FNDC Shunt Question

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provo
Forum Guru
Posts: 252
Joined: Thu Jul 27, 2006 4:34 pm
My RE system: Sixteen Evergreen EC-120
(4 strings, total 1920W)
Eight Rolls S-550 (2 strings, total ~800Ah @ 24V)
One FM80
One VFXR3524A
Hub 10
Mate3s
Trimetric
Location: Sierra foothills

FNDC Shunt Question

Post by provo » Sun Mar 15, 2020 10:08 am

In my present system, the shunt for the Trimetric is between the the battery and the (combined) FM and FXR.

When wiring up the FNDC with two shunts, it appears one shunt is between the battery and the FM, and the other is between the battery and the FXR. So the actual amps in and out of the battery isn’t measured directly by a single shunt, but must be calculated in the FNDC from the values coming from two shunts. Do I have that right?

Does that mean the inherent inaccuracy of a shunt is doubled with an FNDC vs a Trimetric as far as battery monitoring is concerned? That might be a small price to pay for having separated data on FM and FXR, but I’m just wondering….

gtarolli
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Posts: 312
Joined: Thu Dec 21, 2017 10:39 am
My RE system: Outback: off-grid
- 1 GS8048A, Mate3, FlexNetDC, Hub10.3
- 2 FLEXmax 80
- 1 FLEXmax 60
- 30 (10x3) 300w panels (9000w total)
- 6 SimpliPhi 3.4 kWh LI batteries (400ah , 20kWh)
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Location: Wainiha, Kauai, HI

Re: FNDC Shunt Question

Post by gtarolli » Sun Mar 15, 2020 10:47 am

Possibly, but not necessarily. Errors can also cancel each other out.

provo
Forum Guru
Posts: 252
Joined: Thu Jul 27, 2006 4:34 pm
My RE system: Sixteen Evergreen EC-120
(4 strings, total 1920W)
Eight Rolls S-550 (2 strings, total ~800Ah @ 24V)
One FM80
One VFXR3524A
Hub 10
Mate3s
Trimetric
Location: Sierra foothills

Re: FNDC Shunt Question

Post by provo » Sun Mar 15, 2020 10:57 am

gtarolli wrote:
Sun Mar 15, 2020 10:47 am
Possibly, but not necessarily. Errors can also cancel each other out.
True! In my case, the FNDC would be most valuable as a battery monitor that could talk to Optics. I was thinking of using the shunt that's already there for the FNDC also, but then I realized it's in the wrong place for the intended FNDC usage.

gtarolli
Forum Guru
Posts: 312
Joined: Thu Dec 21, 2017 10:39 am
My RE system: Outback: off-grid
- 1 GS8048A, Mate3, FlexNetDC, Hub10.3
- 2 FLEXmax 80
- 1 FLEXmax 60
- 30 (10x3) 300w panels (9000w total)
- 6 SimpliPhi 3.4 kWh LI batteries (400ah , 20kWh)
- 1 Honda 3000 generator + one spare
Location: Wainiha, Kauai, HI

Re: FNDC Shunt Question

Post by gtarolli » Sun Mar 15, 2020 11:23 am

I don't remember seeing any complaints about accuracy, and from my observations (3 shunts), I don't see any gross issues. The largest issue in OptricsRE is that inverter loads are truncated (not rounded) to the nearest amp (per leg), which can lead to a pretty significant error over time. If they were rounded, it would tend to even out in most cases, but truncation leads to accumulated error. Which is why To Load can often be significantly below From Battery. The other issue is inverter inefficiency. I usually ignore To Load and focus on From Battery which I think comes from Shunt A (battery->inv) in most systems.

raysun
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Hyundai 355 watt panels (6 - 3 strings of 2 in series)
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Re: FNDC Shunt Question

Post by raysun » Sun Mar 15, 2020 12:11 pm

gtarolli wrote:
Sun Mar 15, 2020 11:23 am
I usually ignore To Load and focus on From Battery which I think comes from Shunt A (battery->inv) in most systems.
Me too. After 4 years of trying to correlate the two, I've accepted the system works as the system works, and the key metric is battery draw. I glance occasionally at To Load, just to make sure the relationship between the two metrics is maintaining the same basic ratio, but I don't waste much time chasing down the AC power consumption rabbit hole.

provo
Forum Guru
Posts: 252
Joined: Thu Jul 27, 2006 4:34 pm
My RE system: Sixteen Evergreen EC-120
(4 strings, total 1920W)
Eight Rolls S-550 (2 strings, total ~800Ah @ 24V)
One FM80
One VFXR3524A
Hub 10
Mate3s
Trimetric
Location: Sierra foothills

Re: FNDC Shunt Question

Post by provo » Sun Mar 15, 2020 12:33 pm

I'm confused about To Battery and From Battery. Is either one of those values in the Optics graph just taken from a single shunt, or they both calculated from both shunts (assuming no third shunt)? So if From Battery comes from the bat --> inv shunt, what happens when the inverter/charger is sending current back into the batteries? Does that get reported as To Battery?

And what about the Ah and SOC line -- presumably that's calculated from both shunts, right?

As for accuracy, if we really get ±0.25% from these shunts, I guess it doesn't matter if we see that once or twice.

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