Building an RV Electrical System? Read These!

[DiploStrat]

There are many, many threads here seeking the Holy Grail, an effective electrical system for an overland camper. The right answer will vary a bit, depending on loads, autonomy, etc., but, at the present time, it all turns on one thing - charging a battery. We all want a battery big enough to cover our needs and a means to recharge it.

There are many sites which address these issues, but the best that I have found are these.

Handy Bob: http://handybobsolar.wordpress.com

Bob has perhaps the best analysis of what is needed to:

-- Charge a lead acid battery, and,
-- Build a solar kit to do this.

I don't always agree with his brand choices, but he has reasons for every choice he has made and he freely explains his choices and prejudices. Bob's focus is on solar based systems. Key points:

-- Charging voltages need to be high enough to get the job done. Most are too low.
-- You need an hour counting meter to tell you what is happening.
-- Most solar kits have too many solar panels and not enough wire.
-- Most RV's are way underwired.

Smart Gauge: http://www.smartgauge.co.uk

Scholars can debate whether the Smart Gauge can actually work, but the detailed analysis of the science of battery combining for charging on this site is outstanding. Drill down through the Technical Info tab and you will find everything from wire gauge calculators to why diode based isolators are a bad idea to an understandable explanation of Peukert's Law. While the Smart Gauge is available in the US, the Smart Bridge is not. Chris Gibson, the founder of Smart Gauge, came up in the canal boat world, where there is no room to mount a solar array. (And besides, the sun never shines in Blightey. ) All of his material is focussed on battery charging from a vehicle alternator.

Both of these sites will take a long time to read and absorb, but for what most of us want to build, a system that can harvest the greatest number of amps from our engine and a solar kit, it is well worth the read.

 

[dwh]

Totally agree.

Key points:

Here's one of HandyBob's pet peeves that you left out:

-- ANY shade on a solar panel WILL cut the output...even a little on one corner can cut it by AS MUCH AS 1/3 OR MORE!




I don't remember offhand if HandyBob covers the actual reason why it happens. For those who are interested...

All solar cells put out power with about .5v of pressure. Bigger cells put out more amps, but with the same .5v pressure.

To get high enough voltage to charge a battery, you have to string the cells together in series. Most "12v range" or "12v nominal" solar panels use a string of 36 cells in series to get 18v.

Put shade on 1 of those cells and now the string is a 17.5v string. That voltage drop isn't cool, so when they build solar panels, they install bypass diodes which will cut out a string of cells if the voltage drops.

Lots of solar panels are made with 2 or even 3 of these series strings, which are then paralleled. And to save money, most manufacturers just use one bypass diode per string. So if you get shade on even 1 cell, it could end up getting that entire string bypassed.

If your panel has 3 strings and one is bypassed, the amperage output drops by 1/3. If it has 2 strings and one gets bypassed, the amperage output drops by 1/2. (Voltage stays the same as long as there even one string putting out full voltage.)





This guy's panel has 3 strings.

 

[southpier]

I've run across Bob's site and thought it good information. can understand why he's annoyed at the industry. don't know enough about electrics or solar to disseminate his information, but I believe there's a ready audience for anyone who can.


and I think the best advice is "... don't touch that string!"

 

[DiploStrat]

When it comes to shade ...

every little bit hurts. I got up on the roof of my truck to run some tests. Bottom line, it does not take much to cut your output.

I have heard it argued that you should wire your panels in series for the voltage boost. Depending on your controller, this may make sense in a fixed or semi fixed set up, but for an overland camper, it is a terrible idea. Wire those puppies in parallel. And, all other things being equal, you are a bit better off with more small panels (say 100w each) than with fewer big panels (say 200w each) for exactly this reason. Now, with a smaller roof, I might well recommend 150w panels to get squeeze more output out of less area, but do keep an eye out for antennae, fan covers, and the like. There are enough trees in the world without making things worse with your own junk!

 

[Xterabl]

I am confused about something, and maybe you can help, DiploStrat.

If I have two panels, say they are 50W each and have 3 "strings" each.
If they are in series, then I have effectively a single 100W, 6 "string" panel.
I think I understand from dwh's post that if 2 of these strings (let's say on the same panel) are blocked, then power drops to 4/6*100 = ~67W.
Now, if I have them in parallel, then the one panel with 2 blocked strings becomes a 1/3*50 = ~17W panel.
Meanwhile, the other panel is firing on all 3 strings and is at 50W.
Adding these two together, I have 67W...same as if I had them in series.

I am kind of novice at this so I probably missed something. I appreciate a few minutes of your time to help me understand better.
Thanks!

 

[DiploStrat]

A Series of Parallels

dwh can probably go deeper into this than can I, but basically:

-- Series wiring "adds" things together, as in, two 2v batteries, in series, produce 4v. In parallel they produce 2v, but have more power. The problem with a series is that if one part of the series fails, they all fail. Remember the old Christmas light strings? They were made up of something like ten 12v bulbs. Wired in series you could plug them into you 120v wall outlet. But if one bulb failed, the entire string went out.

-- The manufacturer of your panel arranges the cells in series and/or parallel to deliver a desired output at a given voltage. You have no control over this although, at least theoretically, if you could get the information, you might choose a panel that had more parallel strings over one that had more series strings. I simply took the panel that my builder chose.

-- You do have a choice of wiring the assembled panels in series or parallel, within the voltage range that your solar controller will accept.

-- Why might you wire them in series? Because the higher voltage will suffer less loss in the wire run from the panel to the controller. That is, you will get better performance IF your controller will accept the higher voltage.

-- Why might you wire them in parallel? Because shading a single panel will only cost you the output of that panel; your other panels will not be affected.

In my case, I have five relatively high voltage panels (http://www.amsolar.com/home/amr/page_13_20/gs100_solar_panel.html), wired in parallel. For the reasons mentioned above, I prefer to go with heavier wiring to compensate for the lower voltage rather than risk the whole (or a larger part of) the array due to a stray shadow. And since I park under trees, you may be sure that one or more of my panels is always covered. (Another reason for cramming on all the panels you can.)

Does this help?

 

[4x4BNB]

I've run across Bob's site and thought it good information. can understand why he's annoyed at the industry. don't know enough about electrics or solar to disseminate his information, but I believe there's a ready audience for anyone who can.


and I think the best advice is "... don't touch that string!"

He's mainly upset at the RV dealer/solar install people in his area that KNOWINGLY sell equipment that is not up to task...their motive is to sell more panels and other equipment....

 

[4x4BNB]

every little bit hurts. I got up on the roof of my truck to run some tests. Bottom line, it does not take much to cut your output.

I have heard it argued that you should wire your panels in series for the voltage boost. Depending on your controller, this may make sense in a fixed or semi fixed set up, but for an overland camper, it is a terrible idea. Wire those puppies in parallel. And, all other things being equal, you are a bit better off with more small panels (say 100w each) than with fewer big panels (say 200w each) for exactly this reason. Now, with a smaller roof, I might well recommend 150w panels to get squeeze more output out of less area, but do keep an eye out for antennae, fan covers, and the like. There are enough trees in the world without making things worse with your own junk!

Also...if you run an MPPT you can buy smaller 24v (sometimes cheaper than 12v) panels....

 

[dwh]

Diplo's exactly right.

You wire in series to get a higher voltage from the array. This you almost always do with a large fixed location array - such as on a house or cabin - for several reasons...

A) Your battery bank is wired for 24v or 48v (or higher) so you need to get your array voltage above that.
B) You can use thinner copper wire with higher voltage.
C) You've got an MPPT controller.


But for a mobile install, you generally don't need to charge anything higher than 12v (or 24v on some trucks) so you don't need to rig the array in series to raise the voltage of the entire array (and even if you did need 24v (actually 28.8v), you can buy panels wired for that voltage as mentioned in the post above).

You also aren't going to run the wire very far, so the savings from thinner copper isn't all that significant.

Even if you have an MPPT controller, you won't get any advantage rigging the array in series. Even in parallel, the array voltage is going to be higher than the battery voltage, so the MPPT will still be able to do its magic.


So...no real advantage from rigging the array in series.
But...as Diplo noted, by rigging the array in parallel, you (more or less) isolate the PV modules (solar panels) from each other so that if one drops its voltage (usually from a bit of shade), it won't affect the others.

 

[southpier]

He's mainly upset at the RV dealer/solar install people in his area that KNOWINGLY sell equipment that is not up to task...their motive is to sell more panels and other equipment....

my reality is that 95% of salespeople don't have product knowledge beyond what I can read on the product label. this may be encouraged by corporate so there's less liability in the event of a mishap. makes everyone dumber but the liars.

too bad we live in such a litigious society.

 

[Xterabl]

dwh, but can you address my back-of-the-envelope quantitative analysis, which implies there is no difference in shadowing effect between series and parallel, since each panel consists of a couple-few "strings" of cells? Normally you do this in most of your postings (which i always appreciate) but you have overlooked in this case, I think.
Diplo's response invoked a lot of qualitative hand-waving (no offense intended), which is fine for some folks; but do you have anything quantitative?

The only other thing I can think of, to counter my quantitative argument, is that: if, when wired in series, a single *cell* in a particular panel is shaded, than that entire panel does not produce; whereas for parallel panel wiring, you only lose the string of 2-3 cells. Is that truly the case? The whole panel is lost if a single cell is shaded when wired in series?

 

[DiploStrat]

The only other thing I can think of, to counter my quantitative argument, is that: if, when wired in series, a single *cell* in a particular panel is shaded, than that entire panel does not produce; whereas for parallel panel wiring, you only lose the string of 2-3 cells. Is that truly the case? The whole panel is lost if a single cell is shaded when wired in series?

Exactly right.

How much loss? How much shade are we talking about?

If you wire your panels in series, and one panel is shaded, you lose all of the panels. At least theoretically. In the real world, any shading is enough to cut the output, but rarely is a single cell or panel completely dark, so there is always some current being produced.

Series wiring:

[Panel]+[Panel]+[Panel] > Controller>Load

In this case the panel output, in volts, is added to produce a higher voltage.

Parallel wiring:

[Panel] \

[Panel] - > Controller>Load

[Panel] /

In this case, the voltage remains constant, but the current increases.

Completely break the series at any point and you break the circuit. Cover one panel in a parallel set up and you lose only that panel.

How much? Depends on:

-- How much shade, and,

-- How your panels are wired together, and,

-- How your panels are wired internally. If your panel is wired as one series of cells, then cover one and you lose all. If, on the other hand, your panel is wired as several parallel strings of cells, then you will lose only one of them.
You really have to ask the manufacturer of your panel.

Want to guess? Then count the cells on your panel, guess that they are 2v each and then figure how they add up to the rated voltage of the panel. So, in my case, looking at this panel: http://www.amsolar.com/home/amr/page_13_20/gs100_solar_panel.html, which is rated at around 17v, I would guess that there are four parallel strings of 2v cells in series.

So how much do you really lose? As people have noted, at lot. Why don't you lose it all? Because a single cell is rarely completely covered.

You can play with this: http://pveducation.org/pvcdrom/modules/shading

So what are the take aways?

-- At least a part of any panel is probably wired in series.

-- In the real world, you are probably better off wiring your panels in parallel with each other.

Hope this helps.

 

[4x4junkie]

If you wire your panels in series, and one panel is shaded, you lose all of the panels. At least theoretically. In the real world, any shading is enough to cut the output, but rarely is a single cell or panel completely dark, so there is always some current being produced.

Series wiring:

[Panel]+[Panel]+[Panel] > Controller>Load

In this case the panel output, in volts, is added to produce a higher voltage.

Parallel wiring:

[Panel] \

[Panel] - > Controller>Load

[Panel] /

In this case, the voltage remains constant, but the current increases.

Completely break the series at any point and you break the circuit. Cover one panel in a parallel set up and you lose only that panel.

Do most panels not have bypass diodes within their circuit that allows full current to continue flowing from other panels connected in a series circuit should part of one panel become shaded?

The two panels I have (UL-Solar) incorporate two such diodes each (one per each 18 PV cells). This limits the loss to that of just 18 cells should one cell become shaded (resulting in a drop of about 25% in total power harvest, or from ~35 Vmp to ~26.5 Vmp).

In a parallel setup however, my total harvest would drop by nearly 50% in the event of a single shaded cell (loss of 18 cells puts the panel's output voltage below that of the battery voltage, therefore that one entire panel is unable to produce any meaningful current into the battery).

It would seem in this case the series connection is more robust against shading. I know UL-Solar isn't a top-tier brand name (I think they're Chinese), I would've thought a majority of panels made these days would have these diodes.

Want to guess? Then count the cells on your panel, guess that they are 2v each and then figure how they add up to the rated voltage of the panel. So, in my case, looking at this panel: http://www.amsolar.com/home/amr/page_13_20/gs100_solar_panel.html, which is rated at around 17v, I would guess that there are four parallel strings of 2v cells in series.

Cells are typically ~0.5 Vmp each. Panels with Vmp ratings of 17-18 will typically have 32-36 cells in series.

 

[DiploStrat]

I Simply Don't Know

I don't know enough about the internals of a panel to comment beyond my supposition. My panels appear to have 32 cells, but the large white dividers may or may not be the actual cell divisions.

I do know that even the slightest bit of shading causes a tremendous drop in output; more than you would logically expect. Thus my panels are wired in parallel and arranged to limit the chances of shading from vents, antennae, and the like.

The request was for something more scientific than waving a hand. Because of all of the variables, that is hard to produce.

Your wiring choice must also consider your controller's ability to take a higher voltage.

 

[dwh]

dwh, but can you address my back-of-the-envelope quantitative analysis, which implies there is no difference in shadowing effect between series and parallel, since each panel consists of a couple-few "strings" of cells? Normally you do this in most of your postings (which i always appreciate) but you have overlooked in this case, I think.
Diplo's response invoked a lot of qualitative hand-waving (no offense intended), which is fine for some folks; but do you have anything quantitative?

The only other thing I can think of, to counter my quantitative argument, is that: if, when wired in series, a single *cell* in a particular panel is shaded, than that entire panel does not produce; whereas for parallel panel wiring, you only lose the string of 2-3 cells. Is that truly the case? The whole panel is lost if a single cell is shaded when wired in series?

Sure.

4x4junkie's post above is technically correct. Theoretically, his two panels in a parallel setup will lose (in his case) 50% if a string is bypassed, and in series, his two panels would only lose 25% if one string is bypassed.

But of course, there are other issues (aren't there always ).

For one thing, diodes can fail. Older panels had a junction box on the back, where you connected the wiring, and the bypass diodes were usually in there, and could be replaced if one failed. A lot of newer panels don't have replaceable diodes, so if one fails, then you lose that whole string in the panel permanently.

Another problem, is that stacking up the voltage by wiring in series, puts the maximum power point a lot higher on the voltage curve, and if you don't have an MPPT controller, then your panels output will be a lot less when limited to whatever the battery voltage is (it's a loop, so with any controller except MPPT, the voltage of the entire loop - battery and solar) will be regulated by the battery).