Solar Charger - Page 2 - Fiberglass RV

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Old 01-04-2013, 05:17 PM   #15
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mmeyer's Avatar
Name: Michael J
Trailer: U-Haul VT
Posts: 493
Figure I would just go "old school" and measure voltage through 3 or 4 precision resistors using the decade resistance box, from which I can calculate amps , graph it up and see what I get. Knowing amps and volts I can calculate watts. The panel does not have any open circuit voltage or short circuit amp data.

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Old 01-05-2013, 05:49 PM   #16
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Name: Brian
Trailer: Boler (B1700RGH) 1979
Posts: 4,999
Originally Posted by Thomas G. View Post
To test the solar panel, disconnect the wires from the panel to the controller mounted on the side of the battery box. Attach an ammeter lead to each wire end while in full sun. That panel is about 5 watts, so you should get about 0.4 amps or so.
Tom, did you calculate 0.4 amps as 5 watts divided by 12 volts? That is not valid, because the panel output will only be 5 watts under and ideal load, and that's not your ammeter.
  1. Connect just an ammeter (which has nearly zero resistance) and you measure the short circuit current, at nearly zero voltage. The panel can't push this much current through any load; in full sun it should be significantly higher than 0.4 amps for a 5 watt panel.
  2. Connect just a voltmeter (which has nearly infinite resistance) and you measure the open circuit voltage, at nearly zero current. The panel can't push any current through a load at this voltage; in full sun it should be much higher than 12 volts for a "12volt" panel (about 20 V is typical).
  3. Connect the panel to a charged "12V" battery, check both the current flowing through the panel and battery and the voltage, then multiply current (in amps) by voltage (in volts) to get actual panel output power (in watts). A fully charged battery should be at a voltage which is reasonably close to ideal for a panel intended to charge 12V batteries; in full sun the panel should produce close to its rated output.

The later posted video may explain this, but I rarely bother with YouTube videos, and especially not on a wireless internet connection.

Update: I did try watching the video... a minute and a half before he said anything about testing, another minute to actually start (although they showed the panel specs - without explanation - during that time), and then it stalled. I'm guessing it might have been useful somewhere in the last three minutes.

1979 Boler B1700RGH, pulled by 2004 Toyota Sienna LE 2WD
Information is good. Lack of information is not so good, but misinformation is much worse. Check facts, and apply common sense liberally.
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Old 01-05-2013, 05:54 PM   #17
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Name: Kinga DeRode
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Originally Posted by Brian B-P View Post
Tom, did you calculate 0.4 amps as 5 watts divided by 12 volts? ...........
Yes, I recommended this as a quick and dirty test to see if it was putting out anywhere near the original amperage. My own UHaul panel only put out about 100 mA in full sun - that is when I decided to replace it.
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Old 01-05-2013, 06:03 PM   #18
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Name: Chris
Trailer: U-Haul
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Well I figured out that it might not have been putting out what it was supposed to. I washed the trailer today and it seems that the thick green layer of crud might have been keeping it from working very well. A good wash and hooked it to the group 29 and let it see what it can do.

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