I am an electronics know-not-much 🙂 I do understand the basics of current/voltage/amps etc.
I have an array of 10 x 15watt x 12v (max 17.5v) amorphous solar panels here in the UK. I want to arrange them in 5 banks of two (5 pairs) and when light levels are low (they are a lot in the UK especially during winter) I want to switch each pair from parallel operation into series operation. At the low light levels that exist during the long morning & evening periods and during heavy cloud the panels tend to produce 8-9 volts, which is not much use for charging, But if they switched to series operation, each pair then would produce a useful 16-18 volts. When the sun shines fully I want them to switch back to parallel so as not to over volt the charge controller.
My question is such a control circuit easy or difficult? I have searched & googled but cannot find such a control circuit..
Best Answer
Such a control circuit is possible, but a better idea is to leave the panels fixed in one configuration and deal with the resulting change in voltage with the right kind of switching power supply. If you want to get fancy, you could even implement maximum power point tracking, or something reasonably close.
The switching power supply can't make more power than what the panels produce, but it can convert (with a little loss) from whatever voltage and current the panels want to produce to a different controlled voltage or current (with the other limited by the available power).
If the series configuration always produces more voltage than you need even in low light, then that's how you should configure the panels. The switching power supply then always makes a lower voltage, which keeps it simple. That is called a "buck" regulator, with much information about that out there.
It would be helpful to say what voltage you ultimately want, and what current range is useful at that voltage.
Added:
You now say this is to power a 12V system, which presumably runs from lead-acid batteries although you didn't say that.
One useful feature of lead acid batteries is that they can take reasonable charge current even when full and still regulate the voltage well enough for most purposes. Given that, a really simple solution is to wire the solar panels in series to get the higher voltage you talked about, which is always a bit more than the 12V battery level even on cloudy days (when light is really low the voltage will be lower, but then there is so little power to be irrelevant), and just connect this to the 12V rail with a Schottky diode. That will not use the panels most efficiently in high illumination, but probably not so bad on cloudy days from the numbers you give.
A buck converter that runs the panels at the best efficiency for the given insolation and then dumps whatever current it can onto the 12V rail should be a bit more efficient. With a decent converter design, the extra loss in the switcher should be more than offset by running the panels at their optimum efficiency.
However, if you have more than enough power in full sunlight and the real problem is when it's cloudy, maybe the dumb series connection (with a Schottky diode to prevent reverse current when dark) will do it. I'd probably be tempted to try that first and see what you get and how efficient the whole system is on cloudy days when it really matters.