm, the actual voltage drop will be minuscule. I doubt you will be able to measure any voltage drop.

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Just now, nicknorman said:

The fuse is thin, but only over a very small length. So on a per-length basis, the voltage drop is significant. But since the length is only perhaps 10mm, the actual voltage drop will be minuscule. I doubt you will be able to measure any voltage drop.

Great answer, that explains it.  Thanks, i’ll fit the fuse, now that concern is allayed and thinking about the reason for needing it

 

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1 minute ago, Keajre said:

Is that a realistic possibility?  So thinking it through, you are protecting against the positive cable (say the 70mm2 cable) touching the hull which is the earth also.  So then hundreds of amps would turn the cable into a bar heater and then possibly a fire depending on what it came into contact with.  Hmm actually as I type it out, maybe it is important.   The only time that could really happen would be if someone was doing work and forgot to turn off the isolator.  Seems unlikely, but then again if it happened, pretty bad.

Yes the probability is low, but the consequence very severe. Insulation on fire, glowing red or white hot wires, cable welded to the metal, thousands, not hundreds, of amps flowing.

It could happen due to someone doing some work, vibration loosening off connectors, abrasion damage from vibration, trauma such as a severe collision etc. All quite unlikely, but possible. A fuse doesn’t really have any significant down side.

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6 minutes ago, nicknorman said:

Yes the probability is low, but the consequence very severe. Insulation on fire, glowing red or white hot wires, cable welded to the metal, thousands, not hundreds, of amps flowing.

It could happen due to someone doing some work, vibration loosening off connectors, abrasion damage from vibration, trauma such as a severe collision etc. All quite unlikely, but possible. A fuse doesn’t really have any significant down side.

Hmm yes I see.

 

Should there be two fuses, one from each off the two positives that go into the isolator / switch, or just one on the output of the isolator / switch.  Just that if only one of the output of the isolator, if either of the two positive cable connections to the isolator came loose and touched hull, it would cause the same issue. Or is that overkill?

Edited by Keajre
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3 minutes ago, Keajre said:

Great answer, that explains it.  Thanks, i’ll fit the fuse, now that concern is allayed and thinking about the reason for needing it

 

Another way of thinking about it is power dissipation. So let’s say that at 400A, the fuse dropped 0.1v. Doesn’t sound too bad but that is 40 Watts. 40 Watts being pumped into a tiny bit of metal (the business part of the fuse) will make it very hot. Melting hot! So in reality the voltage drop at 400A will be much less than 0.1v, more like 0.02v (I’m just guessing numbers here) and at a more reasonable and normal inverter load, probably below the measurement threshold of a normal digital voltmeter.

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6 minutes ago, Keajre said:

Great answer, that explains it.  Thanks, i’ll fit the fuse, now that concern is allayed and thinking about the reason for needing it

 

I notice that your wiring diagram does not show a Galvanic Isolator or Isolation transformer ?  I trust you will be fitting one or the other to your installation.

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Like this 

Engine Bay v2.0.jpg

1 minute ago, Flyboy said:

I notice that your wiring diagram does not show a Galvanic Isolator or Isolation transformer ?  I trust you will be fitting one or the other to your installation.

Yes I have a galvanic, thanks for noticing though,its fittings to the earth of the 240 shore line, this is 12v system only

3 minutes ago, nicknorman said:

Another way of thinking about it is power dissipation. So let’s say that at 400A, the fuse dropped 0.1v. Doesn’t sound too bad but that is 40 Watts. 40 Watts being pumped into a tiny bit of metal (the business part of the fuse) will make it very hot. Melting hot! So in reality the voltage drop at 400A will be much less than 0.1v, more like 0.02v (I’m just guessing numbers here) and at a more reasonable and normal inverter load, probably below the measurement threshold of a normal digital voltmeter.

Right, I see, good explanation, makes sense

Edited by Keajre
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