Combiner Box Help

[philatio]

I didn't want to hijack this the thread ...

PV Combiner Box Voltage To Inverter

Hi all, I am working on putting together a system for my shop and I had a question about voltage going to the inverter from the PV combiner. If I am connecting 2 strings of 6 panels in parrallel does the PV combiner combine to the total voltage of the 2 strings? So if each PV string going into...

diysolarforum.com

I have started my dive into battery backup. I am using the same MPP Solar LV6048 mentioned in the linked thread above.

The batteries (4 x EG4-LL 48V) and inverter are setup already and running fine. I'm taking this one step at a time. I currently charge the batteries from AC. This is not ideal. I want to use solar only.

I also have a separate grid tied system that I would like to pull some panels from and use to charge the batteries on the separate LV6048, off grid inverter.

The panels are REC 370AA Alpha Black. VOC is 44.1

The inverter manual states:
Number of MPPT 2
Max. PV Array Open Circuit Voltage 145Vdc
PV Array MPPT Voltage Range 60~115Vdc
Min. battery voltage for PV charge 34Vdc

If I understand this correctly, I could put 1 x 3 panels in series in each of the MPPT's for a total of 2220 watts of input.

If I want more than this, I would need a combiner box. I am clueless when it comes to these.

I would like to do 3s2p. That would keep that voltage below the max 146vdc at 132.3vdc.

What would the new wattage be? That would be 1 each 3s2p into each of the 2 MPPTs for a total of 12 panels.

What combiner box? 6 to 1? I'm guessing I would need 2 combiner boxer?

Just trying to understand how this works.

Thanks in advance.

 

[rmaddy]

You don't need a combiner box for anything less than 3P. Once you get 3 or more strings in parallel then you need to combiner box to fuse each string.

But your plan of putting 3 panels in series may not be valid. Voc goes up as the temperature goes down. At 3S your panel array would have a Voc of 132.3V with a max of 145V. Your 3S array would exceed 145V if the panel temperature ever got below about 20ºF/-7ºC. If you live where it ever gets that cold then you could fry your SCC.

But to answer your questions, if you put 6 370W panels in 3S2P (or any arrangement) then it is simply 6 x 370W = 2220W. If you use a total of 12 370W panels then it is of course 4440W total.

If you do live where it gets down to 20ºF/-7ºC then you could arrange the 6 panels in 2S3P. It is still the same wattage but a lower voltage and higher amperage. Since it is now 3 strings in parallel you would need a combiner box. If you did two separate 2S3P arrays for each of the two MPPT trackers then you would need 2 3-to-1 combiner boxes.

 

[philatio]

Thanks for the feedback.

I have 10.4kw of solar on my roof and am over producing considerably more than my billing cycle net metering is giving back.

I'm in Dallas, so it might get down to the 20's a few days a year, but I do want to be on the safe side and I understand that panels produce more at colder temps.

I want to be able to charge the batteries back up in a decent amount of time. Charging with AC as I currently am, and maxing out a 20A circuit, it takes too long to charge the batteries up.

How can I calculate charge time based on input wattage?

 

[rmaddy]

If you have 4 48V 100Ah batteries then that is 4 x 51.2V x 100Ah = 20480Wh of battery capacity.

If this is being charged with 4440W of solar panels then it would take 20480Wh / 4440W = 4.6 hours. But this assumes the batteries were 100% depleted (should never be that low) and your are getting 100% power from the solar panels (will rarely ever happen).

If your batteries were 80% depleted then you need to replace 20480Wh x 80% = 16384Wh. If you happen to be getting 2000W out of your panels on a given day, for example, then it would take 16384Wh / 2000W = 8.2 hours. That's solar hours. In the winter that could be 3 days.

So it will take anywhere from a few minutes to a week depending on how depleted the battery is and how well (and long) your panels are producing on a given day.

 

[philatio]

Thank you again.

Another very helpful response.

I like the idea of the 2s3p configuration to be on the safe side.

Instead of 2 x 3 to 1 combiners, could I use a single 6 to 2 combiner?

 

[rmaddy]

If you can find a 6 to 2 combiner that is really two 3 to 1 combiners then that will work.

 

[philatio]

Thanks.
watts247 has a 6 to 2.

Am I understand correctly? The combinber box takes care of putting the 3 strings of 2 panels into parallel.

 

[philatio]

OK, no need to answer the question above.
I found another post by you @rmaddy that explained it.

Combiner box question

Would a small combiner box double my amps if I attached two strings of solar panels in series? Would the combiner box act like a series/parallel conbection?

diysolarforum.com

Is my math correct?
each rec370AA panel has a VOC of 44.1V and an ISC of 10.55amps.

With 2s3p I will get 88.2V at 31.65amps?

 

[MisterSandals]

I like the idea of the 2s3p configuration to be on the safe side.

Deleted: I misread Voc. Thanks rmaddy for correcting my mistake!

 

[rmaddy]

I do not see the need to go with 2S3P over 3S2P.

Please see my math in post #2 regarding possible excessive Voc in 3S.

 

[rmaddy]

With 2s3p I will get 88.2V at 31.65amps?

Yes, the Voc will be 88.2V and the Isc will be 31.65A.

 

[philatio]

I could risk a 3s2p. According to this max voltage calculator at the coldest day on record in Dallas Texas was in 1899 at -8F (-22c).
For a string of 3 panels the max voltage at this temp would be 147.22V.

The inverter absolute max is 145VDC.

The coldest temp in the last 20year was the freeze of Feb 21 which was -17c. That would drop the voltage to 145.64.

Average coldest day including the Feb 22 freeze is -8C for a voltage of 142.78.

As @rmaddy states 2s3p is the safest way to go.

If I purchase a 8 to 2 combiner box, for future expansion, but only use say 2 or 3 pv inputs on it for now would that work leaving a couple inputs empty for now?

 

[rmaddy]

If I purchase a 8 to 2 combiner box, for future expansion, but only use say 2 or 3 pv inputs on it for now would that work leaving a couple inputs empty for now?

If you might go to 2S4P in the future then getting an 8-to-2 combiner box now saves a lot of money and effort later on. I'm pretty sure it is fine to leave a slot or two unused. Just make sure there's no unsealed holes. You want to keep it weather proof.

 

[philatio]

I'm slowly learning / preparing for if or more like when my utility scraps net metering like a lot of places are doing.

I have 10.4kw grid tied on my roof currently producing 40 to 60kwh a day. I plans on slowly transferring that pv power to complete off grid.

One step at a time to get it right.

Thanks for your input @rmaddy