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Haltzy
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# Posted: 14 Sep 2015 12:34 - Edited by: Haltzy
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Hey Guys;
Quick question...
I currently have 6x100W panels feeding my MPPT CC, i'm looking at changing the wiring pattern to provide a 24V feed to my CC. 3x2 panels.
I have read multiple posts and sites that stated this would reduce the power production... This makes no sense to me.
Thoughts?
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MtnDon
Member
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# Posted: 14 Sep 2015 14:54
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So the existing panels are connected all in parallel to the controller? And you are thinking of rewiring into a system of series and parallel? .... three pairs in series and those three in parallel?
Note: questions like this need more complete information about the equipment. For the panels the rated values for the Vmp, Voc, Imp, Isc are as important, even more important than the rated watts of the panels.
Then the ratings for the controller are also important; Maximum watts it can handle, maximum input voltage, Range of output voltages, maximum amps. The make and model# of the controller would be very handy as well as a link to the user manual.
Without that info you could get a recommendation that would be incomaptible.
Quoting: Haltzy I have read multiple posts and sites that stated this would reduce the power production... This makes no sense to me.
MPPT controllers have a loss that becomes greater as the input voltages get higher but the battery voltage remains the same. However, that can be offset by more efficiency on cloudy days.
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Haltzy
Member
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# Posted: 14 Sep 2015 15:32 - Edited by: Haltzy
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Thanks MtnDon,
Quoting: MtnDon So the existing panels are connected all in parallel to the controller? And you are thinking of rewiring into a system of series and parallel? .... three pairs in series and those three in parallel?
You are correct that is what i was thinking. I need to retain a 12v battery bank. Currently my off-grid system is running very well, and the only reasons I was going to go to 24v setup was to account for line loss since I'm going to be moving the panels away from the CC.
Here are the panel specs.
Rated Power 100W
Solar Cells Mono-crystalline
Max. power voltage, Vm 18.2V
Max. power current, Im 5.5A
Open circuit voltage (Voc) 22.7V
Open circuit current (Isc) 5.89A
I have a cheapy ebay CC. Dont laugh, it has got us buy thus far.. It is the MPPT 40amp version.
http://www.ebay.ca/itm/201227157984?_trksid=p2060353.m1438.l2649&var=500420636709&ssP ageName=STRK%3AMEBIDX%3AIT
Thanks for the feedback.
Haltzy
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MtnDon
Member
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# Posted: 14 Sep 2015 16:34
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One spec I don't see listed for the controller is what is the maximum input voltage.
The panel Voc is 22.7. For 2 in series that = 2 x 22.7 = 45.4
To allow for cold weather and NEC rules we need to take that 45.4 and multiply x 1.25 and again by 1.25
45.4 x 1.25 x 1.25 = 70.93 volts. Round that to 71 volts.
We would want the maximum input voltage of the controller to be at least 71 volts in order to safely cover the voltage that extreme cold can produce from 2 of those panels in series. The controller very well may be able to handle that, but I don't like to assume or guess.
Does the user manual have that info? Maybe the linked data has that hidden someplace where I missed it? ???
What is the new maximum distance from PV to CC going to be?
Just out of curiosity does the CC location have the same ambient temperature as where the batteries are located? I notice the CC states it has temperature compensation. It does not state if that has a remote sensor to be placed at the battery or if it uses the temperature of the CC unit. That can make a difference.
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Haltzy
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# Posted: 21 Sep 2015 14:05
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MtnDon, thanks for the reply;
Max input Voltage 150V, had to go the the developer, the seller didnt even know. That is what you get for buying cheap equipment i guess.
Apart from distance (aprox 15') i am also moving to 24v to accommodate a higher wattage. The CC can handle an additional 500W at 24. I have had two panels unhooked when i was running them at 12v.
The CC comes with a temperature probe, at this time the entire system is in a wooden enclosure. With the way it is setup, the battery storage location, CC, Inverter and controls should all be close in temperature which would be directly impacted by outside ambient temperature.
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MtnDon
Member
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# Posted: 21 Sep 2015 15:08 - Edited by: MtnDon
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Quoting: Haltzy .....higher wattage. The CC can handle an additional 500W at 24.
When charge controllers specifications make a reference to 12 / 24 / etc. voltage figure and state that the higher voltage makes it possible for more watts (of pv panels) they are meaning that with a higher voltage battery bank, you can use more watts total of PV input to the charge controller.
That is because charge controllers also have an amperage limit. The amperage limit is the maximum rating of the amps that the controller can put out into the battery.
For example, if the max amps is 40 and it is connected to a battery bank of 12 volts that means 40 amps x 12 volts = 480 watts. The specs on that 40 amp CC state you can use 500 watts of panels; a safe rounded up figure. With a 24 volt battery bank; 40 amps x 24 volts = 960 watts. The specs state 1000 watts; again a safe rounded up figure.
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MtnDon
Member
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# Posted: 21 Sep 2015 15:20 - Edited by: MtnDon
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So yes, changing the panels to a series / configuration has some advantages as far as wire sizing for the PV to CC connection. But it will not change the fact that the battery configuration means that the controller can only handle an input of up to 500 watts.
With more watts connected and the batteries still at 12 volts, the CC may just waste some of the available power. It is fortunate that the CC has handled the excess input with no equipment failure.
What can also be happening is that when the panels are receiving less than optimal sunshine they are simply not producing their maximum power rating (Vmp and Imp). That is a more or less normal state of affairs. Panels are usually operating at less than their rated capacity, unless they are on a tracker and you have bright clear SW skies.
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creeky
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# Posted: 21 Sep 2015 17:12
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ditto with what mtndon said re: voltage. the thing that kills CCs is too high voltage. of course with your CC big deal. what a bargain. and i don't know? 100v max is pretty standard on the cheap CCs. I don't know about yours. Check your manual.
yes. going from 12v to 24v on parallel/series panels but still to a 12v battery bank: you will see some current conversion loss. whether it's more or less than the line loss of running longer cables is hard to say.
typically tho. a solar panel set up in good sun creates more power than the batteries need. that's why i have things like a dishwasher, clothes washer and air conditioner that i run on sunny days. the batteries are charged or near charged. start the dishwasher. turn on the a/c. its free power.
so getting your panels to a sunnier location will more than make up for any small loss of efficiency converting from nominal 24 to 12v. and is an excellent idea. imho.
and by upping the voltage the money you save on wires to carry that current will just be money left in your pocket book. or wallet. or my bank account if you drop the money and the winds blow the right way. or ...
having said that: as mtndon points out. you are limited by your CCs amp capacity.
so your CC can use 24v and convert it to 12v (mppt rules!) but you're still limited to the power going into your batteries at 12v. 14.5v* x 40 amps is 580 watts. (14.5 being your "absorb" setting for your batteries). Your 600watts of panels is perfect.
Many good quality CCs can handle what I call "overpaneling". this is something i've talked about for many years. overpaneling means you put more panels on your controller than it can handle.
This means you could have 750watts. remember. 80% or more of your solar system gathering time is going to be morning, evening, cloudy, hazy ... well you get the point ... so very rarely do solar panels actually make peak power. so overpaneling gets you more power in the same amount of time. this is particularly important during seasons that aren't all sunshine and long days.
well. i'm going on here a bit. to sum up:
1. 24 to 12v will incur conversion loss. this loss is, with a good controller, in the 2-4% range.
2. you will get some of that loss back (or more) by saving money on wire;
3. and by being able to put your panels in a sunnier spot away from the CC and the batteries you will gain even more power.
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Haltzy
Member
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# Posted: 21 Sep 2015 18:09
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Thanks guys, as expected great feedback.
I will get everything wired up this weekend, and will get some pictures to share.
Always looking for ways to optimize the system. The future (long term) plans include upgrading the battery bank and inverter to 24V, which also contributed to the decision to move to 24V PV.
Cheers;
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