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Old 03-12-2014, 11:26 PM   #1
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Question Solar panels in series or parallel - but why not both?

OK, so here's what I'm understanding on the decision between series and parallel connection with an MPPT charge controller.

In series, there's more voltage and less amperage, thus reducing your I2 (squared) R losses through your connecting cables. However, a shaded spot on one panel will drop your output in series configuration much more (proportionately) than it would with a parallel connection.

Thus, I'm concluding that series connection should be the default, unless your panels are partly shaded (and you don't feel like moving the trailer and you might get caught chopping the tree down). If your panels are partly shaded, then a parallel connection would be a better alternative than taking the shading hit on a series configuration.

So, why not make the series / parallel connection changeable from inside the trailer? It would be pretty simple with one switchable lead and a 12V DPDT relay in a connection box on the roof.

I cannot be the only person who's thought of this. Has anyone else connected their panels so as to be selectable between series and parallel?
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Old 03-13-2014, 02:53 AM   #2
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It could be done - however, a couple factors to consider:

1. That relay would need to be housed in a weather-proof location; which possibly adds cable length to the installation. Resistive losses through the additional cable and the relay contacts may offset any benefit you may get from a serial connection.

2. it's common that the charge controller usually has a configuration for 12 vs. 24 volt PV system, so that would need to be changed as well.

3. The extra relay component and connections are additional points of failure.

4. Not a large factor, but when energized, that relay consumes current - that could otherwise be used for charging battery or other loads.

I also considered a series system, but for the shading issues you mention, and for simplicity, I set my system up as a 12V system, combining the panels in a roof-top junction box, and generously sizing the trunk cable to minimize loss.

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Old 03-14-2014, 12:01 AM   #3
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With two panels, I'd have to have a combiner box anyway, so just adding a much heavy duty relay wouldn't be much of an issue. I can put together some pretty sturdy connections, so the actual relay contacts would be the real point of likely failure.

I believe the MPPT charge controller automatically senses the voltage and adjusts, so that might work out ok as well.

If I do work up the enthusiasm to do it, I'll let y'all know how it works out.
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Old 03-14-2014, 05:52 AM   #4
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Here is the major problem with a switch to change from series to parallel,you cannot go from full output on a solar panel to open circuit it WILL damage the panel. Not to mention burning switch contacts and arcing You must cover the solar panels completely with some kind of blind(cardboard,blanket,etc) BEFORE rotating the series parallel switch . Most solar panels can bypass the shaded panel , some have a bypass circuit and some have a blocking diode so that when the sun goes down you don't drain your batteries ,it's easy to see just what's the best thing to do just look at house solar systems and look at the hydro set up around the world, there all using high voltage for a reason ,the hydro electric people are using high voltage 10000 or more voltage to a transformer and dropping it to 220 volts very close to your home ,they do it because low voltage means high amps and if you have high amps you need massive wire size (copper prices) not to mention the (voltage)losses in the wire due to resistance , ok so house solar systems are set up with there panels in optimum position either a fixed or tracking system setup, trailers are used mostly parked out of the sun for cooling (in the summer) and u.v. damage ( parked under a carport or in garage) I think the best thing would be to mount as many solar panels you can on the roof of the trailer (they still produce power but less on cloudy days or in the shade ) and wire them in series, get a mppt charge controller such as the midnite solar (the kid ) is a new model just came out, I have a midnite solar charge controller for my house (the classic) it's more then just a regular charge controller it's fantastic, go to the midnite solar web site lots of solar info, and i would install the highest input voltage inverter I could For instance if you have two 12 volt batteries get a 24volt inverter,if you have just one 12 volt batterie your stuck with a 12volt inverter most airstreams use two 12 volt batteries so you can use a 24volt inverter ,make sure you buy a pure sine inverter not a modified sine wave inverter ,a modified sine inverter will damage some electronics and overheat motors etc not to mention noise like static on some radios and t.v. ,, now if you want optimum you panels must be portable and you can move them around in the sun rays , you can put a solar panel plug some place on your trailer at ground level or just plug directly on your batteries but you need to store the panels in your tow vehicle and worry about theft.i would go for mounting on the roof of trailer, I think your Idea of a switch from series to parallel is great it maybe can work with proper electrical devices but you still must cover your panels when switching from one to other.
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Old 03-14-2014, 06:32 AM   #5
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You could compensate for the I-squared*R loss of the 12v parallel configuration by using larger diameter power cable.
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Old 03-14-2014, 06:55 AM   #6
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Yes (wayward) the larger the cable less the resistance but the newer airstreams are solar prewired with small gauge wire so to get around this you can wire in series and use a updated mppt charge controller , some can take as high as 250 volts input from panels and drop to 12 volts ,24 ,36,,48 volts ,the problem with large gauge cables is cost ,and hard to make sharp bends . Also you have to have very large cables from the battery or batteries to the inverter five or six feet of cable. My first solar system was 12 volt never again , my house solar system is 150 volt from solar panels (series parallel combo) with a 48volt inverter it's fantastic .when I install solar on my trailer it will have a 24 volt inverter ,a midnite solar (the kid ) mppt charge controller and solar panels in seres.
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Old 03-14-2014, 09:13 AM   #7
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Quote:
Originally Posted by drboyd View Post
Thus, I'm concluding that series connection should be the default, unless your panels are partly shaded (and you don't feel like moving the trailer and you might get caught chopping the tree down). If your panels are partly shaded, then a parallel connection would be a better alternative than taking the shading hit on a series configuration.
If your panels have bypass diodes (as most do), there's no benefit to having them in parallel in the event of partial shade.
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Old 03-14-2014, 09:27 AM   #8
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Scamp- You said that you cannot go from full output to open circuit without damaging the solar panel. My cabin solar setup has a disconnect switch. I believe is even required by the NEC, so the circuit could be de-energized for servicing. Are you sure that operating such a switch can really damage the panels?
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Old 03-14-2014, 02:03 PM   #9
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The panel can be damaged when under load and a arc happens when the circuit is opened, but if you have a proper d.c. Circuit breaker your fine and yes nec code requires all solar systems to have a disconnect d.c. Circuit breaker switch the circuit breakers are usually inside the disconnect switch box and it has to be near the solar panels outside this is so the fire fighters can turn the power off outside near the panels so no one gets a electrical shock and there is also a solar panel circuit breaker switch inside at the solar electrical panel ,you should also have a surge protector at the solar panels outside this is used to protect agains high voltages from lighting strikes. The switch that was proposed in (this thread)to be used for switching from series to parallel circuit will damage solar panels because there will be arcing at the switch contacts .
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Old 03-14-2014, 03:15 PM   #10
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<snip>The switch that was proposed in (this thread)to be used for switching from series to parallel circuit will damage solar panels because there will be arcing at the switch contacts .
A circuit breaker would show arcing as well, and a fuse would probably arc just before it's completely gone, too.
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Old 03-14-2014, 03:40 PM   #11
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Not sure if there is a special circuit breaker for solar there might be a clamping diode in the circuit ( this will prevent arcing) I know in automotive a clamping diode is used to prevent arcing in the a.c. Compressor circuit, but according to midnite solar and other info on the internet this is the way they recommend shutting down the power from solar panels ,I know it's a dc circuit breaker witch is very different then a.c breakers ,Ryan from midnite solar is very knowledgable on this he even has many you tube videos on solar systems and dc circuit breakers ( he is a electrician ,I think a engineer for solar Equipement and product support and he also does training on solar .
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Old 03-14-2014, 05:27 PM   #12
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Here's the dialog between me and the panel manufacturer's tech rep:

ME:
I'm installing a couple of your 100 Watt monocrystalline panels on an RV,
with some Brand X MPPT controller. I'm thinking it would be good to be
able to remotely switch them between series and parallel, with a DPDT relay
right next tot he panels.

The relay would be "break before make," to avoid shorting the panels under
load. However, it would expose them to open circuit under load.

Am I gonna toast my panels if I try this?
TECH REP:
Once you have an open circuit voltage available, there will be no current
flow. So the panels will have an open voltage until the relay completes the
circuit. The BBM relay opens the connection before it closes the other
connection making it safe to do the transition. I donít think this will
damage the panels. The worst that can happen is a dead bypass diode, which
is easily replaceable.
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Old 03-15-2014, 12:27 PM   #13
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My plan is progressing a little. I actually had a Hoffman NEMA 4X box in the garage that I rescued from a yard sale at some time in the mists of the past. I've got a DIN rail 80 amp rated (those are Chinese amps, however) DPDT relay, along with (2) 10 amp DC DIN rail circuit breakers. I got some DIN rail, too - so now I need a couple DIN rail stops and I'm set there.

I ordered a bunch of those supposedly weatherproof strain reliefs in various sizes to get cables in and out of the box.

I'll still use 4 gauge cables leading down from the roof to the controller; I'm now pondering whether the default (no power to the relay coil) state should be series or parallel.

The series Voc of the panels puts me 3 volts over the 42 volt rating of the charge controller, so I do have a tech inquiry in there to see if it's gonna fireball on me or what....
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Old 03-15-2014, 01:47 PM   #14
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Don't forget about hypervoc
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Old 03-15-2014, 03:24 PM   #15
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Don't forget about hypervoc
Just read up on it - thanks!!!
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Old 03-15-2014, 11:26 PM   #16
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drboyd- As I recall I had to make sure my Voc was well under the charge controller max rating (I can't remember the %). The panels can exceed their Voc when more than normal sun brightness hits them, say reflecting off of white beach sand, snow, etc.
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Old 03-16-2014, 07:11 AM   #17
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I think it's somewhere around 15%increase , but I think wiring and charge controller ,breakers so on should be allowed for 25% increase,the panels produce more power when cold(hyper voc) this is why there should be a minimum clearance under the panels for cooling. My (home)panels setup is 6 -220watt panels 3 in series and 2 runs parallel they are called 24 volt panels, there open voltage is higher 36.6 volts per panel ,there short circuit current is 8.1 amps per panel ,total wattage 1320 watts, I have seen 1490 watts produced about 4 days ago under these conditions-10 Celsius ,snow on the ground,full sun but not optimum tilt angle to the sun off by aprox 5 degrees ,and these panels are aprox 6 years old ( the output drops with age) I can produce at this time of the year just over 10 kilo watt hours per day just to give you the idea this supplies all my energy use during the day at home , no big appliances running 220 volt),for a r.v you don't need all this power ! To me most people need aprox 200watts of power due to rainy days and your limited to most of the time two 12 volt batteries, to store power yes you can get away with less but you have only so much roof area to install panels and no you will not be able to run a.c with solar power on your airstream (you need way to many solar panels and batteries
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Old 03-17-2014, 05:42 PM   #18
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Drboyde I'm not sure on your gauge of wiring from panels if you have a 8 gauge and your running 2 100 watt panels ,your good to hook up in parallel and if your running a 12 volt inverter, your two 100watt panels should produce aprox 16.6 amps and with hyper voc maybe 19 amps and that's if you don't have a long wire run , it would be better if you had 6 gauge wire then you wouldn't have to play with a switch box etc.
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Old 03-17-2014, 08:46 PM   #19
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I actually checked with the controller manufacturer and they said it would work OK. If the controller has a melt-down, oh well.
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Old 10-16-2017, 08:39 PM   #20
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here is a list of potential bypass diodes from one of the major OEM.

http://www.st.com/content/st_com/en/...roductId=SC541

these are Schottky diodes. they are the main ones for this type of application
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