RV Solar - Series Vs Parallel?

gkerlin · January 2014

I'm boon docking here in Quartzsite and a buddy of mine came to me with a question.

He currently has 3 140w Kyocera panels wired in parallel. Max voltage is in the 17.something range.

They are wired thru a Blue Sky Solar Boost 50. MPPT. He was thinking of adding one panel as he was not happy with his daily harvest. He also has them tilted into mid day sun.

Before he did that however I was thinking of two possible improvements…

First wouldn't putting the three in series (about 51.3 volts max) boost his output? Especially in less favorable conditions? I haven't been up on his roof yet but his installer I fear also used smaller wire so the higher voltage might help with the efficiency on the run down to the controller.

My concern - not being familiar with the Blue Sky unit is that according to what I could quickly find on that controller - it has a 57 volt limit. Am I cutting it too close? I don't know how it handles over voltage. I have a tristar MPPT 60 in my rig and they can handle transient overloads.

I also thought that even though he's gaining output from the tilt - later in the day he's losing some since the tilt is then away from the sun and that perhaps the total net gain is a loss.

Thoughts on the swap to series??

Cariboocoot · January 2014

Re: RV Solar - Series Vs Parallel?

Don't do it.

There is no real power advantage in making the change, assuming there are no other problems (which should be identified and corrected). I'm not sure what the specs are on that particular Blue Sky, but many of them have weird limitations such as reduced output current maximum if the input Voltage is higher than system Voltage. He could probably benefit from a better charge controller, frankly.

What is more, you would not want to reconfigure the array beyond 2X Voltage nominal as the greater the difference between input and output Voltage the less efficient any controller will be.

Most RV apps have no issues with V-drop over wiring (unless the size is very wrong) but do have issue with occasional partial shading and in that case parallel works better.

Your TriStar, btw, will handle 150 Volts on input so there is very little chance of Voc being a problem for it even with a higher Voltage array.

BB. · January 2014

Re: RV Solar - Series Vs Parallel?

First question is the output current of the controller roughly what would be expected for the size of the array? Bad panels, bad wiring connections, too small of gauge of wiring, bad batteries, etc. can all reduce the expected output of the array.

As Marc/Cariboocoot says, no reason to put panels in series--Especially with the Blue Sky controllers (in general). Read the manuals closely for the Blue Sky controller that he has. Many times, they are not really designed to take "high voltage" arrays and down convert to battery voltage.

-Bill

gkerlin · January 2014

Re: RV Solar - Series Vs Parallel?

Ok this is where I get confused. If a 17v panel is in less than perfect conditions I thought it's voltage would drop to a point (lets say 12v for example) and you are not going to get any output because they are less than the batteries.

So - if the panels are parallel - they are at 12v and you get no charge. But if they were in series they would be 36v and still capable of putting out.

OR… is that where the mppt comes in? The panels would still put out their amperage just at 12v (reduced wattage) and the MPPT would boost the voltage back up to the appropriate level to allow a charge just at a bit less amperage than coming from the panels? (higher voltage less amps out of the mppt).

Cariboocoot · January 2014

Re: RV Solar - Series Vs Parallel?

The problem is that panels are a current source: they produce current 'first' and let the Voltage fall where it may. So you get this condition where the Voltage will fall way down until the panel can produce its Imp. The Voltage will only rise to Vmp if the Imp is achieved first (this is under load; no load a panel will produce Voc with zero current and this is meaningless when it comes to charging batteries). With a panel connect to a battery minimum Voltage is fixed at battery level and the system will not rise above that point unless the panel has sufficient illumination to produce enough current to force a rise. In other word the panel may have a power point of Imp @ 9 Volts and as such the battery @ 12 Volts will hold the panel above its power point - nothing happens.

MPPT controllers do not boost Voltage up, they convert it down into current. So again you have the same problem where panels that are not fully illuminated although capable of 2X Voc in a series arrangement produce near nothing once they are required to provide current: the Voltage falls.

The only way around this is with a circuit called a buck booster, which is in essence the opposite of an MPPT controller: it will increase Voltage to a set point, but at the expense of current output. And so you would have a case of being able to produce the correct charging Voltage, but at a current so low as to be useless for actually charging the battery.

In short, there's no way to produce the power when the power (in the form of sunlight in this case) isn't there to begin with. You can alter how that power is delivered in terms of Voltage and current, but you can't make it appear from nowhere.

About the best situation you can achieve in an RV set up (12 Volt) would be something like four of those 140 Watt panels configured as 24 Volt strings, two strings in parallel, connected through something like a Rogue 3048 (although they would max that controller in bright sun).

bill von novak · January 2014

Re: RV Solar - Series Vs Parallel?

gkerlin wrote: »

Ok this is where I get confused. If a 17v panel is in less than perfect conditions I thought it's voltage would drop to a point (lets say 12v for example) and you are not going to get any output because they are less than the batteries.

"Less than perfect conditions" in this case means hot temperatures. If the MPPT point of the panel drops below float voltage then their power will start dropping. If OCV drops below float voltage then you will see no charge. (And of course absorption charging requires even higher voltages.)

There are very few panels for which this is a problem. Older "self regulating" panels sometimes had issues with this but I haven't seen them in a long time.

The panels would still put out their amperage just at 12v (reduced wattage) and the MPPT would boost the voltage back up to the appropriate level to allow a charge just at a bit less amperage than coming from the panels?

Very few MPPT regulators will do "boost." I don't know of any commercial charge controllers that will. One of the Outback converters was rumored to have a future boost option but I never saw anything concrete.

BB. · January 2014

Re: RV Solar - Series Vs Parallel?

MPPT (maximum power point tracking) based controllers will vary the current the controller draws and measure the resultant voltage from the array.

They then take the P=V*I equation and find Pmax=Imax*Vmax and then use the resultant Vmax to operate for the next few minutes (then do another "sweep"--At least how I roughly, understand the process).

Varray ideally = Vmp-array for a specific temperature of the cells.

And Vmp>Vbatt-charging+1 or 2 volts for wiring and controller voltage drop. If Vmp-array drops below Vbatt+drops, then the output current falls very quickly to near zero amps.

Pretty much, if a panel cannot "hold" Vmp for the current being delivered (current output of an array is roughly equal to the amount of sunlight hitting the panel/array/cell), it is usually because something has failed (not enough sun, bad wiring, failed controller, failed cell/panel, etc.).

If there is a failed cell/panel wiring, putting more panels in series will not help.

If it is because of a shorted bypass diode, you might get some extra energy (a 30 volt Vmp panel with 3 bypass diodes will give ~20 volts). So, in some cases you may get more delivered energy with a shorted diode in 2-3 series panels. But it is probably just time before another diode fails or you get overheating problems from the shorted diode, etc.

The few Blue Sky controllers I looked at seem to have a limited range of Vmp-array that they would accept.... And you could not, for example, put a Vmp-array=30+ volts on a controller connected to a 12 volt battery bank (I am not sure, but I think that is how they work).

And, it would not be worth buying a new high end MPPT controller for $300-$600 just to try an use a failing $100-$200 panel (that will probably fail worse sometime in the near future).

How bypass diodes are wired (and how many diode bypassed cells there are), is specific to the vendor and model of panel. It is difficult to give a generic description of how a failing panel will behave in different configurations. And if the panel has already begun to fail, chances are it will not be a "stable failure" with years more of useful life.

-Bill

gkerlin · January 2014

Re: RV Solar - Series Vs Parallel?

Ok I talked to him further. He gets 24amps at peak sun so I think he's getting good power out of his panels. That's pretty much their max output.
He's just not getting enough amp hrs by the end of the day back into his batteries. He needs more capacity. He's considering a 4th panel.

He currently has #10 wire going down from the roof to the controller (about 10') and the same from controller to batteries (about 12') .

I'm thinking if he adds the fourth panel and wires all 4 into two series strings - those two strings wired into parallel he'd have about 35.4v and 7.9amp per string In parallel he'd have 35.4v and 15.8amp running down 10' or so in the existing #10 wire. He'd then switch to something a size or two heavier for the 12' or so from controller to batteries.

Sound reasonable?

BB. · January 2014

Re: RV Solar - Series Vs Parallel?

Assuming I am reading the correct manual:

http://www.blueskyenergyinc.com/uploads/pdf/Manual_BSE_SB50L.pdf

It looks look like you can put 2x "12 volt" panels in series for Vmp~35 volts and, after you set a dip switch or two, the controller will properly take the "24 volt" array and down convert it to a 12 volt battery bank. Of course, the panels need to be "identical" (Imp match within 10% for series connections, and Vmp match within 10% for parallel connections)... If the 3rd panel is "too different", then you should not mix and match with existing panels on one charge controller.

My question would be "is one extra panel" going to be enough. That is "only" 33% more energy.... Many times, people need much more (factor of 2x or more). Accurate measurements of energy usage vs energy collected will help avoid extra work.

-Bill

Chris · January 2014

Re: RV Solar - Series Vs Parallel?

Just looking at this voltage loss chart
http://www.solar-electric.com/wire-loss-tables.html

It looks like he should be just barley good with the #10 coming from the panels......but to the batteries I think you will need #4. Remember when calculating the distance you have to X2 (both directions count)

RV Solar - Series Vs Parallel?

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