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 Testing Solar Panels		  New Topic  Topic Locked
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Enchantment II
1st Mate

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USA
88 Posts
Initially Posted - 04/12/2017 :  11:59:45  Show Profile
Any ideas on how to test the power generated by a solar panel. I have a Coleman 5w hooked to my battery. I am not sure how much ,if any power it is generating?
Joseph Henderson
82-CAT 25 TR SK

dasreboot
Admiral

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803 Posts
Response Posted - 04/12/2017 :  12:27:09  Show Profile  Visit dasreboot's Homepage
hook a voltmeter to it. output in direct sunlight should be above 13 volts. make sure that there is a blocking diode so when the output drops below 12v that the battery does not discharge.
Todd Lewis
Eowyn 87 TR/WK C25 #5656
ARWEN 84 TR/SK C25 #4031
www.mainsailsailingschool.com
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islander
Master Marine Consultant

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USA
3992 Posts
Response Posted - 04/12/2017 :  12:47:30  Show Profile
A 5w panel is a Trickle charger. Under ideal conditions and unregulated it will put out around 17-22 volts and about 0.3 amps. Don't be alarmed about the volts because the amp output is very low so It can't overcharge the battery
You can check it with a multi meter for output. Make sure it has a blocking diode to prevent the battery from draining power back to the panel at night.Most have the diode built in. I have a 7.5w panel plugged in to a pick up I rarely use through through the winter and it does a great job of keeping the battery fully charged.
Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound


Edited by - islander on 04/12/2017 13:15:28
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Voyager
Master Marine Consultant

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USA
5231 Posts
Response Posted - 04/12/2017 :  19:41:49  Show Profile
Solar panels are rated in watts but the number is misleading. If you purchase a 5W panel what that means its open circuit voltage multiplied by short circuit current. So if your 12VDC panel puts out 20VDC open circuit, the short circuit current will be 0.25A. So under load charging your battery, when the panel delivers 12VDC at 0.25A, it's actually delivering only 3W.
Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain — Milford, CT
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Davy J
Master Marine Consultant

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USA
1511 Posts
Response Posted - 04/13/2017 :  06:24:39  Show Profile
You can test both the output volts and amps:

https://www.youtube.com/watch?v=Hey1QZkYFSQ
Davy J


2005 Gemini 105Mc
PO 1987 C25 #5509 SR/SK
Tampa Bay
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Voyager
Master Marine Consultant

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USA
5231 Posts
Response Posted - 04/14/2017 :  06:04:25  Show Profile
Yep, like I said, it's rated by Voc and Isc, so how else could they rate it? By maximum power transfer under load.

Assume you had a resistor that you could vary in resistance between say 100 Ohms and 2 Ohms. Connect the panel to the resistor with an ammeter in series and a voltmeter in parallel with the resistor.

At 100 Ohms and 20V (approx Voc), using Ohms Law V = IxR, the current would be 0.2A. 20x.2=4W

So now reduce the Ohms to 75, meaning more current, but a lower voltage. Say the voltage drops to 18V, the current is now 0.24A so power increases to 4.32W.

As we know from our calcs above at 12V we get about 0.25A so the equivalent resistance is about 48 Ohms. The power delivered to the battery is about 3W.

So we know that the panel is capable of producing ~60% more power but we can't harness it all for a 12V charging circuit using a direct connection or a simple on/off charge controller.

In our application we're charging up a battery that sets the output voltage, and as a result of that we're stuck with much less than peak current. But it's better than nothing!

There are some more complex and costly switching charge controllers but with a 5W panel, it's not cost justified. If you're a liveaboard with 200W or more, then it makes sense for sure.
Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain — Milford, CT
Edited by - Voyager on 04/14/2017 06:08:27
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Enchantment II
1st Mate

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USA
88 Posts
Response Posted - 04/14/2017 :  18:29:05  Show Profile
Thanks everyone for your input.
Joseph Henderson
82-CAT 25 TR SK
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Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.