Solar panel recomendations for a 24V system ideally with DSSR20.

[Jhon]

I build a 24V 272Ah battery using cell I bought on Alibaba with an SBMS0 and am now ready to add solar panels.  I am located in Southern Ontario, about 2 hours drive from Toronto.  I will be putting those on my shed roof and the available space is 1.8 meter * 4.25 meter (6 feet * 14 feet)  A little overhang  (200 mm) would not be a big issue.  Looks is also an issue as I would prefer all black.

I love the SBMS0, so ideally I would like to use them with DSSR20's but I am not sure how to interpret the specifications.  60 cell panels are recommended but it is also possible to use 72 cell panels, what are the criteria as I would like to get the most bang for my buck as well as have it look great.  A slight overhang would make it look better, but I would need 72 cell panels, but would that be wasteful?  The other option I considered is to run the panels in series and use a mppt solar charger and keep the batteries inside (which would eliminate the need for heating the batteries in winter).  I would also need the SBMS0 to be able to tell the solar charge controller to cut of the charge when the battery is fully charged, ideally without the need of an external (solid state) relay.

I would love recommendations on the "best" way to proceed and where to buy the panels. I would prefer to buy them locally, but I am quite happy to buy from Alibaba and wait for the sipping, if there is nothing reasonably priced available locally in North America, preferably Ontario.

Now this is my covid project, I have been interested in this for years and I can only learn so much from just the forums or youtube, at some stage, I just have to try in order to be able to progress.  I learned a lot from building the battery and I feel I only just scratched the surface.  I would like to maximize the power produced and if need to I can add another set of cells in parallel.

[Dacian Todea]

If you care about the look then there is no harm in using 72 cell panels is just that the extra 12 cells you will pay for will have no impact of charge current but it is extra roof space and it looks better.

MPPT will be more expensive even if you consider the small savings on cables to have the battery inside the house. But if you decide to use an MPPT then Victron ones have remote ON/OFF either with a cable you get separately for the smaller models or directly included in the larger models.

I got my panels from here https://www.solarshoppingmall.com/24V-Solar-Panel_c_25.html but right now it does not seem they have a large selection.

They have a 390W  72  cell panel that is 2m long made with half cut cells (better with shading) but it is not black.

There are now also occasionally 66 cell panels that may be just the right size for your roof of you care about aesthetics.  

[Jhon]

Thanks for your reply Dacian, that was helpful.

I understand how to calculate voltage drop etc, what I don't know is what the optimal input voltage and current is for the DSSR20, the panels list things like Max power voltage and Max power current, do I calculate the voltage drop (3V) from that?  Or do I use Open circuit voltage and short circuit current?

I am trying to decide my best option. Keeping the battery in the basement means no heating requirements, just have to bring in 40Amp DC over about 30 meters if I go with the DSSR20's, (which I certainly my preference)  Otherwise I will have to run 2 lines to the multiplus, one 15Amp for optional charging overnight and one 30Amp for the load.   Can I use #12AWG landscape wire for each panel and join the wires in the DSSR20 and accept the 4Volt drop, or is there a better and cheaper alternative?

[Dacian Todea]

You can see the DSSR20 as an ideal diode plus a solid state switch in series so there will be almost no voltage drop on the DSSR20 (just a few mV) The important part is your battery that is 25.6V nominal but most of the time it will be around 26.5 to 27V and so the panel when hot needs to have at least this 27V + depending on cables 2 or 3V drop on those so around 29 to 30V max power point voltage with a 60 to +70C panel. The max power point for 60 cell panels is around 32 to 33V at STC meaning panel at +25C but for panel to be at +25C ambient needs to be around -10C.

But PV panels are constant current sources so voltage of the panel if connected to battery will be the same as battery voltage plus the voltage drops on the wires. So in most cases (mostly dependent of PV panel temperature) the 60 cell panels + DSSR20 + 8s LiFePO4 battery will result in a 90 to 100% efficiency so an average around 95% and thus it is an ideal match.

If you go for a panel with exact same cells but 72 of them then those extra 12 cells will not contribute with anything but the open circuit voltage of 72 cell panels is below 51V so it is not a problem for the DSSR20 is just that a 72 cell panel has 20% more cells and so 20% higher power rating than a 60 cell panel and also a 20% higher cost but charge current will be the same so will be the charge power thus there is no advantage other than aesthetics (in your case) to pay 20% extra. 

So DSSR20 can handle 60 cell and 72 cell panels with no problem is just that there will be no difference in power provided to battery.

[Ken McKittrick]

Seriously, I am confused

Dacian you just blew my mind! I've been lurking for 6 months  or so and was wondering exactly how this part worked.

So if I have 6 of  these (https://store.santansolar.com/product/santan-solar-t-series-250w-snail-trails/) setup as 2s3p feeding into an MPPT  I would expect to see ~60vdc with ~28 amps of current going into a 24V battery bank?

How does that work with the SBMS0? The what happens to the extra 23 or so Vdc?

Thank You

[Dacian Todea]

Ken,

Those are older panels so they have a bit lower max power point and my struggle a bit in summer when is hot but usually is not that relevant as there is plenty of excess energy then.

With 6 of those panels you will use 3x DSSR20 each of them will have two parallel panels and so battery will be charged at around 40 to 60A depending on how cold the panels are.

With an MPPT you will need the MPPT to be rated at least 60A and so it will be fairly expensive (it will also need remote ON/OFF control so likely a Victron MPPT).

The MPPT will be about as efficient as the 3x DSSR20 in cold days (assuming those old panels) and in summer the MPPT may do a bit better just because those are older panels with lower max power point and when those are hot the max power point voltage will drop below battery voltage so charge current will be reduced no longer working that close to max power point.

[Ken McKittrick]

Dacian

Than you for the clue. It's weird I don't think I've EVER seen anyone really talk about Solar Panel power outputs in real world conditions/situations. I did see one youtube video last week on this.

[Jhon]

Thank you Dacian.

Just so I am sure I understand correctly, let me put it in my own words.  The most efficient way is to connect two panels in parallel for 20Amp using the Max power current as a guide. And the Max power voltage should be at minimum something like 28.4V at the DSSR20  To calculate that I would have to look at what the panels produce at 35C ambient summer temperature, minus the voltage drop in the wires?  Anything more than that is a waste of money from an efficiency point of view?

And thanks for the tip, 66 cell (or 132 half cell) panels would be an ideal fit.  I am talking to one supplier on Alibaba who might be willing to sell only 4 of those panels.  Most of them want to sell them only by the pallet load, which is understandable considering how fragile those things must be.

Thanks Ken, your questions and Dacian's answers were also helpful.

[Dacian Todea]

Jhon,

The STC spec is for +25C PV panel temperature in summer your panels will be 60 to 70C at least so you need data for those conditions if you care much about max of energy in summer. I have 260W panels not much better than the 250W discussed here and when they are hot in summer I get just maybe 7A from each panel but is not so relevant as battery gets full anyway very early in the day and I have plenty of excess (unused energy). In winter when I need most of the energy I get 8 to 10A from the panels.

No the 66 cell panels will be a waste of money and space (10% to be exact) as new 60 cell panels 120 half cells rated 300W plus will already have high enough max power point voltage to work great both in winter but also in summer.

[Jhon]

Thanks Dacian, I am trying to understand how it all works, I am an old electrician, electronics and solar is all magic to me, so guess my thinking is wrong and as such I probably formulate my questions the wrong way.   I am not too worried about a little less efficiency over summer, the longer days will make up for that.

What voltage should I be looking for at the input of the DSSR20, after losses? I imagine it is anything over 28.4V and that I calculated that from the Max power voltage?  My cable run is 30 - 40 meters and 10Gauge with 20Amp would drop the voltage by 5.24V.     What I am trying to work out is at what point a 60 cell panel is not ok.  If I have to I can just go with your recommendations and I am sure I will be fine, just trying to learn more, which is the main purpose of all of this.  I am also happy to read up on this if you have recommendations that my old and now much slower brain can follow.  (I can still just comprehend at electrician level, not at electrical engineer level, but I am still very curious)

Would love to give you a call and pick your brain some time lol.

[Dacian Todea]

If you will have such a long PV cable then you may want to consider a 66 or even a 72 cell PV panel. My cables are about 15m and with that 60 cell panels are fine but at 40m you will need at least 66 cell panels to compensate and since 66 cell panels are not that common you can just get 72 cell panels the difference from 60 cell to 72 cell panels in price is still much lower than thicker cables at that length.  With 72 cell panels you will not worry about the voltage drop no matter how hot are panels in summer you will still get 9 to 10A from each panel.

The PV panels contain the STC (Standard Test Condition) max power point voltage maybe even the curve but that is for panel at +25C (meaning ambient will need to be around -10C depending on wind speed).

https://upload.wikimedia.org/wikipedia/commons/5/55/I-V_Curve_T.png

You can see in that photo how a typical solar cell (a single one) will have voltage fluctuation with cell temperature the reason for that is that a solar cell is nothing else other than a diode so the diode will just short all the current if left open circuit.

[Jhon]

Thanks again Dacian.

That store you suggested is getting some B grade 300W panels for something like 160 dollars (Canadian) + tax and if I put the battery in the unheated garage, I have a run that is about 11 - 15 meters.  Shorter if I don't go underground.  I can parallel 2 panels together for 20Amp and use the 10Gauge wire which would be a voltage drop of 2V.  The panels are not what I was hoping for, but I can't justify the price difference.  I looked at some 132 cell panels from Alibaba, which would have been nearly perfect, but those guys are not really interested in shipping 4 panels.  If those B grade panels don't look right, I will replace them and keep them for the camper van.

I already ordered 2 of the DSSR20's, and it will be fun to put it all together.  I will insulate the battery box and add a heating pad, was thinking of using a reptile heater which comes with thermostat and run that of the inverter.  My final question is, does the SBMS0 cut of the charge when the external temperature senor (already installed) drop below 1C?  If not, should I add another circuit in series to disable charging from the DSSR20 and the Victron multiplus?

[Dacian Todea]

Jhon,

By default the temperature limits are set outside the measurement limits thus it will not take any action. If you set a low temperature limit (you can set at any temperature say +1C) then both charge and discharge sources will be disabled so likely you do not want that or if you do you want this as secondary level protection. You should just have a small 10 to 15W heater in the insulated box controlled by a thermostat set at +10C and this should be able to maintain the battery above +10C in any ambient temperature while 4 of those panels should provide sufficient energy for the heater even in cloudy/overcast days. 

[Jhon]

It will be a secondary protection especially since I have no data on how reliable a heater will be and the garage does get below freezing when external temperatures are below -8C.

Thanks so much for all your advise and making some great products, much appreciated.

[Jhon]

I found the solar panels, from a supplier nearby by googling the modules from the supplier you mentioned.  (160 dollars for 300W panels.)

But I have a problem with the solar shunt, for some reason it is not registering the solar current.  I have double checked the wiring, I tried switching positive and negative and I even tried switching the shunts on the input, but to no avail and I am at a loss.  Is there anything else I could be looking at?  I looked on this forum and no one else seems to be experiencing this problem, so I assume it is me.

[Dacian Todea]

Please remove that resistor from the ADC1p as you will influence the battery current measurement.

Get that resistor and connect it to battery+ directly as the resistor needs to be as close as possible to battery+ as his role is same as a fuse for protection.

It will have been helpful to see a photo of the current shunts also as maybe they are not connected correctly.

The PV1p will connect on the side of the shunt connected to PV panels (DSSR20 or whatever charger you are using) and the PV1n will connect to the side where both shunts intersect and where you have your loads connected see page 3 in the user manual. 

[Jhon]

Thanks Dacian.  I didn't think it would matter much with such a large resistor, but you are probably right and I will change it.

As for the shunt wiring, I double checked the wiring, I also tested them with my meter. switched the shunts (I connected the main shunt sense leads to the pv and I still did not get a reading, I also reversed the wiring thinking maybe it wouldn't read a negative voltage across the shunt...  I searched for any other settings that might have prevented reading the values.  But at the moment the shunt is buried beneath some other things in the box and I can't get enough light to take a picture.

[Dacian Todea]

Jhon,

Yes the resistor will sure have a significant influence in the current reading accuracy.

Is the PV shunt directly connected to Battery shunt  with in turn if connected directly to battery+ ?

Yes PV shunt will not read negative voltage so sense wires need to be connected correctly.

What have you tested with your metter ? The voltage drop on the current shunt ? What was the result ?

What do you have connected to PV shunt ? What sort of current is expected ?

Where are the two ends of PV shunt connected ? 

[Jhon]

Thanks Dacian

"Yes the resistor will sure have a significant influence in the current reading accuracy." Thanks, I have changed it and ran a new separate wire to the battery positive terminal

"Is the PV shunt directly connected to Battery shunt  with in turn if connected directly to battery+ ?" I wired one end of the solar shunt to the output of the DSSR20's and the other end to the "main shunt", which is connected to the battery +  The point where the two shunts are connected is wired into the inverter/charger.

"Yes PV shunt will not read negative voltage so sense wires need to be connected correctly."  Thanks

"What have you tested with your meter ? The voltage drop on the current shunt ? What was the result ?"  My meter only goes down to 200mV both had a similar voltage drop of 1,5 mV The main shunt is 300A/75mV, the battery shunt is 250A/75mv

"What do you have connected to PV shunt ? What sort of current is expected ?" 2 pairs of 300W solar panels and I expect between 30A and 40A  (The same current as what I would see from the main shunt without load)

"Where are the two ends of PV shunt connected ?" I wired one end of the solar shunt to the output of the DSSR20's and the other end to the "main shunt" and the Victron multiplus.

I am sure I am doing something wrong, I just fail to see what it is....

[Dacian Todea]

John,

Another John has just posted a diagram of his setup please check that https://groups.google.com/g/electrodacus/c/RFw2X9wF1Cg/m/iRiOi8-EBwAJ  Maybe you can see any diffrence from your in relation to current shunts connections as his diagram is correct and detailed.

I guess PV shunt is 250A/75mV (you said main shunt and battery shunt but they are the same thing).

So is the battery shunt seeing the 30 to 40A coming from PV panels but not the PV shunt ?  

Just disconnect all loads and leave just the PV panels connected (assuming battery is not full and accepts a charge) the current shown by the PV shunt should be the same as the one shown by battery shunt.

In ADC setting menu I guess you set the correct resistance values for PV shunt (also PV shunt set as 1 meaning ON).

There can be so many possible reasons including no current from DSSR20 as they are not connected correctly.

[Jhon]

"

Another John has just posted a diagram of his setup please check that https://groups.google.com/g/electrodacus/c/RFw2X9wF1Cg/m/iRiOi8-EBwAJ  Maybe you can see any diffrence from your in relation to current shunts connections as his diagram is correct and detailed."  Yes, the same setup.

"I guess PV shunt is 250A/75mV (you said main shunt and battery shunt but they are the same thing)." You are correct.  (Shunt is too big, but I had that lying around since Mississauga sorting center held my package there for over a month, I bought another one. )

"So is the battery shunt seeing the 30 to 40A coming from PV panels but not the PV shunt ? " Correct,  I will take a screenshot.

"Just disconnect all loads and leave just the PV panels connected (assuming battery is not full and accepts a charge) the current shown by the PV shunt should be the same as the one shown by battery shunt."  I did, see screenshot.

"In ADC setting menu I guess you set the correct resistance values for PV shunt (also PV shunt set as 1 meaning ON)." Nope, but I did just now and now I it is working as expected!

"There can be so many possible reasons including no current from DSSR20 as they are not connected correctly."  I know, but after a few days of struggling. I decided to ask for help and thankfully received it.  (The discrepancy between the Main and solar shunt readings is the multiplus.)  Problem solved...

But I noticed something else, any idea why cell 5 isn't bleeding? (is that the correct term)

Update:

It's "bleeding" now but wasn't for about half an hour prior.

[Dacian Todea]

The cell 3 was the highest voltage cell in the second photo so that had the balance current applied and due to that cell balance current the voltage reading was lower thus did not showed as the highest cell even if it was. Every few seconds the cell balancing is disabled so that SBMS0 can see the real cell voltage.

Same in first photo the cell 5 showed up as the highest voltage but in reality all other cells except cell 8 where higher that is why all other cells had the cell balancing current enabled. If you have waited for 3 or 4 seconds you will have seen that cell 5 was no longer the highest cell voltage as the cell balancing current will have been disabled to read the real voltages.

[Jhon]

Thanks again for your help Dacian, much appreciated.