For completely off-grid SMA-powered system, why do you need both PV Inverter and Battery Inverter?

[Thingol]

Hello folks,

I am working on the design of my off-grid system in SMA Web Design tool. One thing I find confusing is why I need both a PV inverter (Sunny boy 6 for example) as well as a storage inverter (Sunny Island). I thought that modern inverters integrate DC-AC. My System would be similar to what is shown here in this SMA guide under 2.3.1 Single System. But if you look at that system, there is a PV inverter as well as a battery inverter. Why do you need both? My expectation was that modern inverters are able to integrate DC input from PV panels + batteries and its conversion to AC, SOC monitoring and battery charging all in one unit. Is that not what Growatt off-grid inverters do?

 

[toms]

You can use a DC Solar controller if you want. Midnite Classic, and SMA SIC50 are compatible DC chargers with the Sunny Island.

 

[Thingol]

You can use a DC Solar controller if you want. Midnite Classic, and SMA SIC50 are compatible DC chargers with the Sunny Island.

Thanks Toms! Yeah, I am attempting to create a DC coupled system like the one shown here. What I find confusing is that the configuration does not appear to be supported in SMA Design tool- checking again to see if I can add SMA SIC50. I also thought that the sunny island had a charge controller within but I now understand that this function would have to be handled separately (SMA SIC50) just like with traditional systems. Cheers.

 

[Zil]

Depending how large a system you want. One or many charging sources can be used to charge a battery bank. The battery bank will power an inverter to provide alternating current. I don't understand the need for an inverter for each charging device.

 

[toms]

I don’t see anyone saying there is a need for an inverter for each charging device?

With SMA, you need Sunny Island inverters in parallel to cover your load requirements. (ie one per 6kw load) then you can use DC coupled chargers, or AC coupled (Sunnyboy inverter) chargers to match your PV to your storage.

 

[Hedges]

With DC coupling, your battery must be able to accept all charge current available. Sunny Island can't regulate that, only control phases of charging and voltage. So if you have too much PV and too little a battery, could cook it.

With AC coupling, Sunny Island uses frequency shift to request reduced power. You can have quite large PV system and small battery. SMA recommends at least 100 Ah per 1000W of PV, but my battery is considerably smaller. I think the issue is primarily how quickly it can respond to a decrease in load.

Also with AC coupling, you can make use of AC power from both PV inverters and battery inverters, good for larger daytime loads.
For instance, you can have 24 kW of PV inverter and 12 kW of battery inverter.

 

[Thingol]

Thanks all. The thing is, I was not at all familiar with the so called AC-coupled systems as everything I worked with before was DC coupled with a charge controller, battery bank then inverter. To have a DC coupled system with the Sunny Island for a 6kw PV array, I would need at least 3 Sunny SMA SIC50 chargers that will direct my the PV DC into the Sunny Island (see SMA SIC50 charger specs here). The total price of all 3 chargers would be well-near that of a single 6KW PV inverter that can AC couple to the Sunny Island. If I were to stick with SMA, then the ideal solution would be an AC coupled system with a Sunny Boy PV Inverter making AC from PV output and feeding all load (load here including Sunny Island's battery bank). SMA magic then decides how to feed house load (from Sunny boy PV inverter when PV output is at max OR from Sunny Island battery inverter or both...). But this is expensive...

The next best thing would be the Victron Easysolar 5KW. This is truly an all in one off-grid solution. It combines the MPPT charge controller, pure sine wave inverter and AC distribution unit all in 1. Earlier on, I mistook the Sunny Island as being equivalent to this- but no with Sunny Island, you need a separate PV Inverter (ideally) or Sunny Island charger (much less ideal). I would go with Victron EasySolar but Victron's support, service and warranty appears to be a far cry from SMA's and so I am still undecided.

 

[Hedges]

Are you in the US, with 120/240V split phase 60 Hz?
Or Europe/other parts of the world 220V 50 Hz?

In the US, Sunny Island is 120V so you either need two of them or a 120/240V transformer to work with 240V Sunny Boy.
SIC50 is harder to come by here.
Plenty of used and new-in-the-box surplus Sunny Island and Sunny Boy, a real bargain.
One 6kW Sunny Boy can sometimes be had for less than a single 3kW Midnight Classic charge controller.

Elsewhere, Sunny Island is 220V so only one needed.
I don't know the used/surplus market there.

Sunny Island/Sunny Boy makes a great system. It adjusts PV production to match battery charging and load. It controls regulation of lead-acid batteries very well. For Lithium it is best to have a compatible BMS (not cheap), and current is then regulated. (Charging lithium with charge controllers that don't receive instructions from BMS would be a problem.)

For a system larger than about 5kW, you wouldn't want all in one - moving several 75 pound to 140 pound units individually is easier, unless you have forklift access.
I've got about 1200 pounds of inverters (and 1000 pounds of batteries) installed.

 

[Thingol]

Are you in the US, with 120/240V split phase 60 Hz?
Or Europe/other parts of the world 220V 50 Hz?

In the US, Sunny Island is 120V so you either need two of them or a 120/240V transformer to work with 240V Sunny Boy.
SIC50 is harder to come by here.
Plenty of used and new-in-the-box surplus Sunny Island and Sunny Boy, a real bargain.
One 6kW Sunny Boy can sometimes be had for less than a single 3kW Midnight Classic charge controller.

Elsewhere, Sunny Island is 220V so only one needed.
I don't know the used/surplus market there.

Sunny Island/Sunny Boy makes a great system. It adjusts PV production to match battery charging and load. It controls regulation of lead-acid batteries very well. For Lithium it is best to have a compatible BMS (not cheap), and current is then regulated. (Charging lithium with charge controllers that don't receive instructions from BMS would be a problem.)

For a system larger than about 5kW, you wouldn't want all in one - moving several 75 pound to 140 pound units individually is easier, unless you have forklift access.
I've got about 1200 pounds of inverters (and 1000 pounds of batteries) installed.

Thanks a lot for the input!! Actually the project site is in Kenya so we are talking 240V/50hz. How do the "new in-the-box" Sunny Boys and Islands come about? If you have any links to some websites, I'll certainly have a look. Also, my system is pretty small: 6KW. The system you have got is massive!! I do not foresee installation difficulties with anything below 100Kg. What do you think of the Victron Easy Solar all in 1?

 

[Hedges]

eBay in the US has a lot of 120V Sunny Island due to DC Solar bankruptcy auction. They have been between $1500 and $2500 each recently.
Sunny Boy also on eBay but from other vendors. Some are used, some are old models not providing the features required by latest code for grid connection. Sunny Boy is usually good for 240V or 208V, 60 Hz, and should be adjustable for 50 Hz as well. That may require a password and interface device. Some newer models maybe just a switch, no password needed unless they've already been operated.

Unlike Sunny Boy, I don't think Sunny Island can simply be reprogrammed for 220V, because I presume it has a different transformer inside. But it could be programmed for 50 Hz (I think) and used with a transformer to make 220V.

Craigslist is another place I see PV equipment including SMA offered.

One of the deals is a trailer with two forklift batteries, two Sunny Island, one Midnight charge controller, 2400W of PV panels. Some have a diesel generator. That's a complete mobile power system. I've seen them offered around $10,000 if no generator.

I made mine big just because I could. There were bargains. US models are limited as to how much PV inverter can be connected to a grid-tied Sunny Island because its 56A relay is used at 120V. Same hardware elsewhere, 56A at 220V. I put in two extra Sunny Island mostly to get 112A 120/240V feed through from the grid. So my whole house is the "protected load", didn't need to separate out some loads.

There may be even smaller Sunny Islands (48V battery) for non-US markets. Also Sunny Boy Storage (400V lithium).
Be prepared for "sticker shock". I don't know if any deals are available.

Sunny Island 4.4M / 6.0H / 8.0H | SMA Solar

www.sma.de

Sunny Boy Storage 2.5 | SMA Solar

www.sma.de

Sunny Boy Storage 3.7 / 5.0 / 6.0 | SMA Solar

www.sma.de

I haven't used Victron. Some people say they are great.

 

[toms]

With DC coupling, your battery must be able to accept all charge current available. Sunny Island can't regulate that, only control phases of charging and voltage. So if you have too much PV and too little a battery, could cook it.

Just to clarify, both the Midnite classic and SIC50 can be controlled by the Sunny Island to prevent battery overcharge when DC coupled.

As has been pointed out, for bigger systems (over 2kw PV), AC coupling is a better option.

Where DC coupling is great with SMA is to add on extra PV as a DIY option with second hand panels after you already have a completed professionally installed AC coupled system.

 

[Hedges]

Any charge controller can charge the battery with its own profile, and should have a temperature sensor (at least for lead-acid). By using a shunt, Sunny Island can measure the current and keep track of state of charge, but it has no control over current (only voltage).

Either SIC50 or Midnight Classic can be fitted with a data interface so it daisy chains with the Sunny Island. In that case charging phase and voltage are controlled by Sunny Island, which has a temperature sensor.

But in either case, Sunny Island has no control over bulk charging current coming from the charge controller. Sunny Island might draw zero to 100A depending on how much AC load. With 100 Ah of battery, a charging current of 20A (0.2C) might be optimum. Is your PV array and charge controller capable of delivering 40A, 2kW? If so, the battery will be subjected to 40A (0.4C) charge current in the event Sunny Island isn't powering any AC loads. The one thing Sunny Island can do is choose not to add additional charging current from a generator (or AC coupled loads)

With AC coupling, Sunny Island uses frequency shift to tell AC coupled inverters to adjust their power output to exactly what is needed for loads plus battery charging. You could have a 10 kW PV array instructed to deliver 7kW, powering 5kW of AC loads and 2kW of bulk charging. Then, as battery voltages comes up and it enters absorption stage, then float, the AC coupled PV further reduces its current.

I suggest having only enough DC coupled PV to be within allowable battery charge current, not over. The rest (or all) PV should be AC coupled.

That's for SMA. I understand Victron has communication between charge controllers and battery inverter to regulate charge current.

 

[the_colorist]

To elaborate on Hedges point with the shunt: I have installed and calibrated shunts on SI systems and although I have not tested this, as far as I know, the SI will measure the total current entering the bank and not exceed the total maximum set in the SI. So if the max is 40A (set in the SI settings) and it's reading 30A charging from the CC, it will only add 10A. Now again, I have NOT tested this but I believe this to be the case.

In that situation, a person would need to manually set a max charge rate on the CC. Also with the Classics, as far as I remember, if you have a WhizBang Jr installed, you can set separate charge and load max amperages. Again untested but I remember a note about that from either MidNite support or perhaps someone else on their forum.

A 6KW SB + 6.0H SI will set you back around $4K before installation.

A Victron Multiplus II 5KW with a 250/100 Charge Controller would be close to ~$2,400 before accessories/installation. Now that is only a 5KW inverter with a 100A charge controller. The 250/100 is specced for a nominal input of 5800W. The output wattage will be dictated by the battery voltage x 100A - losses.

IE 48V * 100A * 0.99 = 4,752W

You do have issues here with the maximum charge current. I'm not sure if Victron has a way to compensate for loads vs charging amps. I should investigate this.

You could also consider a 5KW Victron Multiplus II with a Fronius Primo 5.0-1 5KW PV inverter. This would be overall cheaper than SMA for an AC-Coupled system. Around $2800 before installation and accessories. If I was recommending something of the above options, this would probably be it. The high voltage strings (up to 1000V) make the PV installation much cheaper.

The max PV you can AC-Couple to the Multiplus II 5KW is 5KW (1 to 1 rule). So you could install a bigger pv inverter but it wouldn't do you any good, you would have to limit its output to 5KW. With the 5KW Fronius, you could still connect 6KW of panels. It's called over-provisioning and we do it commercially all the time. It would help you in the evenings and on cloudy days.

The Multiplus II EasySolar unit does look nice. I was never very impressed with the original series like the 5K you mentioned. Just my opinion though. The Multiplus II Easysolar 3K is not a super bad price either.

For me, the biggest issue with the all-in-one units is repairability and downtime. For example if the internal charge controller fuse blows, I believe it has to be repaired by them. With a Victron multi-component installation, you'll be able to instantly replace or limp along on another part (even from another brand) while having something repaired or replaced. That is just my opinion but I'm an installer so I have a bit of a different point of view perhaps. I want to be able to get my client up and running instantly when something goes down. In my humble opinion the all-in-one units, for as tempting as they are, make that a little more difficult.

Personally I'm installing the Victron Multiplus II 5KW units with SMA 4KW Sunny Boy inverters. I'm also installing SI6048's but that's for larger systems. Overall I like the price of the Victron system. The SI's I can still get for under $2700 (less if refurbished) but Victron MP II units have a lower zero-load wattage (18W) which helps on small systems. Plus the monitoring solution is much cheaper (CCGX or Venus GX) than SMA's Data Manager M + COM Gateway.

Plus for $2700, I can almost install 2x Victron MP II 5KW. So 10KVA as compared to a single 5,750W SI6048.

 

[DAM]

I have an SMA Island 8.0 and 2 sunnyboys working in ac coupling (regulated by frequency as was said ago About 10kw PV and 45kwh of (880AH) in lithium batteries with management with a BMS (Emus BMS.
Bms does cargo management by informing SMA Island by Can-Bus communication. My system is set up in off-grid mode. The whole system works fine but would like to add more PV and DC load.
I know that both the SMA SIC 50 and Midnigt communicate directly with Island (CAN-Bus also think) but I got an mpp and liked to add to my system.
Tension add with victron mppt controller (victron 150/45). From what I read behind I can see that is perfectly possible!
Currently you know, from the data that Bms transmits to you, how much it loads/downloads and acts in agreement.
If you add Mppt victron (you have no communication with Island) I believe it will act according to the settings programmed on the controller itself. What's going to happen in the system? How will Sma know about the load data?
Thank you

 

[Hedges]

Given a shunt, Sunny Island will know about both charging and DC loads. (sorry, thinking of lead-acid. Lithium relies on BMS)

If you have a shunt and a charge controller that talks to Sunny Island (SIC50 or Midnight Classic), that charge controller has to be on the correct side of the shunt to avoid being accounted for incorrectly. Forum member TrukinBear had a failure related to that.

I saw the WhizBang Jr, thought it ought to serve to let Midnight deliver just enough for optimum battery charge current plus loads, but didn't find any such documentation. Good if it does, then you can do a large DC coupled system with small battery if you want.

 

[the_colorist]

If you add Mppt victron (you have no communication with Island) I believe it will act according to the settings programmed on the controller itself. What's going to happen in the system? How will Sma know about the load data?
Thank you

This might be worth breaking off into another thread rather than hijacking this one but the biggest issue I see here is the EMUS BMS. It's unlikely that it can talk to both the Victron and the SMA system on the same CAN bus network due to them both using the same CAN-IDs (0x351 etc) on the bus with slightly different formats of the values (as I recall). It would be worth asking EMUS about this. If they can talk to both at the same time, you are set. Just use a Venus GX and connect it to the CAN bus network with the SI and EMUS. Otherwise, your Victron MPPT will be charging without any control from the EMUS BMS. You could approximate the settings and hope for the best (they do have a lithium profile per se) but I would not advise it.

What type of lithium are you using?

 

[toms]

@DAM If you set your DC Charger bulk and float voltages half a volt lower than the Sunny Island’s charger settings, it will work fine.

 

[the_colorist]

That might very well work but in this instance (IMHO), it's not quite that simple. The SI isn't using any settings per se. It's receiving commands from the BMS in realtime. Probably every 210MS or so. Depending on EMUS's charging algorithm, the target voltage may change throughout the charge cycle. This is how some of the commercial systems work, manipulating voltage rather than manipulating the max current values.

For example, the commands from the EMUS to the SI include the following:

 

[DAM]

Meu sistema tem uma corrente metros em as de lítio baterias em si (que é o EMUS BMS "BUS BAR" medidor de This: https://www.electricmotorsport.com/...e/1/image/9df78eab33525d08d6e5fb8d27136e95/e/ m /emus-bb-drcs.jpg). É diferente a partir da origem SMA (Sunny Ilha Shunts) shunt (que eu não tenho). Poderia meu atual BUS BAR ser usado para remover informações de presente e elevar para a SMA? O que é que eu acho ilha SI8.0_12 atualmente realiza carga e descarga de acordo com os dados da BMS transmite para você (corrente, tensão, .... Will BMS continuar a transmitir este adicionais atuais e tensão dados após a adição do MPPT controlador vitron para o sistema?

 

[the_colorist]

Google translate:

My system has a current meter on the lithium batteries itself (which is the EMUS BMS "BUS BAR" This meter: https://www.electricmotorsport.com/...e/1/image/9df78eab33525d08d6e5fb8d27136e95/e/ m /emus-bb-drcs.jpg). It is different from the SMA (Sunny Island Shunts) shunt origin (which I don't have). Could my current BUS BAR be used to remove gift information and raise it for SMA? What is it I think SI8.0_12 island currently performs loading and unloading according to the data the BMS transmits to you (current, voltage, .... Will BMS continue to transmit this additional current and voltage data after adding the MPPT vitron controller for the system?