2 parallel @ 200w serial?

[thiel]

All,

On my trailer, I'm going to over-specify the solar since we are out east (lots of shade) and I figure go big out of the gate. 400w total. We are not heavy boondockers, we are using the stock batteries for now (will upgrade slightly with AGMs at some point)... and we don't use much power. So, we really are doing this so we never have to worry about using the furnace in shoulder season, so we can use state parks for weekends without hooking up, and so we can have the trailer "ready to roll" all winter if we want to head out.

On my 25FB, the best mounting locations are at the two ends of the trailer.. 2 panels at each end. I think that works well too since if one end is shaded, the other is likely to be in the sun.


This is a DIY install. I'm handy so not worried about doing it, but also I want to keep things as simple as possible (but no simpler). SO... here's the question:

Should I wire this as two sets of series-wired panels... 24v at each end of the trailer? That way I could use the stock 10g prewire harness: simplifies my install and still keeps me at about 2% loss.

Thoughts?

Thanks!

[Al and Missy]

The limiting factor is probably the run from the charge controller to the battery as that will always be 12V. 10ga wire has .001 ohms per foot. In my case the round trip run from the controller to the batteries would around 30'. 400 watts at 12V is around 34A. Voltage drop in 10ga wiring would be about .001x30x33 or right at 1 volt. That seems high. Even the 24 volt run would have around a half volt drop.

A lot depends on the geometry of your installation, however. If you could run big wire from the panels to the controller and have a short run to the batteries, that could be good. I'd at least run big wire from the panels to the entry point junction box or all the way to the controller if possible.

If you decide to run new wires, go big or go home. #4 would be about o.25 volts and #2 would be 0.15 volts.

As to the serial or parallel question, the Gone with the Wynns blog (I think) referenced a fairly well run test on series v. parallel connections and blockage. If only one panel gets blocked, the impact to a two panel configuration is much worse for a series configuration. However with the isolation of your panels at each end of the trailer, and if the pair at each end are close together you have to consider the probability that only one will be blocked. IF that is assessed to be low, then series at each end should be OK, at least in my opinion.

Al

[GMFL]

Run them all parallel. That way if one gets some shade it will not knock the others out of the game also

[pteck]

Pro shops will tell you always parallel for RV installs. I generally agree.

What's not said much on these boards:
1) Solar very seldomly operates at full output. Majority of the time, the output is significantly less. Therefore, potential losses are much less than what is computed for full output.
2) Serial configuration itself results in efficiency losses. Even with an MPPT controller. MPPT controllers have documented efficiency functions. The further the input voltage is from the output, the less efficient. Something in the mid 90% efficiency for parallel wiring. And low 90s for serial wiring of 2 panels. And lower still for 3+ panels. So this has to be weighed against the wiring losses.

With 4 panels and using the pre-wire, I'd probably opt for series parallel. Though I'd probably still give all parallel wiring some serious consideration, especially if panels are in any way aligned to the sun differently.

[alano]

I would wire two panels in parallel using the 10 AWG wire. The panels will operate at about 18 V and two in parallel will be about 11 A. So if you have 50' of 10 AWG, that's .05 ohms, or 0.55 V, representing a voltage loss of 0.55 V / 18 V = 3%, hardly significant.

When you go to four panels I would consider a series/parallel configuration which will reduce the wire losses to about 2% and make the DIY much easier.

[Al and Missy]

Originally Posted by alano

I would wire two panels in parallel using the 10 AWG wire. The panels will operate at about 18 V and two in parallel will be about 11 A. So if you have 50' of 10 AWG, that's .05 ohms, or 0.55 V, representing a voltage loss of 0.55 V / 18 V = 3%, hardly significant.

When you go to four panels I would consider a series/parallel configuration which will reduce the wire losses to about 2% and make the DIY much easier.

While I know very little about the operation of solar charge controllers, I would think that even though the percentage loss is low, unless the charge controller has remote voltage sensing, it will think the batteries are at a higher state of charge than they are. That, to me, is more significant than the percentage of voltage loss. If the batteries are at 12.1 volts and the controller thinks they are at 12.6, that's a problem.

Al

[daleyocum]

Given that you aren’t crazy intense boondockers and will have only limited capacity to store the energy anyway I’d vote for just using the 10ga wires with a parallel configuration and call it a day. If you do want to run new wires to increase the effiency you might find this thread helpful. I’ll be running mine down the vent pipe in the bathroom.

[alano]

Originally Posted by Al and Missy

While I know very little about the operation of solar charge controllers, I would think that even though the percentage loss is low, unless the charge controller has remote voltage sensing, it will think the batteries are at a higher state of charge than they are. That, to me, is more significant than the percentage of voltage loss. If the batteries are at 12.1 volts and the controller thinks they are at 12.6, that's a problem.

Al

Al, since the battery current is higher than the input current with an MPPT controller it's important to physically place the controller close to the batteries and use a heavier gauge wire, say 6 to 8 AWG. When this is done correctly the voltage drop between the charger and batteries is negligible. But I bet there are installations that are less than ideal.

[thiel]

Thanks for all the comments y'all.

I found this handy voltage drop calculator online:
http://www.calculator.net/voltage-drop-calculator.html

Using this to generate some numbers, I think my choice is between these two configs:

-- ALL FOUR PARALLEL --
All four panels would come across the roof to a combiner box, then a single set of cables down the fridge vent to charge controller at the front of the trailer. (The MPPT controller would be right near the batteries that that will be wired with thick wires.).

I think that means (and please do correct me if I'm doing this wrong). That I'd have 18v and approximately 22amps running from the combiner box on the roof to the charge controller. I'm going with round numbers and assuming that's 20' of cable (so 40' round trip). (I am ignoring the wires on the roof.

So... using the calculator above: 18v, 22amps, 40' means...
Using 10 AWG = 4.89% loss
Using 8 AWG = 3.06% loss
Using 6 AWG = 1.94% loss


-- USING TWO PAIRS OF 200w PANELS, EACH PAIR WIRED IN SERIAL --
Here the numbers would be 36v, 22amps, 40' roundtrip:
Using 10 AWG = 2.44% loss
Using 8 AWG = 1.53% loss
Using 6 AWG = .97% loss

-----

Some questions:

-- At lower outputs, are the loss percentages also lower, proportionally? E.g. if I cut my output in half, does the loss % also get cut in half?

-- Does having a higher input voltage makes it "easier" for the charge controller to put more of the available power into the battery?

-- Sorta unrelated... but do I need a battery charge monitor? I know I need a solar charge controller, but do I also need a charge monitor?


THANKS!

[wulfraat]

You want all 4 parallel. If you do series/parallel your output will get crushed if even one corner of a single panel is shaded as it will severely drag down voltage (and hence watts) from the entire array.

I had a similar setup and experimented wiring both ways.

All 4 in parallel all the way.

[daleyocum]

+1 on staying parallel.

Also, unless you are upgrading your batteries you won’t have a place to put all the energy you do generate if you are in full sun. I’ll bet you are fully charged by early afternoon with 400w. If you aren’t in full sun then your loss with the 10ga wire is much less. I’d start out with the 10ga prewire and see how it goes. You can always upgrade later.

[alano]

Originally Posted by thiel

Some questions:

-- At lower outputs, are the loss percentages also lower, proportionally? E.g. if I cut my output in half, does the loss % also get cut in half?

-- Does having a higher input voltage makes it "easier" for the charge controller to put more of the available power into the battery?

-- Sorta unrelated... but do I need a battery charge monitor? I know I need a solar charge controller, but do I also need a charge monitor?


THANKS!

1. Yes, at lower current levels, your losses will be lower proportionally.

2. No, it's the other way around - the higher the input voltage, efficiency tends to drop, but it depends on the operating point and manufacturer.

3. Yes, this should be mandatory! No really, a battery monitor is highly recommended to get an accurate measure of the SOC (state of charge) of your batteries.

[wulfraat]

Btw - I think you are fine with 10g wire and 400w worth of panels. I had 310w with my factory 10g wire.

You will never ever see maximum output as you’ll loose 30% efficiency just by virtue of the fact that they are flat on the roof and will not always be at the ideal operating temperature.

I was content to see 13amps @ ~14.2v to the batteries in ideal conditions with 310watts of flat panels on my roof (two 155watt panels). Maximum theoretical output would be 21.8amps @14.2v for my setup.

[waninae39]

+1 on staying parallel.

[afneill]

I am a little late to the party, but I.too would run it all parallel. I recently added 4 100w panels to my unit, 2 mounted on the trailer and 2 portable. While in the process of adding the pvs, we went to Big Bend NP. I was far enough along that we took only the 2 portable panels, which I wired in series. My experience there caused me to wire all in parallel rather than series parallel as I had originally planned. Even with portable panels in Big Bend there was almost always some shade on one of tje panels. Trees, picnic table, charcoal cooker, people, vehicles. Easier to go parallel than move panels around all day long. I used 10ga from panels to mppt controller, then 6ga to battery. Not giving a lot of hard data just my limited experience. Good luck with whatever u decide.

[switz]

The marketing label says it is a 100 watt output solar panel. The reality is one would be extremely lucky to get 75% as that marketing number is generated on paper, not outside with real sun.

The rule of thumb is therefore 75%. So my five 100 watt solar panels on the roof of our 2015 23D International Serenity could possibly, under great conditions, create 375 watts of power. I have seen a maximum value of 350 watts on a really bright cloudless day with no overhead obstructions whatsoever.

We have the battery within three feet of the Magnum converter/charger with 750 mcm copper stranded wire connections. The wires coming down from solar panels junction box on the roof are #6 stranded copper.

YMMV