Adding a shunt

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Adding a shunt

Postby Mike Curran on Tue Feb 21, 2017 10:25 am

Hello - I'm adding a shunt to separately measure my charge controller (MX60) amps and need some guidance on where it goes schematic-wise. I have an existing shunt for all battery current (wired between Batt negative and negative bus), so when I break out the MX60 wiring from the existing negative DC bus, does the new shunt go between the bus and the MX? Or between the battery negative and the MX?

Hope this makes sense! Thanks in advance.
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Re: Adding a shunt

Postby unicornio on Tue Feb 21, 2017 10:58 am

hi, there!
Mike Curran
I would put it between the negative bus and the charge controller, and I would change the current shunt and put between the negative of the inverter and the negative bus, so you can measure the current that consumes/charges the inverter, and the current to the charge controller to batt...

there are some schematics diagrams with 2 and 3 shunts at outback, so you can see this...

hope this help!
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Re: Adding a shunt

Postby Mike Curran on Thu Apr 06, 2017 11:44 am

Thanks, Uni - I'm finally getting around to doing this and your suggestion pretty much aligns with the schematic in the FNDC manual. However, to do it right I'd have to add another shunt to account for a few small DC loads I have attached to my negative bus. This would require a somewhat major rewiring of my PSDC cabinet which I'm loathe to do right now...

All that said, can you look at the attached schematic and tell me what the downside of wiring it as shown would be? In this setup, shunt A would measure ALL battery current, while shunt B would measure charge controller current only. Will the FNDC still work properly in this setup, or does it expect the sum of all shunt currents to equal total battery current (not the case in my proposed setup)?

Any other opinions out there would be welcome as well! Thanks all.

20170406_142938.jpg


PS the FM80 charge controller shown is new as well, forgot to mark it as such before taking the photo.
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Westinghouse Solar: 30 (2x15) 235W panels with Enphase M215 microinverters. 6,450 nameplate watts AC, grid-tied.

Re: Adding a shunt

Postby solartribe on Thu Apr 06, 2017 2:27 pm

the FNDC is not smart enough for that. It expects all shunts to be in parallel, not series.
Leave your loads on the inverter shunt and install a new shunt in parallel with the old one (for the charge controllers only).
Or just run everything through the old shunt.... the data will be accurate, just not specific.
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Re: Adding a shunt

Postby Mike Curran on Thu Apr 06, 2017 3:27 pm

Thanks Solartribe - That's the direction I was leaning. I'll put the new shunt between the battery negative and the new charge controllers, and leave everything else unchanged.
Mike Curran
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Posts: 108
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My RE system: Outback: 8 (2x4) Evergreen 180's into one MX60, 2 grid-tied GVFX3524's classic stacked for 120/240VAC, 12 Surrette/Rolls 2V x 1766Ah, Hub4, Mate3, FNDC, RTS.

Westinghouse Solar: 30 (2x15) 235W panels with Enphase M215 microinverters. 6,450 nameplate watts AC, grid-tied.

Re: Adding a shunt

Postby Mike Curran on Mon Apr 17, 2017 2:40 pm

Okay - New shunt installed in line with current flow to battery from MX60 charge controller.

Does anyone know if the operation of my system has actually changed as a result of this addition? Because it (the system) was accounting for MX60 current before, I'm assuming through an internal measurement in the MX60. So does the system somehow detect the presence of the new shunt and substitute data from it in place of the MX60's current data?

If not, I'm wondering what benefit I've gained from adding a shunt. Any thoughts out there?
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My RE system: Outback: 8 (2x4) Evergreen 180's into one MX60, 2 grid-tied GVFX3524's classic stacked for 120/240VAC, 12 Surrette/Rolls 2V x 1766Ah, Hub4, Mate3, FNDC, RTS.

Westinghouse Solar: 30 (2x15) 235W panels with Enphase M215 microinverters. 6,450 nameplate watts AC, grid-tied.

Re: Adding a shunt

Postby OffgridHiker on Mon Apr 17, 2017 3:59 pm

My understanding is that the benefit of adding the shunt is now you have a known resistance which can be used to measure the current using Ohm's law.
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Re: Adding a shunt

Postby Mike Curran on Mon Apr 17, 2017 4:08 pm

OffgridHiker wrote:My understanding is that the benefit of adding the shunt is now you have a known resistance which can be used to measure the current using Ohm's law.


Agreed, but... current was already being measured (somehow, assuming accurately) by the MX60. What did I gain if the more accurate (again, assumed) shunt current measurement is not being incorporated into the system's operation?
Mike Curran
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Posts: 108
Joined: Sat Aug 28, 2004 6:28 pm
Location: Chagrin Falls, Ohio
My RE system: Outback: 8 (2x4) Evergreen 180's into one MX60, 2 grid-tied GVFX3524's classic stacked for 120/240VAC, 12 Surrette/Rolls 2V x 1766Ah, Hub4, Mate3, FNDC, RTS.

Westinghouse Solar: 30 (2x15) 235W panels with Enphase M215 microinverters. 6,450 nameplate watts AC, grid-tied.

Re: Adding a shunt

Postby OffgridHiker on Mon Apr 17, 2017 6:30 pm

The FNDC is presumably more accurate at measuring the voltage difference and the resistance it use is perhaps more precise and therefore should be more accurate than the MX/FM.

What do you mean by "not incorporated"?

I have a very similar setup as yours and the FNDC reports more precisely on opticsRE for example (battery tab). For example the CCs will display X.X while the shunts are shown as X.XX.

While the CCs will tell you how much is produced the FNDC will tell you what portion of that source went to the batteries vs load/inverters.

Hopefully someone more knowledgeable can chime in. :-)

Hth.
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Re: Adding a shunt

Postby Mike Curran on Mon Apr 17, 2017 7:25 pm

OffgridHiker wrote:What do you mean by "not incorporated"?


I guess an example would be for when your batteries are absorbing and return amps are being measured so the system knows when to resume float mode. Does the FNDC use the shunt current measurements to determine the absorb cutoff? Or does it use each charging device's internal current measurement? Maybe I'm answering my own question because I suspect it's the former, shunts are used. That'd be a good thing, IMHO.
Mike Curran
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Posts: 108
Joined: Sat Aug 28, 2004 6:28 pm
Location: Chagrin Falls, Ohio
My RE system: Outback: 8 (2x4) Evergreen 180's into one MX60, 2 grid-tied GVFX3524's classic stacked for 120/240VAC, 12 Surrette/Rolls 2V x 1766Ah, Hub4, Mate3, FNDC, RTS.

Westinghouse Solar: 30 (2x15) 235W panels with Enphase M215 microinverters. 6,450 nameplate watts AC, grid-tied.

Re: Adding a shunt

Postby EMCF on Tue Apr 18, 2017 6:29 am

Shunts are used by FNDC to monitor current primarily. Calculations of SOC, etc come next. Shunts has very little or nothing to do with the charging operation of charge controllers.
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Re: Adding a shunt

Postby sparksalot on Wed Apr 19, 2017 7:46 am

Shunts and a FNDC do measure, calculate and report/display DC amps/currents amounts in BOTH directions for tallies and graphs relative to the constant changes in system currents through time.

A FNDC can accomodate up to 3 shunts per 1 system and connects to a Mate, and then if desired will connect to OpticsRE, but for either both FNDC and Mate must connect into a HUB.
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