DIY solar charge controller

[LukeB]

I have been working on a homemade solar charge controller, well designing it, and I have been trying to figure out the most efficient way of doing it.

I could just have a mosfet switch the panel on and off to reduce the average voltage and charge the battery but this will waste the energy from the sun when the mosfet is off.

I thought that maybe I could use a buck converter to reduce the voltage down to 14v and charge the battery that way. But wont this still waste energy while the mosfet is off. (see solar charger plain buck converter)

I did some googling then and found that a capacitor over the solar panel will store the voltage while the mosfet is off and allow it to be used later. (buck with capacitor before buck) Will this work??
Why is there no need for an inductor before the mosfet to store the current produced by the panel?? (see charger with inductor and capacitor)

[JS]

Yes, you nees a cap and and then a conventional converter, boost or buck, whatever you need for what you want to do with the energy.

The optimal voltage at the panel depends on a lot of consitions and changes constantly during operation, temp, sunlight angle, shades, etc. You might be ok with a constant voltage or whatever it outputs to go to the converter but if you want to improve your efficeny and get the most energy out of the panel at any given time it worth to check. Sensing voltage and current, every once in a while doing a sweep for some predefined range, picking the maximum power point, staying there for a while and repeat when I/V response changes or some timeout.

JS

[LukeB]

What is the benefit of using a buck converter as compared to simply a switching mosfet.

[JS]

Efficency.

As I said you want to control the current and voltage delivered by the panel somehow. It works as a reverse diode, with some exponential law, at the knee of that curve you have the highest power delivery, the longer you stay there the higher power you get from it and you want that because bigger panels are more expensive to be wasting their energy delivery.

Now, to whatever you want to use that energy for, it will most likely need a different voltage/current than the one delivered by the solar panel. For that you need a DC to DC converter to deliver your energy optimally, to charge the battery as fast as you can within the safe operating area of the battery and the power delivery from the panel.

Now, if the thing is one of those cordless LED sticks for the backyard you are probably find without much efficency in mind, the thing has all day to charge up, you mostly need the light for the first few hours of the evening, and if the day was too dark to fully charge the battery you're not likely to use yard very much as it isn't likely to be a nice clear night. In that case you could pick a cell with the right voltage and current for the battery and add a switch to disconnect them when battery is full. At that scales the saved panel by a complex system might not worth the complexity or not save anything at all.

JS

[LukeB]

Ok that makes sense. I guess my question is is what is the purpose of the inductor? Is it really needed? The PWM will drop the voltage so why have an inductor?

[JS]

Ok that makes sense. I guess my question is is what is the purpose of the inductor? Is it really needed? The PWM will drop the voltage so why have an inductor?

Why do you need an inductor in any DC-DC converter?

You would be shorting two capacitive circuits at diifferent voltage otherwhise, getting a huge peak in current, a horribly bad energy transfer and an uncontrolled voltage at both sides. Baaaad idea... Pwm is fine for inductive and or resistive loads which doesn't care much about the enery deliver, but how much enery and thats it. Like resistive heaters, low power LEDs, electric motors, etc.

For a battery charger you want controlled voltage or current depending on charging condition, and for the panel source as well, so you get a cap at the input and a cap at the output. If you put a resistor to transfer the energy by a pwm ypu waste energy so you put an inductor and a diode and you transfer the energy from one side to the other in a smooth and efficent way. You can get away without the output cap as you have the battery to take care of that, but you might still want some capacitance so the controller doesn't go crazy if the battery is missing.

JS

[LukeB]

Oh I see. That makes sense. Clearly my electronics knowledge is still growing. Haha I know just enough to be dangerous.

Thanks for your help though that really helps me out with my project.

[Siwastaja]

The inductor is used to convert between two different voltages. When you apply voltage over an inductor, you start storing the energy as a magnetic field. Then, the same inductor allows you to convert this magnetic field back to electricity - the great thing is, this can happen at different voltage!

Some circuits won't need it either because there already is an inductor in the circuit - such as in motors. Some won't need it because they are circuits that are not sensitive to the actual voltage. For example, a heater generates heat more quickly with higher voltage, but still works just fine (with the same 100% efficiency) - so you just PWM the voltage to generate right amount of heat, on average.

But, charging a battery requires just the voltage the battery happens to be at, at that time. Similarly, a solar panel produces best efficiency at a certain output voltage (which is not exactly a constant). So, to match these two different voltages together with high efficiency, you need an inductor somewhere.