Aggregating Current Measurements from Two Different Shunts

[GeorgeW]

A clearer diagram for my previous post...

Note that I drew two shunts - that won't work as it doubles the total shunt resistance. A single shunt rated for the desired dc current and millivolt output is needed. Shunts can be expensive.

[goboatingnow]

Originally Posted by GeorgeW

The OP indicated "the OUTPUT of two shunts".



It's not an unreasonable question and under some configurations might actually work but would most likely cross-connect the load conductors with instrument gauge wires.

No , it’s makes NO sense a series shunt is simply a single shunt of different resistance , you cannot measure two different currents with any form of “ series “ shunt

You must certainly cannot connect the sense voltages together either that’s meaningless electrically

What the OP can do is simply parallel the negatives of both banks and use a single shunt

[Dockhead]

Originally Posted by goboatingnow

. . . What the OP can do is simply parallel the negatives of both banks and use a single shunt

That's what I can't, actually, do, as per the OP.


Someone has written me a PM stating that the Mastervolt battery monitor can aggregate the currents from two different shunts. Don't know why he didn't post this extremely useful information. I'm checking it out now.

[goboatingnow]

Originally Posted by Dockhead

That's what I can't, actually, do, as per the OP.


Someone has written me a PM stating that the Mastervolt battery monitor can aggregate the currents from two different shunts. Don't know why he didn't post this extremely useful information. I'm checking it out now.

The OP could do this

[GeorgeW]

[QUOTE=goboatingnow;3644114]No , it’s makes NO sense a series shunt is simply a single shunt of different resistance , you cannot measure two different currents with any form of “ series “ shunt

You must certainly cannot connect the sense voltages together either that’s meaningless electrically
/QUOTE]


Your statements do not represent what I wrote nor do they reflect what I drew in my comments numbered 30 and 31.

[GeorgeW]

Originally Posted by Dockhead

One problem I've had since combining the banks is I have no way to measure current going into and out of them, since there is no single cable for either negative or positive which I can shunt.

I tried to figure out a way to aggregate the readings from two shunts, but couldn't figure it out so dropped the idea.

I realize that I could do this with a R-pi and four or five lines of code I could probably write myself, but is there any other way to do it? Can the shunts be simply wired in series -- how is the output signal structured? Is there an ammeter which can aggregate the readings from two shunts?

I will be happy of someone has some insight.

The OP wants to measure total current going in and out of the batteries but into each battery independently.

The metering instruments will not need to have shunts connected to the battery negative.

The shunt(s) will be placed on the positive side of the batteries.

Case 1:
The OP already has two identical shunts rated 500A/50mV.
The OP already has a 50mV meter calibrated with a +/- 500A scale.
The maximum combined panel load is 500A.

A single shunt placed where Shunt1 and Shunt2 are located will work.
------------------------------------------------------------------------------

Case 2:
The OP already has two identical shunts rated 250A/50mV.
The OP already has a 50mV meter calibrated with a +/- 500A scale.
The maximum combined panel load is 500A.

Place 2 shunts in parallel (not series) in the locations shown in post 31.
-------------------------------------------------------------------------------

Case 3:

The OP already has two shunts but they are not identical.

Buy a new shunt with suitable ratings unless one of the existing shunts meets the current requirement of the maximum combined panel load.
-------------------------------------------------------------------------------

Case 4:

The OP already has two identical shunts rated 500A/50mV.
The OP already has a 100mV meter calibrated with a +/- 500A scale.
The maximum combined panel load is 500A.

Connect the two shunts in series (not parallel) as shown in the diagram in post 31. Minimize the resistance of the connection between the shunts, possibly by bolting them together. A single new shunt would be more accurate.
----------------------------------------------------------------------------------

Case 5:

The OP has neither shunts nor meters and must purchase all new components.

Purchase a battery monitor that uses current transducers rather than shunts. They are much easier to install than shunts as long as their inside diameter is large enough to slip over existing cable terminals and no alterations will be needed in the existing dc cabling. Being able to monitor the banks separately, as well as summing the AmpereHours, may provide some insight into the health of the two banks.

-------------------------------------------------------

If shunts are used:

The dc cable from the panels would be re-routed from the batteries to the shunt(s) which would preferably be located somewhere near the mid-point of the cable that connects the battery banks together.

I think that covers most of the possible configurations and provides some insight into what could be done. Connecting multiple shunts together is likely to degrade accuracy to some degree.

Shunts are quite robust however they do degrade over time especially if operated for long periods of time at their maximum current ratings.

My personal choice would be to use a monitor current transducers on each bank.

It's 3am. Good night.

[goboatingnow]

Originally Posted by GeorgeW

A clearer diagram for my previous post...

Note that I drew two shunts - that won't work as it doubles the total shunt resistance. A single shunt rated for the desired dc current and millivolt output is needed. Shunts can be expensive.

Attachment 259984

This is a single shunt solution and one I advocated right back at the start. Putting two in series is meaningless as all you are doing is changing the shunt sensitivity. One single shunt is all that is required

[goboatingnow]

[QUOTE=GeorgeW;3644153]

Originally Posted by goboatingnow

No , it’s makes NO sense a series shunt is simply a single shunt of different resistance , you cannot measure two different currents with any form of “ series “ shunt



You must certainly cannot connect the sense voltages together either that’s meaningless electrically

/QUOTE]





Your statements do not represent what I wrote nor do they reflect what I drew in my comments numbered 30 and 31.

31 is a meaningless waste of a second shunt as it does nothing other then change the sensitivity of the shunt

[ClarkW]

Mastervolt has solved this problem. I have a Mastervolt display that displays the combined current in amps and the combined Ahs for two separate shunts connected to my house battery bank. One shunt was limited to 500A so this was done by Mastervolt to allow them to work with higher currents. My stern thruster takes 650A when operating. The shunts have a network connection to the display and the setup to combine them is done in the configuration for the display. My boat is 2008 and this is original equipment. Here are links to what I think are the current versions of the products:

https://www.mastervolt.com/products/...unt-500-czone/

https://www.mastervolt.com/products/...ay/easyview-5/

Clark

[goboatingnow]

High side current shunts are fine but if you are using electronic metering which most battery monitors are nowadays you must ensure the meter is capable of high side measuring, most are not


Putting shunts in parallel is a bad idea as you cannot be sure if the accuracy is good enough especially at small currents

[Dockhead]

Originally Posted by goboatingnow

The OP could do this

I am the OP

[Dockhead]

Originally Posted by ClarkW

Mastervolt has solved this problem. I have a Mastervolt display that displays the combined current in amps and the combined Ahs for two separate shunts connected to my house battery bank. One shunt was limited to 500A so this was done by Mastervolt to allow them to work with higher currents. My stern thruster takes 650A when operating. The shunts have a network connection to the display and the setup to combine them is done in the configuration for the display. My boat is 2008 and this is original equipment. Here are links to what I think are the current versions of the products:

https://www.mastervolt.com/products/...unt-500-czone/

https://www.mastervolt.com/products/...ay/easyview-5/

Clark

Thanks very much; hot tip

[botanybay]

Originally Posted by ItDepends

You can get a Victron Venus or Cerbo system that, with a recent update, supports multiple shunts. You'll need to check to be 100% sure it does exactly what you want, but my understanding is it will.

You probably know this, and I understand that you are stuck with a suboptimal physical setup, but pulling loads out of different places of a battery bank is a recipe for balance problems, especially with the individual parts of the bank widely separated with long lengths of wire. Not likely to be a major problem if the loads and charging currents are small, but especially if you charge the bank fast, different individual batteries will end up at different places and in the long run some will undercharge and some overcharge.

Really!!! I have been planning to write the code in my Color Control GX for years but never got around to it.

I have two shunts and two BMV 701s which measure both sides of my battery bank. I do midpoint monitoring of each string independently and definitely was able to see when the batteries were aging and when one call in one string dropped out I immediately got alarms :-) All good.

However, I have not been able to aggregate the values so I assume 2x on all values and that is close.

If they have added the ability to aggregate that will be wonderful.

When I talked to the Victron Guys their concern was everyone has different goals with aggregation (eg. display the worst case percentage, the average percentage, the best case percentage... Worst midpoint, best midpoint, or whatever.) and why they had not gotten to it.

I will see if the most recent update provided the option, do you know where to look in the menus?

[botanybay]

I have considered the following:

I have a leftover double shunt where it is two shunts with a common bar for the DC buss and two connections, one for each battery. It came off of a Link 2000R which monitored and adjusted the alternator based upon state of charge, but that was a long time ago on a different boat.

So two 50mv shunts, one for each battery with the side opposite the battery being a common copper block. I have two Victron BMV 701s (Amp Hour Monitors from Victron), one on each shunt which works nicely but I have to multiply all of my currents and amp hours by a factor of two assuming both sides are active.

Note that on the positive side I can isolate either of the two batteries and the system works at half capability.

So, to make the system work better how about a third BMV 701 which is wired with two 100k Ohm resistors in series between the battery side of the two shunts. These are both 50mv shunts @ 500A have a resistance of E=I/R --> R=E/I = 50e-6V/500A = 0.0125 Ohms

I have attached a GIF of the circuit.

Essentially if both shunts are reading the same value then there is no current through the 100k ohm resistors and the voltage is the same as either shunt:
10mv on shunt 1 and 10mv on shunt 2, no current through the 100k ohm resistors so midpoint is at 10mv.

With only shunt 1 active the midpoint of the two series 100k ohm resistors is half the voltage on shunt 1 as shunt 2 has no current and so the battery side is approximately 0 (yeah, a small current flows I = E/R = 10mv / 200k ohm = 0.00000005 amps :-) )

So the midpoint behaves as if the battery sides of the shunt has been shorted together and acts like a 50mv @ 1000A shunt.

So, if I program the midpoint BMV 701 to have 50mv = 1000A it should properly measure the overall consumption on the two banks without impacting the other two shunts.

Why pick 100k Ohms for the resistors? I am guessing that the input impedance of the BMV is on the order of 1Meg to 10Meg Ohm, by making the series resistors an order of magnitude less the mid point voltage is not impacted significantly by adding the BMV between the midpoint of the series resistors and the common side DC negative bus.

Just remembering back to basic circuit theory and I don't think I have messed anything up. Since the shunts are a single combined block there is no risk of lifting one side of the Shunt to DC common but even if I did the resistance is such that the current flow would be tiny.

Any reason this simple solution is a bad idea.

(I do like the other devices which can perform the summation correctly but I also like that my Color Control GX is uploading this data to the cloud all of the time and I can check on the boat from anywhere on the planet :-) )

[hzcruiser]

Originally Posted by GeorgeW

This configuration would work, possibly with some constant error due to the series connection of the shunts. Connections to the shunts would be at the battery end and at the load end only, nothing between the shunts. Meter will see the combined voltage of the two shunts in series. A very low resistance connection between the shunts would be preferable to minimize error due to voltage from in the connections. Meter full scale will need to match sum of shunt maximum mv output or be custom calibrated (Sharpie?) or digital on appropriate scale.

. Panel1. . . . . . . . . . . . . . . . . Panel2
..... X ....................................... X
..... xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
........................... X
........................... Xxxxxxxxxxxxxxxxxxxxx
....................... Shunt1 . . . . . . . . . . . . . X
........................... X . . . . . . . . . . . . . Meter
........................... X . . . . . . . . . . . . . .. X
....................... Shunt2 . . . . . . . . . . . . . X
........................... XxxxxxxxxxxxxxxxxxxxX
........................... X
..... xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
.... X ......................................... X
.... X .......................................... X
. Battery1....................................Batter y2


My carefully drawn ascii diagram is being mangled by having whitespace removed when posting. Ignore the periods - the X's show connections.

From what I gathered in your diagram, both shunts are in series, yes, but the whole current from both panels is going through both shunts?
Still doesn't add up in my head, sorry.