Stacking - Other options?

Discussion about OutBack Inverters in Off Grid Applications

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bear_essentials
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Stacking - Other options?

Post by bear_essentials » Sat Mar 23, 2019 12:50 pm

I have two completely independent 8048a Radian Pre-wired Power Centers, with batteries in an off-grid setup. They are 75 feet apart in separate rooms.
Can the two AC outputs be bussed together to feed a single panel board, without configuring them in a master/slave relationship? If this requires a special device, can someone point me towards the correct one?
If the first option is not possible, then can the one's output act as the grid input for the second? That should at least allow me to manually choose which system is supplying power to the panel, via the bypass option.

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Re: Stacking - Other options?

Post by Mike Curran » Sun Mar 24, 2019 8:37 am

This is pure speculation on my part, but it seems that connecting the output of one of your Radians to the other is kinda like an AC coupling arrangement where the "second" Radian (assuming one is energized before the other, second one) will sync its AC output with the other. As such it would seem to be a workable idea, but beware (is it possible?) that if/when either Radian stops getting enough PV to invert to AC, if the other has sufficient PV that's not used by your loads, it may charge your battery through the "weaker" Radian. Or not...

This would be kind of an unorthodox setup and I'd guess that if anyone from OB were to chime in on this (hint), they'd say it's a bad idea...hoping for some expert thoughts.
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Re: Stacking - Other options?

Post by Kurt Lundquist » Mon Mar 25, 2019 7:28 am

If the two inverters are connected to the same battery bank the two Radian AC outputs could be set up in parallel with a master/slave configuration. If they are not on the same battery bank it will not work out.

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Re: Stacking - Other options?

Post by EMCF » Mon Mar 25, 2019 8:13 am

bear_essentials wrote:xxxxxx...If the first option is not possible, then can the one's output act as the grid input for the second?

Yes, that should safely work. Once the recipient Radian accepts the other Radian's AC input, you theoretically ties in the AC out (load). The load fed to either Radian is now being fed to both units.
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Re: Stacking - Other options?

Post by bear_essentials » Mon Mar 25, 2019 12:14 pm

So the follow on question is what parts if any do I need to replace such that the combined output of the to can be safely fed to the distribution panel?
It has been awhile since I read the Radian docs but I seem to recall that the inverter would supplement demands greater than 50amps using grid input. This would seem to allow the downstream Radian to supplement using the upstream Radian.
If IÔÇÖm trying to get as close as possible to the 100amp combined output, are ther some wires or breakers or other components that I need to replace?

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Re: Stacking - Other options?

Post by EMCF » Mon Mar 25, 2019 1:12 pm

I am rethinking my earlier opinion. I think that set-up where the output of one Radian goes to the AC In of the second Radian is a little more complicated than I thought. We want them to share the load equally, just like 2 paralleled generators, but what if the recipient Radian goes pass-thru? That will be just like one Radian unit supporting all the loads. Anyways, if you are brave enough to experiment, put the first inverter in Off-grid mode, the recipient inverter in Grid-tied mode, and hopefully it stays in an Off-set situation wherein battery and AC in are working together to support the loads.
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Re: Stacking - Other options?

Post by bear_essentials » Mon Mar 25, 2019 2:04 pm

I may never exceed the 50amps of a single Radian, so if that is the limit that I must live with then so be it.
At the very least, this setup should allow the downstream Radian to go into pass thru after it has exhausted its battery bank, thus doubling my total AHs. Additionally, it provides a convenient manual bypass of the downstream Radian for service.
It would be icing on the cake if the downstream Radian will also supplement (exceed 50amp output) using the upstream Radian as needed.

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Re: Stacking - Other options?

Post by Kurt Lundquist » Mon Mar 25, 2019 3:14 pm

Are the battery banks same type/age? The best long term solution would be to create a single system with the two inverters at a single location. While in theory feeding one inverter with the other can work I think it is rife with complications. If the downstream inverter is in GT mode it can sell more power back to the first inverter than the inverter can handle if all the loads are off. Even if your throttle the selling back to a reasonable limit do you really want to sell power from one battery bank back to the other? Do you want the charger from the downstream inverter to use charge its battery bank from the other inverter? Even in bypass the inverter can charge so you have to be on top of turning chargers on/off as necessary. In my opinion, operating an off-grid power system is complicated enough as it is. Imagine trying to tell a caretaker/friend/significant other on how to operate the system while you are gone. It seems like you are almost better off keeping the two load panels/systems isolated if you cannot combine the inverters on a single battery bank. My 2 cents.

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Re: Stacking - Other options?

Post by EMCF » Mon Mar 25, 2019 3:58 pm

Yes, Kurt's solution is the best and the "by the book" solution.

But if @bear really wants to experiment, the concerns aired can be mitigated, I think. To avoid selling by the downstream inverter, set the sell voltage to an unreal/unreachable voltage, like 60V, or the sell current to 0A, for double protection.
To avoid/disable one battery bank from charging the other battery bank, disable the charger for both Radians. Both Radians will just be dependent on their charge controller for charging needs. Just brainstorming \:D/
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Re: Stacking - Other options?

Post by bear_essentials » Mon Mar 25, 2019 4:37 pm

EMCF wrote:Yes, Kurt's solution is the best and the "by the book" solution.

But if @bear really wants to experiment, the concerns aired can be mitigated, I think. To avoid selling by the downstream inverter, set the sell voltage to an unreal/unreachable voltage, like 60V, or the sell current to 0A, for double protection.
To avoid/disable one battery bank from charging the other battery bank, disable the charger for both Radians. Both Radians will just be dependent on their charge controller for charging needs. Just brainstorming \:D/
Thanks. I hadn't thought about shutting off the downstream charging function, but that makes sense.

Is there a specific reason to prevent the downstream inverter from back feeding its surplus production to the upstream Radian? Seems to me that the upstream Radian might make use of that surplus power to charge its batteries. Is it because the upstream Radian may not be able to use the power and there is no control (i.e. diversion load?)

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Re: Stacking - Other options?

Post by EMCF » Tue Mar 26, 2019 9:49 am

bear_essentials wrote:Is there a specific reason to prevent the downstream inverter from back feeding its surplus production to the upstream Radian? Seems to me that the upstream Radian might make use of that surplus power to charge its batteries. Is it because the upstream Radian may not be able to use the power and there is no control (i.e. diversion load?)
If we allow back feed to the upstream inverter that is the "selling" that we are trying to avoid to happen.
Also, if we allow that (charging the upstream), it is like charging a battery bank by another battery bank, specially during night time, when there is no charging taking place using the charge controller.
I think your goal is achieve equal load distribution, and if you can achieve that when both outputs of Radian are in essence tied together using the piggyback set-up, you are good to go.
Again, all these are recommendations/professional opinion. Good luck.
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Re: Stacking - Other options?

Post by bear_essentials » Tue Mar 26, 2019 10:13 am

Thanks.

As I am reading up (catching back up to speed) on the Radian modes, I am wondering whether it is possible to setup the downstream Radian in Grid Zero mode and to split/feed the output of the upstream Radian both to the Grid Input of the downstream Radian (for sine wave synchup) and to the output buss bar of the downstream Radian? in Grid Zero mode, it looks like the downstream Radian can be configured such that it never actually tries to use grid power. Theoretically, this would split the load and keep the sine waves in synch. I just don't know whether the inverter might introduce a delay to the sine wave that it produces to the output, which could be a problem.
I probably need Outback's blessing on this before trying it.

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