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Old 26-02-2014, 11:43   #16
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Re: solar output question

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Originally Posted by HappyMdRSailor View Post
Others way smarter than me on panels... Stab? 35ah?

bump.....
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Originally Posted by Dsanduril View Post
We have this discussion every time a solar question comes up. doesn't work with solar panels. Solar panels are a current source. They put out a specific amount of current. I grabbed the specs for a 100W panel. It has an Isc of 6.2A and a Imp of 5.8A at 17.8V. Between 0V and 17.8V it will put out about 6A regardless of the voltage. The amount of power it puts out depends on the voltage, but not the amount of current.

If the panel is operating at 14V (typical float voltage) it will still put out 6A, for a power of 6x14 = 84W. If it is running at 12.5V (depleted batteries) it will still put out 6A for power of 75W. The only way you will ever see 100W is if you can run the panel at it's "preferred" voltage of 17.8V.

All of the above based on STC, etc. and very simplified. Unless you are using an MPPT controller, ignore the power rating. Look at the Isc and Imp current ratings, you will see that current (at 25C panel temperature with 1000 W/m2 sunlight intensity at the panel surface) regardless of the voltage.
Thanks Dsanduril!!!

Lucky for me, my engineering background was not completely compromised by my boozing... broading... and boating.... when I shoulda been paying more attention.... I respectfully submit my "outta the blue stab" as exhibit A.... AND... my complete admission that I am a neophyte panel dude....

I didn't even factor in... fast cars, gambling and golf.... Sheesh!
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Old 26-02-2014, 12:06   #17
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Re: solar output question

I use the following rule of thumb which says that on a sunny day a 100 watt panel will put out 33 amp hours of power.

My guess is that you will be lucky to average 25 amp hours, given sunniness (medium at that time of year), latitude (good for Hawaii, bad for BC), sun angle Isc, Imp and BS factors.

David
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Old 26-02-2014, 12:12   #18
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Re: solar output question

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Originally Posted by Dsanduril View Post
We have this discussion every time a solar question comes up. doesn't work with solar panels. Solar panels are a current source. They put out a specific amount of current. I grabbed the specs for a 100W panel. It has an Isc of 6.2A and a Imp of 5.8A at 17.8V. Between 0V and 17.8V it will put out about 6A regardless of the voltage. The amount of power it puts out depends on the voltage, but not the amount of current.

If the panel is operating at 14V (typical float voltage) it will still put out 6A, for a power of 6x14 = 84W. If it is running at 12.5V (depleted batteries) it will still put out 6A for power of 75W. The only way you will ever see 100W is if you can run the panel at it's "preferred" voltage of 17.8V.

All of the above based on STC, etc. and very simplified. Unless you are using an MPPT controller, ignore the power rating. Look at the Isc and Imp current ratings, you will see that current (at 25C panel temperature with 1000 W/m2 sunlight intensity at the panel surface) regardless of the voltage.

+1, give that doggy a biscuit

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Old 26-02-2014, 12:16   #19
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Re: solar output question

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True that... re 12v... BUT... any charging SOURCE MUST BE well over battery potential.... Alts are 14.5+... Panels are way higher... you gotta figure current A directly with the potential V.... There is no other variation... Kirchoff man.... If he gave us V, we'd have a more refined answer, but power is power... No gettin around that either... Higher V=lower A

Effin A...

Nope. Pwm regulators ( the simple kind ) merely connect the panel to the battery, the voltage of the panel will be the battery voltage , yet charging occurs , go figure. ?

Ps alternator outputs are at the battery voltage , you think a 40 A alternator will out wrestle say a 1000Ah capacity bank.

Rule 0.

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Old 26-02-2014, 12:30   #20
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Re: solar output question

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Nope. Pwm regulators ( the simple kind ) merely connect the panel to the battery, the voltage of the panel will be the battery voltage , yet charging occurs , go figure. ?

Ps alternator outputs are at the battery voltage , you think a 40 A alternator will out wrestle say a 1000Ah capacity bank.

Rule 0.

Dave
Aww man... Your disrupting my beverage.... How is it a 40A alt can charge a 1kah bank if the "potential" of the source isn't higher than the potential of the load???....
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Old 26-02-2014, 12:34   #21
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solar output question

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Originally Posted by HappyMdRSailor View Post
Aww man... Your disrupting my beverage.... How is it a 40A alt can charge a 1kah bank if the "potential" of the source isn't higher than the potential of the load???....

Because a battery is not a perfect voltage source. Here's a thought experiment

Again put a 18v solar panel directly across 13.8v battery, what's the battery terminal voltage ? , since there is no voltage drop on the wires , ( for this gedanken) what's the panel output voltage?

How is this setup charging a battery

Dave
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Old 26-02-2014, 12:34   #22
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Re: solar output question

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Originally Posted by djmarchand View Post
I use the following rule of thumb which says that on a sunny day a 100 watt panel will put out 33 amp hours of power.

My guess is that you will be lucky to average 25 amp hours, given sunniness (medium at that time of year), latitude (good for Hawaii, bad for BC), sun angle Isc, Imp and BS factors.

David

David,

I don't know if this was the type of answer the op was looking for, but it was certainly what I was looking for - a simple and concise ballpark figure. Thanks!
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Old 26-02-2014, 12:38   #23
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Re: solar output question

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Originally Posted by slowshoes View Post
David,



I don't know if this was the type of answer the op was looking for, but it was certainly what I was looking for - a simple and concise ballpark figure. Thanks!

You can't make that sort of computation unless you know the operating point of the panel. If you ignore it , depending on the panel. You could be miles out.

Dave
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Old 26-02-2014, 12:48   #24
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Re: solar output question

Theory of solar panel output has nothing what-so-ever to do with the answer to the question: " how many amp hours we can expect, on average, per day for 100 watts?"

The OP is asking a real world question!

For example - one five day sail from Cape Flattery, WA to Monterrey, CA saw two days of brilliant sunshine and almost calm seas. The solar panel output was close to max, given the sun angle, for six to eight hours a day. Two of the days were heavy overcast with drizzle so there was very little solar output. The last day was heavy, heavy, soaking wet fog until 2 PM then brilliant sunshine.

Another example - in the 10 days at sea from Cape Flattery to San Diego we had four days of heavy overcast, three days of heavy fog, two days of high overcast, and one day of bright sunshine. What does theory have to do with the solar output in those conditions?

A boat using solar panels should plan on seeing little solar panel output the first 10 days of so of a trip from Cape Flattery, down the US coast and then off to Hawaii. There may be some nice solar power available, but in the four trips I've made - the sun made useful power less than 30% of the time, thus my 500 watts of solar panel power.

On the other hand - a sailboat headed down the US west coast will motor quite a bit 'cause the wind almost always dies at sunset and does not rise again until close to noon. Your diesel alternator will recharge the batteries when motoring at night.

Two sailing friends each took a boat from San Diego to Hawaii last summer. They each reported heavy clouds the first couple days out of San Diego and then trade wind conditions with big puffy clouds, ten foot seas, and occasional clear patches. How do you calculate solar panel output when clouds obscure the sun for ten or 15 minutes an hour and the boat changes it's angle to the sun every few seconds?

We adjusted our electrical power consumption to match the state of the batteries:
- turn radar on/off
- turn off the stereo
- less use of reading lights at night
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Old 26-02-2014, 13:00   #25
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Re: solar output question

Sheesh. The point is that depending on where the operating point is a 100w panel could actually be anywhere from 50 w to 100 w , this is before you start factoring in illumination levels and light hours.
A 100w panel is not like a 100w generator

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Old 26-02-2014, 13:17   #26
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Re: solar output question

Quote:
Originally Posted by goboatingnow View Post
Because a battery is not a perfect voltage source. Here's a thought experiment

Again put a 18v solar panel directly across 13.8v battery, what's the battery terminal voltage ? , since there is no voltage drop on the wires , ( for this gedanken) what's the panel output voltage?

How is this setup charging a battery

Dave
Oh ho ho!!! (channeling Dr. Watson)

You mean the intrinsic variances of lower potential in the source bank???
Darnit... We're back to hi to lo potential again???

OK proff.... Terminal voltage of the 18V panel is 13.8 dammit... But... somewhere in there is a cell at 1.84529372158V (i've gotta rockin' DMM)= lower potential... Besides.... it's already bee pounded in my head that panels are a constant current source....

OK smarty pants.... What's the terminal voltage on a bank 12.4v bank as soon as an Alt is pulled in the circuit??? Hmm...???

TEACH ME MAN! Knowledge is the shizzit!
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Old 26-02-2014, 13:26   #27
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Re: solar output question

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Originally Posted by goboatingnow View Post
Sheesh. The point is that depending on where the operating point is a 100w panel could actually be anywhere from 50 w to 100 w , this is before you start factoring in illumination levels and light hours.
A 100w panel is not like a 100w generator

Dave
+1

So many different ways to say this - ignore the watt ratings on panels installed on boats without MPPT controllers. The only thing of value is the current rating, followed by the voltage rating (need enough to be above your battery voltage at your operating temperature).

For the OP's 100W panel it is most likely either a 32-cell or a 36-cell panel. The 32-cell panel will have a 12.5% higher current rating for the same power, but a 2V lower operating point (so something like 16V vs. 18V). That means the 32-cell panel will put 12.5% more Ah into your battery for the same power rating.

On paper the 32-cell panel is a better match for a 12V battery, however, once you start to factor in temperature effects this may not be so (depends on where you are operating). Originally most "12V" panels were 32-cell, but at high operating temperatures they couldn't charge a near fully charged 12V battery. So, we have a broad range of panels available on the market and having the power rating of the module is the least useful of all of the various specifications for the panel.
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Old 26-02-2014, 14:46   #28
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Re: solar output question

Lots of complex electronic engineering, maths, climatology discussed here, but in practice it all comes back to what was said in the first few posts i.e

A good rule of thumb is a quarter to a third of the rated watts in AH per day. Which end of that spread an installation will return depends on how well the panels are sited and how sunny it is on average in your location.
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Old 26-02-2014, 14:48   #29
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solar output question

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Originally Posted by HappyMdRSailor View Post
Oh ho ho!!! (channeling Dr. Watson)

You mean the intrinsic variances of lower potential in the source bank???
Darnit... We're back to hi to lo potential again???

OK proff.... Terminal voltage of the 18V panel is 13.8 dammit... But... somewhere in there is a cell at 1.84529372158V (i've gotta rockin' DMM)= lower potential... Besides.... it's already bee pounded in my head that panels are a constant current source....

OK smarty pants.... What's the terminal voltage on a bank 12.4v bank as soon as an Alt is pulled in the circuit??? Hmm...???

TEACH ME MAN! Knowledge is the shizzit!

Perhaps this might help
Click image for larger version

Name:	ImageUploadedByCruisers Sailing Forum1393451293.995800.jpg
Views:	81
Size:	69.6 KB
ID:	76833

Hint cell equivalent diagrams ( a or b most useful ) only cause you mentioned Mr. Kirchhoff

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Old 26-02-2014, 19:44   #30
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Re: solar output question

You have to be about 3 tenths a volt over of terminal battery voltage, from a charge source for current to flow to the LA battery.

This is based on a new healthy battery. You have to overcome the internal resistance of the bat as well as v-drop between the bat and the charging source. Bad cables, and poor lugs will result in a higher voltage potential needed for amps to flow to the battery. The same goes for a battery as it ages and devlops a higher internal resistance due to age, and sulphating.

An 12 volt alternator spinning at it's design speed will have a no load voltage of about 22-30 volts depending on design, and weather it's delta or wye connected. It's 3 -phase so you will have to do some math to correlated it to a single phase charge source.

The charge potential of any source is the terminal voltage of the battery, as a negative added to that of the voltage potential of the source.

So a LA at 12.4 volts and a source potential of 18 volts, produces a amp potential of 18 v- 12.4 volts = 5.6 volts by its watt potential. As the battery moves higher in voltage the, a voltage limited device will produce less potential current to the battery.

With an alternator this is probably ideal, because the overhead of the alt is a such a great potential. and the battery soon fills and requires less amps.

With solar, we are limited to 18 volts, with pmw. This is where MPPT comes into real effect. With the much higher potential from higher voltage panels. We can see better charge into the bats. An MPPT controller, on an 18 volt panel is going to provide littel reward after losses to the controller.

You just can't compare an alternator to a solar panel.

Lloyd


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Quote:
Originally Posted by HappyMdRSailor View Post
True that... re 12v... BUT... any charging SOURCE MUST BE well over battery potential.... Alts are 14.5+... Panels are way higher... you gotta figure current A directly with the potential V.... There is no other variation... Kirchoff man.... If he gave us V, we'd have a more refined answer, but power is power... No gettin around that either... Higher V=lower A

Effin A...
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