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Old 26-02-2014, 19:22   #31
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Re: solar output question

Originally Posted by Dsanduril View Post

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.
I thought I knew something about solar panels until I read this thread. Instantly blow my mind. There is no way I could do that kind of math. My question is on this particular answer is, if you if you use one of those controllers like a M PPT whatever that is, whatever it's called, can you eliminate some of these calculations?
Who knows what is next.
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Old 27-02-2014, 04:06   #32
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Re: solar output question

Originally Posted by artist View Post
We've just added 100 watts of solar panels to out boat for an upcoming cruise from Victoria BC to Hawaii in May-June. Now working out the amp budget based on our gear. Is there someone who can give me an idea of how many amp hours we can expect, on average, per day for 100 watts?
32-40 on an average day
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Old 27-02-2014, 05:59   #33
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Re: solar output question

Originally Posted by FlyingCloud1937 View Post
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.


When designing

Correct though you weren't asked

The reason is the existence of internal charge resistance , ie the equipotential force is behind a non zero resistance.

If it was a true voltage source , you could never charge it.

That's why I included the equivalent circuit for hints.

Its amazing what people assume they know as opposed from first principles. DC electricity seems accessible at first instance but in fact few really understand it.

Many people don't understand engines either , but rarely try an add another cylinder , the same should be true for DC electrics.

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Old 27-02-2014, 06:12   #34
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Originally Posted by StuM View Post
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.
+1. Best ballpark figure for a boat at anchor on a sunny to partly cloudy day. When you're sailing there's a good chance that the sails will make a lot of shade. but sometimes(rarely) it will hold your boat to a really good angle to the Sun so you get even more power. If you have a MPPT controller and discharged batteries you can get up to 25 percent more. One more thing to add, placement of your panels is critical. The farther away from your mast and other things that cause shade you can get it more productive it will be.
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Old 27-02-2014, 07:09   #35
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Re: solar output question

Originally Posted by goboatingnow View Post
Perhaps this might help
Attachment 76833

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

OK man.... Ya got me.... I had a brain fart and neglected to think about all of the gate valves in the plumbing system...


Is there not a potential difference??? I mean you're not going to charge a 1kah bank with a single cell putting out o.5v no matter what the size (A output) or how close you hold it to the sun????

ammitoounderthewetherfromtheflutobetacklingthisrig htnow?
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