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Old 07-09-2015, 14:18   #16
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Re: Current Measurement?

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Originally Posted by DeepFrz View Post
Two shunts in series will only record the same value. Current is the same through each shunt so you don't need to add the values. If the two shunts are wired in parallel of course you need to add the values if you want the total.
Certainly not the same value - shunts convert current to a voltage value.

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Old 07-09-2015, 14:32   #17
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Re: Current Measurement?

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Certainly not the same value - shunts convert current to a voltage value.

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Deepfrz is correct. In dc series circuit current is constant.
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Old 07-09-2015, 14:39   #18
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Re: Current Measurement?

Dockhead,

I am having trouble with understanding all the consternation on this. Simply re-wire the bank so all neg side current is being taken from one single point.

I have had to do this far too many times to count when installing battery monitors. Yes there is some cost in big fat cable, or a neg busbar or such but other than that it is most often an hour or two of work.
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Old 07-09-2015, 19:16   #19
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Re: Current Measurement?

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Deepfrz is correct. In dc series circuit current is constant.

the discussion is on wiring a single meter in series to 2 different shunts in 2 different banks. the data wires in series therefor getting the addition of both shunts to a single amp reading, single gauge. not wiring the shunts themselfs in series. that would be pointless. but it still wouldn't work. you'd still have to read each one and add them. because of voltage drop in the cables between shunts.
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Old 07-09-2015, 19:36   #20
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Re: Current Measurement?

pretty sure it's not going to work. but would be interesting to try. I think if you unganged the negs together. and each bank had it's own neg bus to loads. it would probably work. but tying them together will cause strange voltage differences btween the shunts.
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Old 07-09-2015, 20:34   #21
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Re: Current Measurement?

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Originally Posted by smac999 View Post
pretty sure it's not going to work. but would be interesting to try. I think if you unganged the negs together. and each bank had it's own neg bus to loads. it would probably work. but tying them together will cause strange voltage differences btween the shunts.
Sorry, no. In your drawing you have two of the sense wires essentially connected to ground, and the shunt on the right connected to ground at both terminals via a sense wire. The load current on the right-hand shunt will end up flowing to ground via that sense wire (as well as through the sense resistor). Bad things might happen, and it's definitely not going to give you a useful measurement.

Either share one sense resistor, or use two meters.
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Old 08-09-2015, 04:42   #22
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Re: Current Measurement?

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Originally Posted by Captain Bill View Post
Deepfrz is correct. In dc series circuit current is constant.
Sure, but it's not the main circuit which is put in series. It's the signal circuit which goes up in voltage (not current) to indicate how much current is going through the shunt.

This is a purely theoretical discussion because more knowledgeable people have pointed out that it doesn't work for other reasons.
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Old 08-09-2015, 04:43   #23
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Re: Current Measurement?

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Originally Posted by smac999 View Post
the discussion is on wiring a single meter in series to 2 different shunts in 2 different banks. the data wires in series therefor getting the addition of both shunts to a single amp reading, single gauge. not wiring the shunts themselfs in series. that would be pointless. but it still wouldn't work. you'd still have to read each one and add them. because of voltage drop in the cables between shunts.
That's it. I suppose it would work to wire them in series to get the sum of the currents, if not for this, correct?

I guess the problem is the very tiny voltages involved.
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Old 08-09-2015, 04:55   #24
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Re: Current Measurement?

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Originally Posted by Maine Sail View Post
Dockhead,

I am having trouble with understanding all the consternation on this. Simply re-wire the bank so all neg side current is being taken from one single point.

I have had to do this far too many times to count when installing battery monitors. Yes there is some cost in big fat cable, or a neg busbar or such but other than that it is most often an hour or two of work.
Yes, thanks.

I actually paid a professional to do this very thing a couple of years ago. A top guy in Cowes, but he failed to connect it properly and it didn't work -- there was some stray lead I guess.

I think you are right that all the consternation is not worth it. I have studied my wiring diagrams again and I think there is, after all, a point where all the negatives come together. If that's true then it should be easy to put a single shunt in. I'll get into it over the weekend.

Otherwise, I'm just going to have two ammeters and sum them in my head, rather than screwing around with some kind of exotic solution.

Thanks.
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Old 08-09-2015, 04:57   #25
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Re: Current Measurement?

I have always found that theoretical questions lead to theoretical answers. The sum conclusion of the matter always leads to early testing of botanical infusions of Gin and mixer of choice.

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Old 08-09-2015, 05:12   #26
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Re: Current Measurement?

Assuming you have a house bank and a starting battery/bank, you have all the tools you need now.

Wire the smartgauge such that the house is the primary or monitored bank and the other bank only is queried for volts (not needed but I hate loose wires).

Wire the Victron to the starter battery AND HERE IS THE BIGGY clean up the 12VDC negative runs so they all come off the Victron shunt. This will give you watts in and out.

Are there other ways to do this? Sure, but this way uses what you have saving money for the cruising/bar kitty.
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Old 08-09-2015, 05:43   #27
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Re: Current Measurement?

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Originally Posted by Snore View Post
Assuming you have a house bank and a starting battery/bank, you have all the tools you need now.

Wire the smartgauge such that the house is the primary or monitored bank and the other bank only is queried for volts (not needed but I hate loose wires).

Wire the Victron to the starter battery AND HERE IS THE BIGGY clean up the 12VDC negative runs so they all come off the Victron shunt. This will give you watts in and out.

Are there other ways to do this? Sure, but this way uses what you have saving money for the cruising/bar kitty.
It's not house/start.

My boat was built with four separate banks -- 1. engine start (12 volt; separate AC charger and separate alternator on main engine); 2. genset start (12 volt; charged by alternator on genset); 3. House (24v; charged by main battery charger and by school bus alternator through Driftgate splitcharger); 4. Service (24v; charged by main battery charger and school bus alternator through Driftgate).

I combined 3 and 4 into one single bank serving both house needs and service (winches, thruster, windlass).

The problem is that the new combined bank has separate negative bus bars.
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Old 08-09-2015, 06:32   #28
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Re: Current Measurement?

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It's not house/start.

My boat was built with four separate banks -- 1. engine start (12 volt; separate AC charger and separate alternator on main engine); 2. genset start (12 volt; charged by alternator on genset); 3. House (24v; charged by main battery charger and by school bus alternator through Driftgate splitcharger); 4. Service (24v; charged by main battery charger and school bus alternator through Driftgate).

I combined 3 and 4 into one single bank serving both house needs and service (winches, thruster, windlass).

The problem is that the new combined bank has separate negative bus bars.
Put a significant load on various DC systems, and then measure with a millivolt meter the voltage difference between these two bus bars. It should be '0.

If it is not zero, then, a lot of work but what I would do, is disconnect all the plus side of all the batteries (put tape around each removed power lead so they don't fly around to a neg post close by, and measure the resistance between these two bus bars.

Open, or a few ohms? Again, should be less than 1 ohm, more like 0.01 ohm or thereabouts.

You only want one true ground on the boat, including the thru-hulls, engine block, shaft, rudder post, quadrant, bow thruster tail pieces, SSB ground plate, breaker panel, mast, etc.

The battery selector switches are only switching between the various plus sides of the batteries.
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Old 08-09-2015, 08:24   #29
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Re: Current Measurement?

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Originally Posted by Ericson38 View Post
Put a significant load on various DC systems, and then measure with a millivolt meter the voltage difference between these two bus bars. It should be '0.

If it is not zero, then, a lot of work but what I would do, is disconnect all the plus side of all the batteries (put tape around each removed power lead so they don't fly around to a neg post close by, and measure the resistance between these two bus bars.

Open, or a few ohms? Again, should be less than 1 ohm, more like 0.01 ohm or thereabouts.

You only want one true ground on the boat, including the thru-hulls, engine block, shaft, rudder post, quadrant, bow thruster tail pieces, SSB ground plate, breaker panel, mast, etc.

The battery selector switches are only switching between the various plus sides of the batteries.
The root problem is that there are two negative buses which feed separate groups of equipment. I could cut the connection between one set of batts and one of the busses, feed it back up to the other bus with a heavy cable, then tie back to the first bus with another heavy cable, but this would introduce long cable runs for heavy power consumers (10 horsepower bow thruster) -- definitely not a good idea.

My through hulls, mast, etc. are not bonded -- per modern practice.
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Old 08-09-2015, 08:46   #30
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Re: Current Measurement?

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The root problem is that there are two negative buses which feed separate groups of equipment. I could cut the connection between one set of batts and one of the busses, feed it back up to the other bus with a heavy cable, then tie back to the first bus with another heavy cable, but this would introduce long cable runs for heavy power consumers (10 horsepower bow thruster) -- definitely not a good idea.

My through hulls, mast, etc. are not bonded -- per modern practice.
What? Im of the school that one ground is the way to go. Everything on my Centaur was ground to the "chassis" so to speak. All batteries were connected to the common ground.
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