Advice in wiring 14 Solar Panels in Parallel to Battery Shed

[Obba41]

Hi all,
I need some help-advise in the best way to cable up 14 solar panels in Parallel to the shed that houses the Inverters and Batteries.
The panels are 190W 24v (Open 37v) approx 8A
I have been given them 2nd hand.
I have existing panels on the actual Battery shed and on the roof of the house.

But as we get into winter (coldish lowest ever 6ºC and low sun angle- Gold Coast Australia), the batts are doing it do it hard and I have to resort to charging them if after 3 days of cloudy weather.
So these FREE panels have a place at the back of the garden.
At the rear of the garden I have 4 sheds laid along side by side. So the diagram shows the line of panels I want to place on the shed roofs.

I was going to either do a run to Bus Bars at the end of the rear sheds and then a run to the Battery Shed.
Or just run a - really - long cable run all the way to the battery shed.

Bear in mind that I would need approx 5mtrs of run once inside the shed to get to the inverters/panels cable layout etc.

I had considered using 6mm Twin and earth (100m roll), divvy it up into two 50mtr double runs and use the 2 reds and 2 blacks plus a single Green/Yellow on each of them (Taking HUGE care not to get the Green/Yellow mixed up)... So in essence 3 individual (6mm²), lines into 2 strings = 18mm²

Looking at suitable cables in the AWG format and others quoting 4mm, which is NOT cross section 4mm²
Considering that 6mm² is about 9AWG.

So any advice on my crazy idea and how on earth I can do it would be appreciated.
I enclose some Vincent Van Gogh diagrams.

 

[brucehoult]

14x 8 amp panels in parallel = 112 amps? Are you kidding?

I wouldn't put more than 30-35 amps through 6mm^2 cable — which is the cable size I'm using with 14 Ioc, around 10-12 amps working. Certainly not in an enclosed space e.g. in conduit. Also normal MC4 connectors are often only rated for 20A.

I'd be putting those in parallel in pairs (16A) and then run one string of 4 pairs (96V .. 144Voc) and one string of 3 pairs (72V ... 111 Voc).  Or, if you want to respect the UK/AU/NZ DIY wiring limit is 120 VDC then do one cable of three pairs and one cable of three triplets (24A ... still ok).

Do you know the temperature coefficient of voltage for those panels?

If not, then estimate -0.25%/ºC or -0.3%/ºC so for 6º C the voltage can be 0.25*(25-6) = 4.75V or (using 0.3%) = 5.7V higher in the cold. So 111 Voc at 25º C will still be under 120V on a cold morning.

What are you feeding it into?

I'd be happy feeding those 3s2p and 3s3p strings into my Pecron E3600LFP (which has 2x 150V 20A MPPTs). That's 2660W total. I actually have 2640W (6x 440) going into it as 3s with panels that are 39.38 Voc (and -0.25%/ºC) and 14.96 Isc.

https://moorabbinbatteries.com.au/collections/portable-power-stations/products/pecron-e3600lfp-portable-power-station-3200w-3600wh

Or of course lots of people would recommend Victron stuff.

[Obba41]

Hi Brucehoult,
Thanks for the reply.
I should have mentioned some more info: I have a 24v system. Too much money/time to go over to 48v
Have a 60A and a 50A solar controllers (Roof 60A - Shed 50A). NONE every go the max, except on a huge Summer high UV day.
These feed two Lipo4 battery banks, which then go into a 5Kw (Continuous), PSW invertor, into the house.

With the suns crossing these 'new' panels facing East (roof faces N, Shed faces W), the distribution of Sun Solar Rays would be fairly evened out.
Objective would be to have the Solar Controllers at 75% through the day. 


I didn't intend to use the MC4's I was going to strip them off. The cable on the panels is 6AWG. AFIAK a MC4 at 37v is rated to 30A.

I can get 4 into 1 adaptors. So pondering the run from 4 panels (adaptor length before the 'join', is approx 2mtrs. Then snip the MC4's off the ends of that adaptor...
But I was hoping to have the least amount of connections/losses as possible.

I did look at putting them in series as you say, but I would need to convert the 100's of DC Volts back down to 24v
Nice and easy using the existing MC4's. But then with 14 panels, I'd have to divvy them up.
If i did look at 12panels, which way would you go (3x4, 4x3, 6x2)?
Stil got to go from those V's to 30v's for the controllers...


That Pecron E1500LFP is a nice piece of kit. AUS $2700 is probably a fair price, but I only need the Panel feed going into my existing setup.

With the parallel run though using those adaptors (can get 2 way as well), I'd only need to start off 'thin' at the far side and then go thicker as I got to shed wouldn't' I ?
That's why I thought of the panels going into Bus Bars at the end of the sheds, then use super thick to the Battery Shed.
That 6mm Twin and Earth 6mm² using all 3 cores for a single Pos or Neg would be 18mm²

Funny. Got the panels free. Got the room to fit them. But looking at $1000 to just cable it

Humm....

[brucehoult]

I did look at putting them in series as you say, but I would need to convert the 100's of DC Volts back down to 24v

The voltage of the PV string is completely unrelated to the voltage of your battery, other than that if the PV input is lower than the battery voltage then you need a less efficient buck-boost converter (or at least mode).

MPPT controller are not expensive, and if they can let you use $200 of wiring instead of $1000 by using a higher voltage and lower current then it's well worth it.

e.g. two of these would do the job: up to 150V DC in, 1000W to a 24V battery.

https://www.springers.com.au/shop/scc115035210-victron-smartsolar-mppt-150-35-1882

[Obba41]

Right ok.

Those Victron controllers can take in 150V, my Plasmatronics PL60 and the other 50A can only take in 30v ish and are not MPPT.

I see now. I was not aware that MPPT solar controllers can take in 100v-150v or higher.

I shall look into these MPPT Solar Controllers now as it certainly seems now to be a better solution to just run two/three 150v strings to the Batt Shed and then into the MPPT controllers, then into the BMS's   

Cheers,
Obba41.

[Renate]

Higher voltage always saves copper, to the tune of the square of the voltage.
It's mostly just a case of safety and more expensive transistors in the converter as a limit.
Copper is expensive, insulation is cheap.
A commercial solar farm is using 1500 V!

[brucehoult]

Higher voltage always saves copper, to the tune of the square of the voltage.

And higher voltage batteries have a similar advantage.

Note that those Victron MPPT controllers can put twice the Watts into a 48V battery than they can into a 24V battery, for the same MPPT cost.

It's mostly just a case of safety and more expensive transistors in the converter as a limit.
Copper is expensive, insulation is cheap.
A commercial solar farm is using 1500 V!

In NZ/AU you can legally DIY up to 120V DC. Over that you should get a sparky to check it and make the final hookup.

Also in AU they have a 60V limit for DIY batteries. Not in NZ but "48V" (which is up to 55V or 56V topping off a charge) is where you want to be anyway.

[Siwastaja]

Hi all,
I need some help-advise in the best way to cable up 14 solar panels in Parallel

There is no "the best way" to cable up 14 panels in parallel - there is just one way, and it's useless. Connecting panels in parallel is just wrong. Don't do it.

Get a proper charge controller which steps down the voltage for battery charging (internal DC/DC conversion) and put as many in series as the the controller max input voltage, and safety regulations allows you to (see the manual). That might be like 2-8 panels, depending on the controller and the panels. If you then want more, build more strings, each with their own controller.