Panel wiring: series, parallel or both?? (Help!)

Mr.Cockburn · January 2012

Hi everyone,

I will be installing 4 kyocera KD185GX-LPU panels to charge 2 6v, 215ah batteries wired for 12volt loads. My charge controller is an Outback FM-80.

Here's the thing... I was going to wire the panels in series being that the array is located about 75 feet away from the cabin (save $ on wire, right?) BUT I read that series wiring is not good at all with shading or snow on panels, etc... Snow is really a concern on my site because we have some for like, 4-5 months a year. As for shading, not so much because I chose the site accordingly. I must also specify that I only go to the cabin on weekends so sometimes snow might cover some parts of the array for days on since I am not there to remove it.

Now I'm a little confused.... Do I go all parallel, (I know this will require massive wiring and quite a lot of dollars!) Or do I go for a series/parallel setup ? Or maybe a series-only setup will do after all?

I really need to maximise my panels as my location is (obviously) not ideal as to peak sun-hours, etc...

Any help you can provide will be greatly appreciated! Thank you.

Kyocera KD185GX-LPU panel specs:
Pmp: 185 Watts
Vmp: 23.6 Volts
Imp: 7.84 Amps
Voc: 29.5 Volts
Isc: 8.58 Amps

vtmaps · January 2012

Re: Panel wiring: series, parallel or both?? (Help!)

As for snow, can you mount the panels vertical? What you lose by not being aimed at the sun can be gained back by reflection off the snow on the ground.

As for wiring the panels in series, that is generally an excellent idea with a MPPT controller... BUT in your case I would worry that you might exceed Vmax for your controller... 4 panels at 29.5 Voc at low enough temperatures could exceed the 150 volt max for your controller.
--vtMaps

westbranch · January 2012

Re: Panel wiring: series, parallel or both?? (Help!)

Use Niels spreadsheet for voltage drop for each combo of panels, including 3 in series and see what you are willing to accept for losses.

http://forum.solar-electric.com/showthread.php?29-voltage-drop-calculator

BB. · January 2012

Re: Panel wiring: series, parallel or both?? (Help!)

Not often I type this... "You have a lot of solar panel for your planned battery bank".

Will not hurt anything, but will require a larger charge controller because of the large(r) solar array.

What type of charge controller are you looking at... the minimum would be the MorningStar TS 45 amp MPPT charge controller (certainly a very nice controller but around $420 or so).

Shading of panels is not great in any case, and as Vtmaps said, mounting near vertical to shed snow can be a very good thing.

For a 75 wire run (one way, some voltage drop calculators require two way length) your two option extremes (all parallel or all series) using a generic voltage drop calculator. First all in parallel:

75 feet, 23.6 Vmp, 4x 7.84 amps = 31.36 amps ; 3% voltage drop * 23.6 = 0.708 volts -> 75 feet @ 31.36 amps 0.7 volt drop with 1 AWG cable
75 feet, 23.6 Vmp, 4x 7.84 amps = 31.36 amps ; 1% voltage drop * 23.6 = 0.236 volts -> 75 feet @ 31.36 amps 0.24 volt drop with 250 kcmil cable

Now, do the same thing with 4x panels in series:

75 feet, 4x23.6 Vmp=94.4 Vmp, 4x 7.84 amps ; 3% voltage drop * 94.4 = 2.832 volts -> 75 feet @ 7.84 amps 2.25 volt drop with 12 AWG cable
75 feet, 4x23.6 Vmp=94.4 Vmp, 4x 7.84 amps ; 1% voltage drop * 94.4 = 0.944 volts -> 75 feet @ 7.84 amps 0.89 volt drop with 8 AWGl cable

Now, because the Vmp of the panels are not around 17.5 volts or so, but 23.6 volts, a MPPT controller is really a good idea for your configuration because you would have more power from your array... But, if you are pushed for costs and need to use a PWM controller, you could use:

17.5 Vmp ideal / 23.6 Vmp = 0.74 = 74% of rated panel power "lost" due to Vmp / Vbatt-charging mismatch
23.6 volts - 17.5 volts = 4.1 volt cable voltage drop "available"
75 feet, 4.1 cable drop, 31.36 amps Imp -> 75 feet @ 31.36 amps @ 3.6 volt drop with 8 AWG cable

So, in theory, you could run 8 AWG cable with a PWM controller and your 4 panels in parallel, and later, if you have the time and money, buy a good MPPT charge controller and rewire the array to 4 panels in series... And get a 33% increase in output from your array (due to series wiring and MPPT controller). And you would not have to pull new 8 AWG cable.

-Bill

Mr.Cockburn · January 2012

Re: Panel wiring: series, parallel or both?? (Help!)

Thank you all for your replies. I plan on adding to my battery bank later on, (buying all new batteries) but for now I only have these two. As for my charge controller, it is an Outback FM-80 MPPT. Mounting near vertical will be hard because I was planning on mounting the array on an existing post about 11 feet high, so I was planning on a 45 degree angle (my lattitude). I guess I could go a bit more for clearing snow but how will this affect my summer performance?? I don't plan on changing angles every season.

So far, nobody has talked about a series/parallel setup, is this because it would not be good?

BB. · January 2012

Re: Panel wiring: series, parallel or both?? (Help!)

Series/Parallel setup will work too... And the wire gauge will be in-between what I estimated above (8 awg would probably work fine with ~2-3% losses).

There is also the issue, I believe, that some MPPT charge controller are most efficient with Vmp-array at ~2x Vbatt-charging voltage. If I recall correctly, somebody here had an Outback series (FM/MX?) charge controller with ~70-100 VDC array input voltage and 12 volt battery bank--It caused the charger to run hot enough that the fan needed to run. When re-wired to ~2x battery voltage, it ran cooler (not an expert by any means, do your reading of the manual and perhaps somebody here can elaborate/confirm/deny).

If you are going to install a larger battery bank, you probably will want/need to revisit the whole 12/24/48 volt battery bank and inverter choice.

Also, when you trench (I assume) for the PV array to battery shed cable run--I would suggest one or several large conduits so you can pull/upgrade cables without having to open the ground up again.

-Bill

Panel wiring: series, parallel or both?? (Help!)

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