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Old 17-06-2022, 01:20   #1
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Aggregating Current Measurements from Two Different Shunts

My boat was designed with an electrical system like what you have on a motor yacht --


No inverter, heavy duty generator intended to be run more or less all the time when the boat is not on shore power.


She had two battery banks each consisting of 4x 110AH 12v batteries wired SP to get 24v x 220AH each.


One battery bank was for house loads, the other for "service" loads -- so, winches, windlass, thruster.


The very first year I had he, I installed a Victron inverter/charger and combined the two banks into one, to get 440AH x 24v. I did this using heavy cabling between the two banks, which are located about 5 meters away from each other.


Each bank is still connected to its own panel with its own original loads; the cabling simply allows all loads to share the pooled capacity.


Some time later I cut and extended the battery boxes and installed 8x Trojan 6v T105's, so now each box is 4x 6v batteries in series.


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.
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Old 17-06-2022, 03:21   #2
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Aggregating Current Measurements from Two Different Shunts

You’ll need a custom board that can add both shunts together, easy Arduino project
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Old 17-06-2022, 06:56   #3
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Re: Aggregating Current Measurements from Two Different Shunts

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.
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Old 17-06-2022, 12:23   #4
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Re: Aggregating Current Measurements from Two Different Shunts

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Originally Posted by ItDepends View Post
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.


Cerbo will support multiple shunts but it will not aggregate them afaik
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Old 17-06-2022, 13:30   #5
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Re: Aggregating Current Measurements from Two Different Shunts

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...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?...
For dumb shunts, technically yes. The shunt output is just the voltage drop across a very carefully configured resistance. Most shunts we use are 0-50mV. So, if you wired them in series you could get out a 0-100mV signal. And if you (by design) never operated above 50% of a shunt's capacity you end up with a combined 50mV signal. But, the wire you use between the shunts becomes a parallel conductor on the negative side of your system, and could see considerable current which a) means it must be large and b) means that voltage drops in that parallel path could throw off the tiny mV signals quite a bit. So, while on paper it might work, in reality it would be a rare beast where it would work. And, as a series connection in this manner the current from the lower side shunt could also flow through the higher side because of the parallel path....

There are mV signal isolators/conditioners that would allow you to do this (thermocouples are an industrial standard and they all put out mV signals). Put an isolator on each signal, then put those in series (or buy a two-channel isolator with an addition function), but all of them I found (on quick look) also up-convert the signal to a higher level because no one really likes tiny signals. That then means that you're looking for a different device to read the output, which means more complication.....
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Old 17-06-2022, 14:07   #6
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Re: Aggregating Current Measurements from Two Different Shunts

Sorry , what on Earth are you talking about.

You cannot wire shunts on different circuits together in series it makes no sense

If the shunts are in series , then all you have done is created one shunt of twice the sensitivity then the original. By definition a series shunt is only detecting the same current

The OP has two shunts measuring two separate currents flows , zero use for any sort of nonsensical series shunt

Secondly the sense voltage of each shunt cannot be assumed to be common referenced , even if it is in this case ( assuming the batteries are paralleled behind the shunts)

To aggregate the sense voltages first requires a differential voltage adder circuit such that the output is the linear sum of both input sense voltages , such an output then represents the combination of the two separate currents,

Alternatively these days adding the sums up in the digital domain is arguably easier, convert each shunt sense voltage to numbers and add them digitally , easy to do in an ardunio etc.

There are several devices in the marine space that support two shunts , but none that add the numbers together to my knowledge
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Old 17-06-2022, 14:36   #7
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Re: Aggregating Current Measurements from Two Different Shunts

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Cerbo will support multiple shunts but it will not aggregate them afaik
If it is not built in to the Venus OS, to would be easy to implement using their "Large" operating system that includes a NodeRed implementation.
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Old 17-06-2022, 23:28   #8
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Re: Aggregating Current Measurements from Two Different Shunts

With banks in different places. And different cable lengths You are Likly going to pull different amps out of each bank. And each bank will be slight different Soc.

I have have seen this with 2 house banks. Combined at the battery switch. The closer bank pulls more current and will be a lower Soc.

So really you need 2 battery monitors.
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Old 18-06-2022, 01:08   #9
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Re: Aggregating Current Measurements from Two Different Shunts

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Originally Posted by Dockhead View Post
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?
The Simarine Pico battery monitor will aggregate, ignore or even subtract shunts as a standard feature. Multiple shunts can be used in the system (we use four). It is a very versatile and programmable monitor, ideal for advanced users.

I think it will do what you want, but check with the manufacturer before purchase.
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Old 18-06-2022, 03:52   #10
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Re: Aggregating Current Measurements from Two Different Shunts

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Originally Posted by noelex 77 View Post
The Simarine Pico battery monitor will aggregate, ignore or even subtract shunts as a standard feature. Multiple shunts can be used in the system (we use four). It is a very versatile and programmable monitor, ideal for advanced users.
I think it will do what you want, but check with the manufacturer before purchase.
Simarine PICO BATTERY MONITOR [278.95€] ➥ https://simarine.net/pico-battery-mo...SAAEgLpnPD_BwE

FAQs ➥ https://simarine.net/support-faq/
Manualshttps://simarine.net/manuals/
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Old 18-06-2022, 04:24   #11
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Re: Aggregating Current Measurements from Two Different Shunts

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Originally Posted by goboatingnow View Post
You’ll need a custom board that can add both shunts together, easy Arduino project

I guessed that; I guess I could even write the code myself. But I don't really want a whole other computer on board just to do this one job
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Old 18-06-2022, 04:24   #12
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Re: Aggregating Current Measurements from Two Different Shunts

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Thanks guys; hot tip. Checking out.
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Old 18-06-2022, 04:31   #13
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Re: Aggregating Current Measurements from Two Different Shunts

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Originally Posted by smac999 View Post
With banks in different places. And different cable lengths You are Likly going to pull different amps out of each bank. And each bank will be slight different Soc.

I have have seen this with 2 house banks. Combined at the battery switch. The closer bank pulls more current and will be a lower Soc.

So really you need 2 battery monitors.
I'm aware of this drawback but didn't have any other choice.

In practice it works pretty well, because the two banks are equidistant from the inverter/charger and connected with heavy cables of equal length. Besides that, there are balance cables between the two banks. SOC runs very close between the two banks.

The ultimate solution to this will be to go to lithium, which can be done in the volume of just one of the battery boxes. I almost pulled the trigger on that a couple of years ago, but ended up not having time to mess with it.

Lithium is totally superior with so many advantages, but for my use case the Trojan golf cart batteries are quite acceptable. I have a lot of charging capacity including a heavy duty low speed generator, so better batteries in practice just means somewhat less generator usage, which would be nice, but is not really critical. I can barely hear my generator in use (and not at all from outside), so it's not a big deal to run it an extra hour a day.

Building a boat from scratch, however, I would definitely go lithium. And possibly even with this boat, if I still have her when the present set of Trojans have reached EOL.
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We must have a turn together . . . . I undress . . . . hurry me out of sight of the land,
Cushion me soft . . . . rock me in billowy drowse,
Dash me with amorous wet . . . . I can repay you."
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Old 18-06-2022, 06:12   #14
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Re: Aggregating Current Measurements from Two Different Shunts

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Originally Posted by Dockhead View Post
Thanks guys; hot tip. Checking out.

You guys are correct:


Click image for larger version

Name:	Screenshot 2022-06-18 160930.jpg
Views:	54
Size:	168.5 KB
ID:	259637


From the Pico manual.


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I believe you refuse to go back without feeling of me;
We must have a turn together . . . . I undress . . . . hurry me out of sight of the land,
Cushion me soft . . . . rock me in billowy drowse,
Dash me with amorous wet . . . . I can repay you."
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Old 18-06-2022, 07:35   #15
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Re: Aggregating Current Measurements from Two Different Shunts

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You guys are correct:


Attachment 259637


From the Pico manual.




A whole other computer on board of course !!
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