How to tell if a solar panel is working

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I have a 25w panel I bought many years ago. Used it for the time and it has been sitting on the shelf since. I am thinking I could use to trickle charge my car battery. How can I test to see if panel still works?
 

tx2sturgis

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If you have a DVM or DMM (digital volt meter or digital multi meter) you can test it when it's receiving sunlight.

You should see around 20 volts (VOC, volts, open circuit) assuming it is a 12v panel.

If the meter can also measure amps, you can measure ISC (amps, short circuit) and with full sunlight you should see about 2 amps...more or less.

If you dont have a meter or this is all unfamiliar to you, then we might need to go to plan B.

Plan B is, do you know anyone who is handy with electronics or electrical wiring?
 

frater secessus

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I am thinking I could use to trickle charge my car battery.

@tx2sturgis correctly advised on how to test above.

I will chime in to say that a direct connection of even a small panel to the starter battery could overcharge/damage the battery (example). A solar charge controller between the panel and starter battery would keep this from happening. Doesn't have to be a fancy or expensive controller. A dead-simple one that has one output voltage in the 13s would likely do the trick.
 

tx2sturgis

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Yep stuff can happen.

However a small panel (usually 20 watts or less) can be hooked up to a FLA (flooded lead acid) battery directly, and provide unregulated charging. A 2 to 5 amp silicon blocking diode is needed to prevent reverse current flow when the sun goes down at night.

YOU become the 'charge controller'. You can arrange the panel so that it only receives direct sunlight a few hours a day, such as tilted east towards the morning sun and never move the panel.

Or pointed due west. Same effect.

Over the decades I have set up and maintained lots of flooded lead acid batteries this way. And yes, you need to periodically check the electrolyte level in the battery, and top off with distilled water as needed. And occasionally checking the battery voltage is a good way to know if the battery is being maintained at the right voltage. Again, YOU become the 'charge controller'...controlling everything about this.

For occasional starter battery upkeep when parked or camped in the boonies, and for counteracting the built-in parasitic drain that new vehicle electronics suffer from, its a great solution.

Of course, it goes without saying, that if the battery will remain un-attended and un-supervised for weeks or months, this is not the best solution.

And I do NOT recommend charging AGM (absorbed glass mat) batteries without a charge controller.
 
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If you have a DVM or DMM (digital volt meter or digital multi meter) you can test it when it's receiving sunlight.

You should see around 20 volts (VOC, volts, open circuit) assuming it is a 12v panel.

If the meter can also measure amps, you can measure ISC (amps, short circuit) and with full sunlight you should see about 2 amps...more or less.

If you dont have a meter or this is all unfamiliar to you, then we might need to go to plan B.

Plan B is, do you know anyone who is handy with electronics or electrical wiring?
Well this girl does have a Multi meter :) Question is does she know how/remember to use it :-(

The solar panel as 2 wires coming off it with battery clips. Do I put mm leads to the clips when panel in sun.?
 

tx2sturgis

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Yep!

Just lay the panel on a table or the ground, anywhere convenient, facing up, around the middle of the day, lets say from 10 am to 3 pm more or less, and then set the meter at DC 20 volt scale, or the next highest scale, (or auto if it has auto ranging)

On some meters the DC scale is marked with a solid line above a set of dashed lines and AC is marked with a 'wavy' or curved line representing a sine wave. Set meter to DC volts, not DC amps. Some meters you just set to 'auto' and it will figure out AC or DC and voltage, and it will indicate all of that info on the display.

Just for info, the DC voltages we are working with here are not hazardous to a human.

Touch the red meter test point to metal on the red clamp from the panel, and black meter test point to metal on the black clamp and then you should see the meter settle somewhere near 20 volts. Examples might be 18.2, might be 19.6, or it might even go above 20 and settle on say, 22.4 volts, somewhere in there. It might fluctuate a bit, but the reading should be somewhere around 18, 19, or maybe a bit above 20. Some smaller panels will read as high as 22-24 volts with no load!

If you get a range of voltages way different from this, say its showing 5.75 volts, well Houston, we have a problem!
 
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I really appreciate you tx2sturgis :) My mm (Greenlee DM-20) has all kinds of settings. But your description of "DC scale is marked with a solid line above a set of dashed lines and AC is marked with a 'wavy' or curved line representing a sine wave. Set meter to DC volts" did the trick.

My little panel showed a consistent 24v in full sun and did go down as I purposefully shifted the panel. So I guess it still works.

Now to figure out what to do with it. I am getting ready to depart sticks&bricks for ft traveling and storage is at a premium. Decisions, decisions.

TY for your help.
 

tx2sturgis

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Very good to know you have a working panel....thanks for the update.

I actually have one of those little Greenlee meters! Oldie but goodie.

And just a note, that meter will not be able to measure the full amperage (DC current) from that panel. The highest DC amps it can measure (without an external shunt) is less than 1 amp.

So it's a good thing we did not try that...

And good luck on the transition to full time!
 

Happy Camper

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I was so tempted to joke and "recommend" the tried and true tongue on the 9 volt battery trick. But I'm positive someone would do it with a 250w panel lol.

Do NOT do this if you understand the reference! And if you don't? Carry on.
 

maki2

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It’s easy just pack up everythingy or anothery that you might think you could possibly find a use for. Then if 2 years later you have not used it donate it to a free pile at the RTR.
 
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