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SMA Sunny Island grid-tie / battery backup with Sunny Boy inverters design discussion

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kbridgeman

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Requirements:
* Modestly large system > 12KW PV eventually to 20KW PV
* Grid-Tie sell-back
* 12KW PV / 8000 watts available when grid-down
* 15-20KW battery
* All UL listed components that will pass inspection.

Option 1: Outback Radian Flexpower with DC coupled arrays.
Option 2: Outback Radian AC bundle with 2 Sunny Boy 6K grid-tie inverters

Assumption: Outback Radian AC frequency shift capability will properly control the Sunny Boy grid-tie inverters.

I am considering the AC coupled solution thinking that if the Radian inverter fails, I can still have grid-tie inverters online presuming I bypass the Radian.
I am considering Radian because everything I read says it just works, " it's a beast", 20 years, yada yada.

Hope it's not obvious that I largely don't know what I'm talking about. Appreciate any constructive feedback, even if it means I said something really stupid here.

EDIT: For new readers, initial discussion as you can see asked about Outback Radian, but discussion exposed several misconceptions on my part and I quickly moved to a full SMA focused design.
 
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Just read an article today regarding Outback having a new system called the Mojave. Maybe worth checking it out.
Based on your requirements, I would recommend you also look at the Schneider Conext Pro 6848. Its a traditional low frequency work horse inverter that can be AC Coupled. Is priced reasonably and does have a built-in charger but NOT any built-in MPPT solar charge controllers, those are sold separately.
Sol-Ark is a possibility also
 
Also consider Sunny Island as your battery inverter.

Key capabilities of battery inverter:
Able to start your motor loads.
Peak shaving, shifting time when power goes to/from grid.

Sunny Island delivers 11 kW surge (for 3 seconds) per inverter. I don't think it has peak shaving features, at least not the current US model.
 
I have a radian GS4048 with a 35kwh battery both ac and dc coupled. A MidNite 200 DC coupled and 24 Enphase Micros and 3 APSystems QS1 quad micro AC coupled. The GS frequency shifts so smooth with the Enphase and works great. The APSystems when frequency shifted just shuts off for 5 minutes and then goes on and off like a switch (not recommended for frequency shifting those micro's in my opinion)..

The GS has to have the upgraded firmware when I did mine 2 years ago. The menus in the Mate3 can be confusing....There are multiple menus off the main, each item in the menu is for components that can be hooked up, but if you don't have that component the menu will still have the item listed, so it is confusing.
 
BentleyJ said:
Just read an article today regarding Outback having a new system called the Mojave. Maybe worth checking it out.
Based on your requirements, I would recommend you also look at the Schneider Conext Pro 6848. Its a traditional low frequency work horse inverter that can be AC Coupled. Is priced reasonably and does have a built-in charger but NOT any built-in MPPT solar charge controllers, those are sold separately.
Sol-Ark is a possibility also
Click to expand...
Thanks BentleyJ, I wasn't liking the required lithium batter with the Mojave. And, it just doesn't have a history.

Heard from this forum that Schneider isn't very supportive of DIY; they want professional installers. But, if I can get past the support question might be worth considering.
 
Thanks Hedges, Perhaps I have misinterpreted some of your posts. I have interpreted from your posts that the Sunny Island doesn't naturally switch from grid-tie to island in the sense that it doesn't frequency shift to keep the Sunny Boys active. I have come to that conclusion because you've said you need an off-grid password to enter into the Sunny Boys (I believe).

Understand, you were answering different questions, and specifically for your situation, so it's possible I applied something out of context.

But, I did like the SI/SB ecosystem, but couldn't get past "running back to my computer" to type in a code. Which didn't have the most "certified" feel to it.

Please explain where I went wrong. Cheers.
 
Thanks newbostonconst; did you add your GS4048 after having a grid-tie micro-inverter system or choose that architecture? Would you choose that architecture again despite the cost?
 
Hedges said:
Also consider Sunny Island as your battery inverter.

Key capabilities of battery inverter:
Able to start your motor loads.
Peak shaving, shifting time when power goes to/from grid.

Sunny Island delivers 11 kW surge (for 3 seconds) per inverter. I don't think it has peak shaving features, at least not the current US model.
Click to expand...
Getting the hang of responding to the requestor...so repeating this response presumably so Hedges can see it.

Thanks Hedges, Perhaps I have misinterpreted some of your posts. I have interpreted from your posts that the Sunny Island doesn't naturally switch from grid-tie to island in the sense that it doesn't frequency shift to keep the Sunny Boys active. I have come to that conclusion because you've said you need an off-grid password to enter into the Sunny Boys (I believe).

Understand, you were answering different questions, and specifically for your situation, so it's possible I applied something out of context.

But, I did like the SI/SB ecosystem, but couldn't get past "running back to my computer" to type in a code. Which didn't have the most "certified" feel to it.

Please explain where I went wrong. Cheers.
 
BentleyJ said:
Just read an article today regarding Outback having a new system called the Mojave. Maybe worth checking it out.
Based on your requirements, I would recommend you also look at the Schneider Conext Pro 6848. Its a traditional low frequency work horse inverter that can be AC Coupled. Is priced reasonably and does have a built-in charger but NOT any built-in MPPT solar charge controllers, those are sold separately.
Sol-Ark is a possibility also
Click to expand...
re: Sol-Ark - was my initial interest due to initial introduction by Engineer775 but issue with high-frequency vs low-frequency inverters and various niggling customer struggles measured against the "it just works" reviews and solid history for both Sunny I/B and Radian has given me pause on that front.

I'm convinced the 12K (9K) with inability to share load across the phase is too small for me, which means I'd wait for the 15K. Waiting isn't really a problem ... so I might consider revisiting this as of course the DIY support seems fantastic.

However, would require getting past my concern that an all-in-one potentially not being a resilient architecture; i.e. if something happens to the all-in-one your system is down. If something happens to your inverter in an ac-coupled system you can reconfigure without the inverter and still generate power while you work to resolve the problem.

Hence the genesis of my question: perhaps it would have been better to simply start with a more simplified question without the focus on a vendor; i.e. : AC Coupling or DC Coupling into an all-in-one?

With the Radian FlexPower, seems like you can swap out the MPPT charge controllers and even the computer control system if something goes wrong, making it a bit more resilient to a failure. But again, looking to test my assumptions.
 
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kbridgeman said:
Thanks newbostonconst; did you add your GS4048 after having a grid-tie micro-inverter system or choose that architecture? Would you choose that architecture again despite the cost?
Click to expand...
I keep adding on as I get time and money....

Note, GS4048 and maybe others have an issue where if you wear down the batteries to much and the unit turns off for low voltage they don't turn back on even with solar coming back on.....you have to charge the batteries....DC coupling takes care of this.
 
kbridgeman said:
Getting the hang of responding to the requestor...so repeating this response presumably so Hedges can see it.

Thanks Hedges, Perhaps I have misinterpreted some of your posts. I have interpreted from your posts that the Sunny Island doesn't naturally switch from grid-tie to island in the sense that it doesn't frequency shift to keep the Sunny Boys active. I have come to that conclusion because you've said you need an off-grid password to enter into the Sunny Boys (I believe).

Understand, you were answering different questions, and specifically for your situation, so it's possible I applied something out of context.

But, I did like the SI/SB ecosystem, but couldn't get past "running back to my computer" to type in a code. Which didn't have the most "certified" feel to it.

Please explain where I went wrong. Cheers.
Click to expand...

That would be about configuring Sunny Boys one time during installation, which don't have any front panel buttons.
A data terminal, PC, or phone with Bluetooth is needed to configure SB. A user password can protect settings. Installer password would be required to change on/off-grid mode in the future; it is changeable first few hours of operation, then becomes locked. Installer password also required to mess with parameters related to grid voltage/frequency/timing compliance to code.

Sunny Island has a backlit LCD display and four button keypad. That is all that's required to configure it and alter any parameters.
(grid tie/generator/standalone, FLA/VRLA/Lithium, battery volts, Ah, auxiliary relays, etc. etc.)
Alternatively, several RS-485 and Ethernet connection options for configuration & monitoring, and can be grid connected with a gateway. Web based monitoring and SMA support.
SI is easy to set up without any other gizmo required.

Older Sunny Boys had three modes: UL-1741 grid tie/grid-backup/off-grid

Backup and off-grid tolerate a wider frequency and voltage range, including if you use a generator feeding Sunny Island. To simplify installation, SMA started shipping them with grid backup enabled, so you just hook up Sunny Boy (AC wires, and if used with Sunny Island RS-485). Nothing else to do in that case; if signal present on RS-485, SB knows SI is present and when to switch from tight on-grid limits to wide off-grid limits. Off-grid, SB remains connected even if voltage sags a lot and frequency goes up to 64 Hz, ready to respond immediately when it returns to normal.

The newer -40 and -41 SB don't have RS-485. But they have UL-1741-SA frequency-watts. With frequency-watts enabled (I think it is an option you have to select through Ethernet or Bluetooth from a phone), the newer SB will respond to frequency shift within tighter limits. I'm not sure, but if frequency or voltage goes far out of range it would disconnect for 5 minutes. Same as any other UL-1741-SA inverter with frequency-watts.

The newer -40 and -41 SB could alternatively be set for strictly off-grid, where they would tolerate wider voltage and frequency variations. That is not for use in a grid-tied system.

So all the hassle is in setting Sunny Boy, not Sunny Island. It is likely any other AC coupled grid tie inverter has the same issue; would possibly need to be updated to current firmware and have frequency-watts enabled.

The Sunny Island/Sunny Boy system seems to work quite well. What I don't think it supports is shifting time of import/export from grid (except by disconnecting from grid, using "grid charge" mode), and doesn't do limited export. The Sunny Boys do support zero export and limited export by means of Ethernet connectivity to a gateway with 3rd party power meter connected. Probably that works fine whether Sunny Island is used or not. SMA talks about playing in spot markets for power and remote monitoring with their Sunny Portal, but I don't use those.

If you do AC coupling only, no DC coupling, you should put a "load shed" relay between Sunny Island and your loads. It is mentioned in manual but not shown in data sheet diagram. That lets loads be disconnected but Sunny Boys remain connected so discharged battery gets recharged in the morning. Sunny Island can't recharge from grid (or generator) unless Sunny Island can generate AC output, so needs a way to make sure loads are less than generation. This is because it doesn't just close relay to grid, it first generates AC, watches grid for 5 minutes, synchronizes, then connects relay.

I went with older model Sunny Boys like 5000US because my grandfathered installation didn't need to be UL-1741-SA, and for a while SMA's compatibility list said the -41 models were only for on-grid or off-grid, not grid-backup. SMA has since included grid-backup for the -41 models (probably because they implemented frequency-watts in a firmware update.)
 
Hedges said:
That would be about configuring Sunny Boys one time during installation, which don't have any front panel buttons.
A data terminal, PC, or phone with Bluetooth is needed to configure SB. A user password can protect settings. Installer password would be required to change on/off-grid mode in the future; it is changeable first few hours of operation, then becomes locked. Installer password also required to mess with parameters related to grid voltage/frequency/timing compliance to code.

Sunny Island has a backlit LCD display and four button keypad. That is all that's required to configure it and alter any parameters.
(grid tie/generator/standalone, FLA/VRLA/Lithium, battery volts, Ah, auxiliary relays, etc. etc.)
Alternatively, several RS-485 and Ethernet connection options for configuration & monitoring, and can be grid connected with a gateway. Web based monitoring and SMA support.
SI is easy to set up without any other gizmo required.

Older Sunny Boys had three modes: UL-1741 grid tie/grid-backup/off-grid

Backup and off-grid tolerate a wider frequency and voltage range, including if you use a generator feeding Sunny Island. To simplify installation, SMA started shipping them with grid backup enabled, so you just hook up Sunny Boy (AC wires, and if used with Sunny Island RS-485). Nothing else to do in that case; if signal present on RS-485, SB knows SI is present and when to switch from tight on-grid limits to wide off-grid limits. Off-grid, SB remains connected even if voltage sags a lot and frequency goes up to 64 Hz, ready to respond immediately when it returns to normal.

The newer -40 and -41 SB don't have RS-485. But they have UL-1741-SA frequency-watts. With frequency-watts enabled (I think it is an option you have to select through Ethernet or Bluetooth from a phone), the newer SB will respond to frequency shift within tighter limits. I'm not sure, but if frequency or voltage goes far out of range it would disconnect for 5 minutes. Same as any other UL-1741-SA inverter with frequency-watts.

The newer -40 and -41 SB could alternatively be set for strictly off-grid, where they would tolerate wider voltage and frequency variations. That is not for use in a grid-tied system.

So all the hassle is in setting Sunny Boy, not Sunny Island. It is likely any other AC coupled grid tie inverter has the same issue; would possibly need to be updated to current firmware and have frequency-watts enabled.

The Sunny Island/Sunny Boy system seems to work quite well. What I don't think it supports is shifting time of import/export from grid (except by disconnecting from grid, using "grid charge" mode), and doesn't do limited export. The Sunny Boys do support zero export and limited export by means of Ethernet connectivity to a gateway with 3rd party power meter connected. Probably that works fine whether Sunny Island is used or not. SMA talks about playing in spot markets for power and remote monitoring with their Sunny Portal, but I don't use those.

If you do AC coupling only, no DC coupling, you should put a "load shed" relay between Sunny Island and your loads. It is mentioned in manual but not shown in data sheet diagram. That lets loads be disconnected but Sunny Boys remain connected so discharged battery gets recharged in the morning. Sunny Island can't recharge from grid (or generator) unless Sunny Island can generate AC output, so needs a way to make sure loads are less than generation. This is because it doesn't just close relay to grid, it first generates AC, watches grid for 5 minutes, synchronizes, then connects relay.

I went with older model Sunny Boys like 5000US because my grandfathered installation didn't need to be UL-1741-SA, and for a while SMA's compatibility list said the -41 models were only for on-grid or off-grid, not grid-backup. SMA has since included grid-backup for the -41 models (probably because they implemented frequency-watts in a firmware update.)
Click to expand...
Lots to unpack there, thank you.

I'll start with the Load Shed Relay. Configured like this? Apologies for lack of diagram; hopefully it gets the idea across.

[Critical Loads Panel] <--> [Load Shed Relay] <--> [Sunny Boy] <--> [Battery Inverter] <--> [Main Panel] <--> [Grid]

Something about this architecture feels off because I don't think I've seen (or can find) a Sunny Island diagram with a critical loads panel.
 
Like that, except I'll call it "battery-backed loads panel" that gets disconnected by load-shed relay.

Sunny Boys should have a breaker, so connect to a breaker panel. Optionally include truly critical loads, if small enough they can run all night together with Sunny Island no-load current draw, using < 10% of battery.

[battery-backed Loads Panel] <--> [Load Shed Relay] <--> [Critical Loads Panel (Sunny Boy, alarm, telecom)] <--> [Battery Inverter] <--> [Main Panel] <--> [Grid]

For whatever reason, SMA offers simplified block diagrams, but details like load-shed relay and which SI terminals AC2 input vs. AC1 output are only documented as text or zoomed in images. There ought to be a one page schematic showing all AC connections including which terminal from grid to load and everything in between. I'm sure most of their business is big installers.

Each SI has two signaling relays, so with a pair of them you'll have four.
Two load-shed levels can be set; one could disconnect discretionary loads like A/C at higher SoC.
Others can start a backup generator (you would need a transfer switch if both grid and generator used), enable battery fan, etc.

What I would like but don't find as a commercial product is loads that do frequency-watts, like heater, water pump, A/C which could linearly ramp up consumption as frequency increases (make use of surplus, before curtailing excess PV production.)
 
Hedges said:
Like that, except I'll call it "battery-backed loads panel" that gets disconnected by load-shed relay.

Sunny Boys should have a breaker, so connect to a breaker panel. Optionally include truly critical loads, if small enough they can run all night together with Sunny Island no-load current draw, using < 10% of battery.

[battery-backed Loads Panel] <--> [Load Shed Relay] <--> [Critical Loads Panel (Sunny Boy, alarm, telecom)] <--> [Battery Inverter] <--> [Main Panel] <--> [Grid]

For whatever reason, SMA offers simplified block diagrams, but details like load-shed relay and which SI terminals AC2 input vs. AC1 output are only documented as text or zoomed in images. There ought to be a one page schematic showing all AC connections including which terminal from grid to load and everything in between. I'm sure most of their business is big installers.

Each SI has two signaling relays, so with a pair of them you'll have four.
Two load-shed levels can be set; one could disconnect discretionary loads like A/C at higher SoC.
Others can start a backup generator (you would need a transfer switch if both grid and generator used), enable battery fan, etc.

What I would like but don't find as a commercial product is loads that do frequency-watts, like heater, water pump, A/C which could linearly ramp up consumption as frequency increases (make use of surplus, before curtailing excess PV production.)
Click to expand...
ah, I see your point. we need a panel to land the sunny boys with breakers. That panel can be considered Tier 1 battery backup critical loads, and the load shed panel is the Tier 2 battery backup critical loads (AC, heatpump, et.al.). i.e. if battery is too low we do not want the heavy hitters drawing current we want that current to getting batteries up to minimum level first.

On the one hand, we went down a rabbit hole there, but on the other hand, that's usually how I design stuff, design/test/adjust/repeat. And, to choose the right products one needs to know how they are going to be used within the architecture.

I'll go further down that path at another time asking about disconnects and emergency shut-down.

Do you still prefer your all AC-coupled system versus a DC-coupled system? I anticipate your answer is yes, but would be curious about your pros and cons rather than guess. What advantage would a DC-coupled system bring, if anything, that you admire?
 
Hedges said:
That would be about configuring Sunny Boys one time during installation, which don't have any front panel buttons.
A data terminal, PC, or phone with Bluetooth is needed to configure SB. A user password can protect settings. Installer password would be required to change on/off-grid mode in the future; it is changeable first few hours of operation, then becomes locked. Installer password also required to mess with parameters related to grid voltage/frequency/timing compliance to code.

Sunny Island has a backlit LCD display and four button keypad. That is all that's required to configure it and alter any parameters.
(grid tie/generator/standalone, FLA/VRLA/Lithium, battery volts, Ah, auxiliary relays, etc. etc.)
Alternatively, several RS-485 and Ethernet connection options for configuration & monitoring, and can be grid connected with a gateway. Web based monitoring and SMA support.
SI is easy to set up without any other gizmo required.

Older Sunny Boys had three modes: UL-1741 grid tie/grid-backup/off-grid

Backup and off-grid tolerate a wider frequency and voltage range, including if you use a generator feeding Sunny Island. To simplify installation, SMA started shipping them with grid backup enabled, so you just hook up Sunny Boy (AC wires, and if used with Sunny Island RS-485). Nothing else to do in that case; if signal present on RS-485, SB knows SI is present and when to switch from tight on-grid limits to wide off-grid limits. Off-grid, SB remains connected even if voltage sags a lot and frequency goes up to 64 Hz, ready to respond immediately when it returns to normal.

The newer -40 and -41 SB don't have RS-485. But they have UL-1741-SA frequency-watts. With frequency-watts enabled (I think it is an option you have to select through Ethernet or Bluetooth from a phone), the newer SB will respond to frequency shift within tighter limits. I'm not sure, but if frequency or voltage goes far out of range it would disconnect for 5 minutes. Same as any other UL-1741-SA inverter with frequency-watts.

The newer -40 and -41 SB could alternatively be set for strictly off-grid, where they would tolerate wider voltage and frequency variations. That is not for use in a grid-tied system.

So all the hassle is in setting Sunny Boy, not Sunny Island. It is likely any other AC coupled grid tie inverter has the same issue; would possibly need to be updated to current firmware and have frequency-watts enabled.

The Sunny Island/Sunny Boy system seems to work quite well. What I don't think it supports is shifting time of import/export from grid (except by disconnecting from grid, using "grid charge" mode), and doesn't do limited export. The Sunny Boys do support zero export and limited export by means of Ethernet connectivity to a gateway with 3rd party power meter connected. Probably that works fine whether Sunny Island is used or not. SMA talks about playing in spot markets for power and remote monitoring with their Sunny Portal, but I don't use those.

If you do AC coupling only, no DC coupling, you should put a "load shed" relay between Sunny Island and your loads. It is mentioned in manual but not shown in data sheet diagram. That lets loads be disconnected but Sunny Boys remain connected so discharged battery gets recharged in the morning. Sunny Island can't recharge from grid (or generator) unless Sunny Island can generate AC output, so needs a way to make sure loads are less than generation. This is because it doesn't just close relay to grid, it first generates AC, watches grid for 5 minutes, synchronizes, then connects relay.

I went with older model Sunny Boys like 5000US because my grandfathered installation didn't need to be UL-1741-SA, and for a while SMA's compatibility list said the -41 models were only for on-grid or off-grid, not grid-backup. SMA has since included grid-backup for the -41 models (probably because they implemented frequency-watts in a firmware update.)
Click to expand...
Hedges, what do you know about pairing the Sunny Island with LiFePo4 batteries? That should have been in my requirements. Docs say you can use Lithium Ion
Hedges said:
Like that, except I'll call it "battery-backed loads panel" that gets disconnected by load-shed relay.

Sunny Boys should have a breaker, so connect to a breaker panel. Optionally include truly critical loads, if small enough they can run all night together with Sunny Island no-load current draw, using < 10% of battery.

[battery-backed Loads Panel] <--> [Load Shed Relay] <--> [Critical Loads Panel (Sunny Boy, alarm, telecom)] <--> [Battery Inverter] <--> [Main Panel] <--> [Grid]

For whatever reason, SMA offers simplified block diagrams, but details like load-shed relay and which SI terminals AC2 input vs. AC1 output are only documented as text or zoomed in images. There ought to be a one page schematic showing all AC connections including which terminal from grid to load and everything in between. I'm sure most of their business is big installers.

Each SI has two signaling relays, so with a pair of them you'll have four.
Two load-shed levels can be set; one could disconnect discretionary loads like A/C at higher SoC.
Others can start a backup generator (you would need a transfer switch if both grid and generator used), enable battery fan, etc.

What I would like but don't find as a commercial product is loads that do frequency-watts, like heater, water pump, A/C which could linearly ramp up consumption as frequency increases (make use of surplus, before curtailing excess PV production.)
Click to expand...
do you have a link on here where you describe your configuration or better have a detailed wiring diagram? Could learn more just looking at that versus asking random questions.
 
I haven't made a diagram. It is based on block diagram of SMA's SI data sheet, but has extra breaker panels upstream and downstream, with various interlocked breakers bypassing and backfeeding for manual options.





On the Tier 2 backup I'd put refrigerator, other kitchen appliances, TV, Computer ...
For Tier 3, heavy loads like A/C, electric heat, clothes dryer. Possibly not a high power load-shed relay, just control thermostat of HVAC.

I'm happy with AC coupling. It comes from starting with most efficient grid-tie net metering, which is having PV voltage a bit above AC line voltage with a string inverter. For grid-backup, normal use is driving 100% of PV to AC. Only when grid is down does power cycle into batteries. My PV array is oversize and battery is undersize (because I don't normally rely on it), and battery charging efficiency doesn't matter. Only about 40% of available PV ever goes to battery. The rest either directly powers loads like A/C or is curtailed.

Having multiple components means system can keep working if one goes down. Multiple SB, one just goes offline. I have four SI wired 2s2p, so if a slave went down I think it keeps working, but might exceed pass-through relay on grid so need to manage. If master goes down I can reconfigure another. (I should wire a couple RJ-45 plugs and extra temperature sensor, maybe an RJ-45 switch, so reconfiguring is simplified.) Having an autotransformer wired to a breaker on critical loads/SB panel would allow operation with just one SI.

The other day I knocked both slaves on second phase offline. Zero PV production and only 120V being produced. I'd been messing around with a circuit on the main panel and created a "bonded fault" which fast tripped three circuit breakers. The slaves saw power go away and return within milliseconds and thought their relay was stuck. I pressed their front panel button, also stopped/started master (which may not have been necessary) and system came back.

If primary mode of use is to charge batteries while the sun shines, use the power at night, then DC coupled is more efficient (a few percent.)

I picked up a couple Sunny Island Charger SIC-40, made by MSTE. With a daughter card, they daisy chain to SI, which controls voltage and lead-acid charge mode. Midnight Classic can emulate the same with an adapter board. It may no longer be offered, because SI changed their protocol. I'm considering picking up a couple in case I want to use a Classic in the future with my (old firmware) SI. Alternative is to connect a battery shunt to SI (but that only communicates current, doesn't provide control.) I've got mine on a "test rig" with one 5048US for now. Considering making it mobile (120V output, so 240V SB is less convenient, went with DC.)

Mixing AC and DC coupling with SI could mean not needing a load-shed relay. If you hit 80% DoD shutdown, SI shuts off, SB shuts off, and only DC coupling is available to recharge back up to 50% DoD. I think you need to manually restart. So I would recommend load shedding anyway. 2-stage is preferred.

Something like SolArk has PV MPPT to high voltage DC rail. Inverter runs off that, and battery charges/discharges with DC/DC converter to that rail. Should offer similar efficiency for PV to AC, maybe better to battery. Maybe not quite as high as DC coupled direct to battery.

SI supports lithium battery. There is a fair size list of commercial offerings. For DIY, REC BMS is available, about $500.

Found on the SMA German web site, scroll down to page 5 for 6048-US


REC, one of their models is for SI

BATTERY MANAGEMENT SYSTEM – REC BMS

www.rec-bms.com www.rec-bms.com

www.offgridsoftwaresolutions.com

REC SI-Q Series BMS 16S - Off Grid & Marine Energy Systems

Select your options to build you own custom BMS kit and save 5% on all accessories! Works with most cell chemistries and 5-16 Cell packs with a 16-62 VDC range.
www.offgridsoftwaresolutions.com www.offgridsoftwaresolutions.com
 
Like Hedges, I also have a couple of Sunny Island inverters as well as two 6kW SB6.0-US-40 Sunny Boy grid tied inverters. We use a 25kWh lithium battery bank and the system works very smoothly.. Sunny Islands can power pretty much anything and everything and at the same time.

I also have an Outback Radian 8048 as a backup unit along with FlexMax 80 charge controllers, but I don't use them.. they're just backup.

Only thing I can tell you beyond what Hedges said is that AC Coupling is a better way to go than DC coupling.. fewer components, fewer problems, more efficient when pumping into the grid.

DC Coupling a lithium ion battery isn't a good idea.. not catastrophically bad, but AC coupling is better. AC coupling prevents energy exchange with the battery system when its full. and while lead acid cells don't mind that kind of shallow cycling, lithium batteries don't like it.
 
Hedges said:
I haven't made a diagram. It is based on block diagram of SMA's SI data sheet, but has extra breaker panels upstream and downstream, with various interlocked breakers bypassing and backfeeding for manual options.





On the Tier 2 backup I'd put refrigerator, other kitchen appliances, TV, Computer ...
For Tier 3, heavy loads like A/C, electric heat, clothes dryer. Possibly not a high power load-shed relay, just control thermostat of HVAC.

I'm happy with AC coupling. It comes from starting with most efficient grid-tie net metering, which is having PV voltage a bit above AC line voltage with a string inverter. For grid-backup, normal use is driving 100% of PV to AC. Only when grid is down does power cycle into batteries. My PV array is oversize and battery is undersize (because I don't normally rely on it), and battery charging efficiency doesn't matter. Only about 40% of available PV ever goes to battery. The rest either directly powers loads like A/C or is curtailed.

Having multiple components means system can keep working if one goes down. Multiple SB, one just goes offline. I have four SI wired 2s2p, so if a slave went down I think it keeps working, but might exceed pass-through relay on grid so need to manage. If master goes down I can reconfigure another. (I should wire a couple RJ-45 plugs and extra temperature sensor, maybe an RJ-45 switch, so reconfiguring is simplified.) Having an autotransformer wired to a breaker on critical loads/SB panel would allow operation with just one SI.

The other day I knocked both slaves on second phase offline. Zero PV production and only 120V being produced. I'd been messing around with a circuit on the main panel and created a "bonded fault" which fast tripped three circuit breakers. The slaves saw power go away and return within milliseconds and thought their relay was stuck. I pressed their front panel button, also stopped/started master (which may not have been necessary) and system came back.

If primary mode of use is to charge batteries while the sun shines, use the power at night, then DC coupled is more efficient (a few percent.)

I picked up a couple Sunny Island Charger SIC-40, made by MSTE. With a daughter card, they daisy chain to SI, which controls voltage and lead-acid charge mode. Midnight Classic can emulate the same with an adapter board. It may no longer be offered, because SI changed their protocol. I'm considering picking up a couple in case I want to use a Classic in the future with my (old firmware) SI. Alternative is to connect a battery shunt to SI (but that only communicates current, doesn't provide control.) I've got mine on a "test rig" with one 5048US for now. Considering making it mobile (120V output, so 240V SB is less convenient, went with DC.)

Mixing AC and DC coupling with SI could mean not needing a load-shed relay. If you hit 80% DoD shutdown, SI shuts off, SB shuts off, and only DC coupling is available to recharge back up to 50% DoD. I think you need to manually restart. So I would recommend load shedding anyway. 2-stage is preferred.

Something like SolArk has PV MPPT to high voltage DC rail. Inverter runs off that, and battery charges/discharges with DC/DC converter to that rail. Should offer similar efficiency for PV to AC, maybe better to battery. Maybe not quite as high as DC coupled direct to battery.

SI supports lithium battery. There is a fair size list of commercial offerings. For DIY, REC BMS is available, about $500.

Found on the SMA German web site, scroll down to page 5 for 6048-US


REC, one of their models is for SI

BATTERY MANAGEMENT SYSTEM – REC BMS

www.rec-bms.com www.rec-bms.com

www.offgridsoftwaresolutions.com

REC SI-Q Series BMS 16S - Off Grid & Marine Energy Systems

Select your options to build you own custom BMS kit and save 5% on all accessories! Works with most cell chemistries and 5-16 Cell packs with a 16-62 VDC range.
www.offgridsoftwaresolutions.com www.offgridsoftwaresolutions.com
Click to expand...
thx. will take some time to consume this. a short term lack of response doesn't mean I'm ignoring it. many questions to come. cheers.
 
MurphyGuy said:
Like Hedges, I also have a couple of Sunny Island inverters as well as two 6kW SB6.0-US-40 Sunny Boy grid tied inverters. We use a 25kWh lithium battery bank and the system works very smoothly.. Sunny Islands can power pretty much anything and everything and at the same time.

I also have an Outback Radian 8048 as a backup unit along with FlexMax 80 charge controllers, but I don't use them.. they're just backup.

Only thing I can tell you beyond what Hedges said is that AC Coupling is a better way to go than DC coupling.. fewer components, fewer problems, more efficient when pumping into the grid.

DC Coupling a lithium ion battery isn't a good idea.. not catastrophically bad, but AC coupling is better. AC coupling prevents energy exchange with the battery system when its full. and while lead acid cells don't mind that kind of shallow cycling, lithium batteries don't like it.
Click to expand...
Thanks Murphy, I am convinced on the AC coupling.

I'm almost embarrassed to ask this question on battery that's confusing me. In SMA docs and your and Hedges references everything is Lithium Ion. I have never seen a reference to LiFePo4 Lithium Phosphate, always Lithium or Lithium Ion.

So, does SMA support Lithium Phosphate batteries?

There is an article from Fortress Power that implies it does, but this is the only reference I've seen.
 
There is a max amount of AC-coupled solar you can put on the output side of a hybrid inverter and it depends on each units spec.

On the grid side of the inverter you can put as many micros/inverters as your breaker box is allowed.
 
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