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Powering a 12v device directly from a solar panel

BroomJM

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I have a 200w solar panel with 22 volts, open circuit. I have a DROK brand buck converter rated for an input of 8-22v with an adjustable potentiometer that allows an output between 3 and 15 volts @ 3 amps (up to 45 watts). I am wanting to power a 12v heating element that is rated at 25 watts, so it basically draws 2 amps. Can I connect the panel to the buck converter, set the buck converter to ~13 volts output, and wire it (with a fuse) to the 12v heating element?

I know I can do that, technically, so I guess my real question is; will this setup work to power the 12v heating element, when the solar potential at the panel is high enough to create sufficient voltage? (Yes, I know this panel is probably larger than I really need.) Is there any danger to wiring something directly?
 
I would say the main danger is whether, since you are right on 'limit' of 22v, if a cold snap and full sun wouldn't put your panel far enough past 22v to smoke the converter. That's the only thing i'd be worried about on the input side.

On the output side, though, i would suggest actually turning down the output voltage a bit, depending on how the converter acts when its input voltage drops. Most of the cheap step down converters can ONLY go down, which means if your input ever drops below your output, it either A. becomes a pure passthrough and the output drops with the input, or B. turns off!

Option A is fine because you can set it to 13v and it will never 'overdrive' the heater (other than putting 13v to a 12v thing, fine in 99% of cases) and when solar output drops below that, the heater will stay on but will simply drop in power. We basically know it will turn off at 8 regardless because of the 8-22 input range.

Option B would be annoying because depending on how much time your solar output spends fluctuating above and below the 13v line, you might get more total heat output over time setting the output to something like 12, 11 etc and having it run more of the time, versus setting it at 13 and having it turn all the way off every time it drops to 12.9. Now, most likely you're way below the 200w panel rating regardless when it's dropping below 13v, but it still might be frequent enough (cloudy days etc) that you might see a noticeable difference in total heat if the converter turned off at 13 rather than letting the heater continue to run at a lower voltage.
 
Assuming that the maximum power point of the panel is somewhere around 18V, the buck converter will never see the open circuit voltage of the panel. I would test the converter with a bench supply to see what happens to it as the voltage gets low so you will know what happens.

If you are still concerned with the maximum voltage you can use a diode to drop about 0.7V.
 
I mean if the heating element has a thermal switch or thermostat of some sort there will be times the 13v output side is basically open circuit in which case panel input side may rise to VOC.. But yeah, minor concern with minor workarounds. (y)
 
There are all manner of inexpensive voltage regulators out there that would do a much better job. You could build it yourself using an LM7812 chip or buy a ready to go unit. Found one, of many on Amazon that takes 72V-12V and regulates it to 12V at up to 10A for ~$12.


<EDIT> Scratch that inverter. It only handles 72, 60 or 48V input. The description is deceptive.
 
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What's your actual objective, BroomJM?

You have a 200w solar panel and you say you want to drive a 25w heater. Surely there's more to the story than that?

If not, then why not buy a cheapy 25w (30w would be better) solar panel and drive the heater with that? The cost of the cheap panel would be offset by you not having to buy converters and the like.
 
What's your actual objective, BroomJM?

You have a 200w solar panel and you say you want to drive a 25w heater. Surely there's more to the story than that?

If not, then why not buy a cheapy 25w (30w would be better) solar panel and drive the heater with that? The cost of the cheap panel would be offset by you not having to buy converters and the like.

My goal is to heat up a metal watering pan for poultry during the winter. Maybe I'm overthinking it. I already own the 200w panel and it's currently not doing anything.
 
I'd be tempted to just hook the heater directly to the panel.
When the heating element gets enough voltage from the panel, it will run, but not pull any more than the 25 watts it needs? I would be more inclined to try that with a smaller panel.
 
The heater cannot use more current than it can use. That said, it may overheat at the higher voltage. Let's do some math with your numbers:

12v / 2 amps = 6 ohms heater element

Now let's assume that the panel will operate around 18v at max power.

18v / 6 ohms = 3 amps
18v × 3 amps = 54 watts

It might be better to put two of the heaters in series so they don't overheat.
 
When the heating element gets enough voltage from the panel, it will run, but not pull any more than the 25 watts it needs? I would be more inclined to try that with a smaller panel.
Technically any voltage higher than 0, the heating element will do something. The watts of heat produced go up and down with the voltage. It is not linear, as you can tell from Justgary's post. Double the voltage is actually 4 times the watts, etc.

If the heating element is constantly immersed in water it will be hard for it to hurt itself because it will "never" get above 212f or so, but if the pan runs dry and it's trying to dissipate twice the heat it was designed for, it might break at some point, or present a fire hazard. So using the step down converter is a valid approach for avoiding that, IF you're worried about not having water in the pan.
 
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The heater cannot use more current than it can use. That said, it may overheat at the higher voltage. Let's do some math with your numbers:

12v / 2 amps = 6 ohms heater element

Now let's assume that the panel will operate around 18v at max power.

18v / 6 ohms = 3 amps
18v × 3 amps = 54 watts

It might be better to put two of the heaters in series so they don't overheat.

Given the objective of warming water for poultry during freezing conditions, two heating elements would likely be preferable, anyway. Thank you!
 
Yeah, I would think a pan of water in freezing ambient temps might be able to exchange 25w to the outside air continuously which means gotta put more than 25 in it to stay ahead. Now a chicken doesnt need a big space to stick its face in, so you could optimize your water pan’s ‘surface area to mass ratio’ to mitigate heat loss to air. ?

In terms of a heating system element “cannot” use more current, uhh.. ohm’s law. More volts more current. Now it may be that its a diminishing returns thing because of a steep positive temperature coefficient, but if i had some unlimited power source i guarantee you i could melt the thing with current. Not relevant to a solar discussion except to avoid the basic misunderstanding of anything reprsenting a ‘fixed’ ‘draw’, because it’s not fixed and the whole concept of ‘draw’ is what i think sends people down this path where they think a load will only take the current it ‘needs’ to operate and other such untruths.
 
The law of unintended consequences. Real sunny days is when you need it least and the water could likely be too hot to drink. What you want is a linear current booster with a thermostat so there will be heat on the worst days which you don't have with the current design.
 
The law of unintended consequences. Real sunny days is when you need it least and the water could likely be too hot to drink. What you want is a linear current booster with a thermostat so there will be heat on the worst days which you don't have with the current design.

Our warmest winter days are when it's cloudy. Very clear sunny days are when it's coldest. When the temps are warm enough to not worry about the waterer freezing, simply disconnect the panel from the heating element.

There is probably only one way to really know if this idea works or not. :)
 
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