Author Topic: Squeezing a second shunt into a narrow E-panel?  (Read 656 times)

jnh

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Squeezing a second shunt into a narrow E-panel?
« on: May 12, 2020, 01:42:28 PM »
Is it possible and practical to fit a second 50mv/500A shunt anywhere within the standard (narrow) E-panel enclosure?  Has anyone managed to do this?

Mine is an MNE250ST (single-pole AC bus bars & bypass assembly), hinged on the left, with an Outback GVFX3524 mounted to its door, a Flexmax 80 to its right (using the bundled charge-controller bracket), and a Flexmax 60 down below, connected by a short conduit. 

I have an Outback Flexnet DC in one of the three panel-mount breaker slots at upper-right, with its Shunt A terminals wired to the E-panel's standard shunt, and Shunt B going to one I added under the inverter's DC terminal cover (sold by Outback for use with their small Flexware 250 enclosure; this fits where the grounding lug would normally go).  I'd like to have a Shunt C dedicated to the charge controllers only, to separate their current from some DC loads now sharing the Shunt A bus-bar.

Space is already tight in this panel, though.  I really wish now that I'd bought the stretched/"Plus" model, but wasn't sure there was sufficient wall space.  All available breaker positions are filled, both DIN-rail and panel mount (two MNEDC breakers for charge controller outputs are behind the FNDC), and three knockouts in back are used by 6/2 & 6/3 Romex runs entering from the wall cavity behind, this stiff wire taking up a lot of space near the top of the panel.

ClassicCrazy

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Re: Squeezing a second shunt into a narrow E-panel?
« Reply #1 on: May 12, 2020, 10:52:14 PM »
If both your existing shunts are redundant ( measuring the same circuit )- you can use the same shunt for both meters . You can have two meters sense wires on one shunt. 

Larry
system one
Classic 150 , 5s4p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system two
Classic 150 ,5s 135 watt Kyocero , 16s Calb 100AH LiFePO4 , Outback VFX 3648 inverter
system three
Midnite KID MPPT 24 DC in to  12 volt AGM batterie

jnh

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Re: Squeezing a second shunt into a narrow E-panel?
« Reply #2 on: May 12, 2020, 11:30:02 PM »
Oh, I should have been more clear in describing my connections.  The two existing shunts are not on the same circuit, and I only have one metering device in use, an Outback FNDC, although a Xantrex Link2000 is packed away somewhere as a spare, left over from my old 12V system.

Shunt B (under inverter's DC terminal cover) measures current to/from the inverter, Shunt A (built into E-panel) "everything else", including charge controllers.  I moved the E-panel's inverter-negative cable to the battery-side of its inbuilt shunt, to avoid double-counting that circuit and confusing the FNDC.  It requires that all shunts share a common terminal on one side, and that this go to battery-negative.

I'd prefer to get the two charge controllers onto a shunt by themselves (and perhaps with the outboard Iota charger connected there also), leaving Shunt A for DC loads only.  As of now power delivered by the charge controllers but drawn off directly by DC loads does not pass through any shunt (remaining on the source/load side of Shunt A), and so isn't tracked by the FNDC.  This doesn't affect battery state-of-charge tracking, since it doesn't flow into out of the battery either, but it'd be nice to have better precision.

The charge controllers have their own current measurement reporting of course, but it seems less accurate.  What prompted my question was recently noticing reported output currents of CC#1 + CC#2 adding up to *less* than what was passing through Shunt A, despite about 40-50W having already gone to DC loads not monitored by the FNDC.
« Last Edit: May 12, 2020, 11:31:50 PM by jnh »

ClassicCrazy

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Re: Squeezing a second shunt into a narrow E-panel?
« Reply #3 on: May 13, 2020, 10:45:23 AM »
wow I will have to put on my thinking cap to figure out exactly how all that works ,

So why not put a Whizbang on the initial shunt  and I think it will give you the information you want .
Are you factoring in inefficiencies of your inverter or charge controllers in your calculations - maybe that will explain the power you don't add up ?
Wait  - I just realized you don't have a Classic so can't use a Whizbang ?   If you were using a Classic with Whizbang I don't think you would  be having the monitoring issue.
So back to your original question - I guess I can't answer since I don't know the E-panel and if you want to stuff more wires and another shunt into it I don't have any idea other than adding another box next to it.

Larry
system one
Classic 150 , 5s4p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system two
Classic 150 ,5s 135 watt Kyocero , 16s Calb 100AH LiFePO4 , Outback VFX 3648 inverter
system three
Midnite KID MPPT 24 DC in to  12 volt AGM batterie

jnh

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Re: Squeezing a second shunt into a narrow E-panel?
« Reply #4 on: May 21, 2020, 01:37:11 PM »
Well, after obtaining an extra MNSHUNT to test-fit, I opened my E-panel last night and quickly realized it would take a major effort to shoehorn that into the already-crowded panel, not really worth it for the small incremental benefit.  With everything I have stuffed in there, it may already exceed NEC box-fill limits, if an inspector ever wanted to be picky about that, so making it even tighter didn't seem like a good way to go.

The spot I had in mind was in between the E-panel's existing shunt and its horizontal ground bus, drilling new mounting holes and connecting a thick copper strap between Bat- sides of the two shunts, but I'd forgotten there were already a number of heavy-gauge DC wires passing through this area, none of which had sufficient slack to be moved out of the way - I'd cut them as short as possible to minimize voltage drop, and replacing all these with longer runs would worsen that, as well as possibly making for less effective lightning-surge protection if the DC- to ground bond connection were lengthened.  So, the Shunt C terminals of my FNDC will remain idle for now.

An Outback FNDC is basically the equivalent of three Whizbang Jr modules, plus a battery voltage measurement channel, though it mounts separately from the shunts and connects to them via twisted-pair.  I did use the Whizbang at another site last year, and it's definitely the easier and more economical choice for systems where only a single battery shunt is needed, and there's a Classic installed - but only one Whizbang is supported, right?  Could two Classics each have their own Whizbang and shunt, say one monitoring total charge current and one inverter current, summing up the measured values for tracking battery SOC and end-amps but reporting them separately over Modbus?  If not, that might be a nice software feature to consider adding, for more complex systems.

Outback charge controllers do try to take their own losses into account when measuring battery current and estimating array current, but their internal shunts being in the Bat+ line requires a differential "high side" measurement, which is inherently more error-prone due to the large common-mode voltage that has to be subtracted out... e.g. 27.005-27.000 if charging at 50A/27V with the shunt dropping 1mA per 10A (just an example... I don't know if that's the actual ratio they use).  Temperature changes in that shunt and surrounding analog circuitry, as they heat up during the day can add some additional error.  They're usually not more than ~2% off, though, which is tolerable.
« Last Edit: May 21, 2020, 01:38:50 PM by jnh »

ClassicCrazy

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Re: Squeezing a second shunt into a narrow E-panel?
« Reply #5 on: May 21, 2020, 09:55:38 PM »
You can put a separate Whizbang and shunt on each Classic  that has Aux2 available.  It will report on that Classic's modbus .

Larry
system one
Classic 150 , 5s4p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system two
Classic 150 ,5s 135 watt Kyocero , 16s Calb 100AH LiFePO4 , Outback VFX 3648 inverter
system three
Midnite KID MPPT 24 DC in to  12 volt AGM batterie

jnh

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Re: Squeezing a second shunt into a narrow E-panel?
« Reply #6 on: May 22, 2020, 05:22:03 PM »
You can put a separate Whizbang and shunt on each Classic  that has Aux2 available.  It will report on that Classic's modbus .

Will the Classics talk to each other to total up both shunt readings for determining SOC and End Amps, though?  Say one shunt on Classic #1 AUX2 has all charge sources going through it, and the other on Classic #2 has all inverters & loads.

ClassicCrazy

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Re: Squeezing a second shunt into a narrow E-panel?
« Reply #7 on: May 22, 2020, 09:38:24 PM »
You can put a separate Whizbang and shunt on each Classic  that has Aux2 available.  It will report on that Classic's modbus .

Will the Classics talk to each other to total up both shunt readings for determining SOC and End Amps, though?  Say one shunt on Classic #1 AUX2 has all charge sources going through it, and the other on Classic #2 has all inverters & loads.
No the Classics will not total up both shunt readings. But if you have two Classics in Follow Me then you can use just one Classic for Ending Amps ( the one that has the Whizbang )  and the other Classic is going to follow it . You can have two Classics in Follow Me share one BTS battery temp sensor.

Larry
system one
Classic 150 , 5s4p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system two
Classic 150 ,5s 135 watt Kyocero , 16s Calb 100AH LiFePO4 , Outback VFX 3648 inverter
system three
Midnite KID MPPT 24 DC in to  12 volt AGM batterie