Tom&Shelly wrote:So if your panel is in full Sun, but not pointed optimally, is it the voltage output that drops? I'm designing a solar power system for our non-electrified garage to charge and maintain the teardrop battery, just did some calculations of the solar angle throughout the year, and now I'm wondering if I made a mistake ordering a PWM controller.
Tom
There is a couple of things at play, one is just simple trig. The sun put out X amount of watts per sq foot. If you panel is at an angle that it only is half the size when viewed from the point of view of the sun there is only half the energy to be converted. But modern panels have a 90 degree ( 45 past perfectly perpendicular each way ) where they dont lose much. See the pic below, this is major factor in loss. Another is reflectance, at larger angles the sun reflects more of its energy off the glass on the front of the panel. This is why flexible panels can be better, they are less reflective.
Most panels are also rated at a much higher voltage than what is used. Most "12 volt" panels will put out around 20 volts open circuit, under an actual load they will do 16 to 18 volts. This is done for a few reason, the big one in manufactures minds is warranty, they say the panel will put out "12" volts, but they put out 16, after a 10 years they might only be putting out 13 volts, but that still above the 12 volt warranty. Another is dirt and shade, a dirty panel can lose a few % points and still put out what they say it will, same with cloudy days. it covers their butt.
Where pwm falls on its face is shade, you drop a leaf on a "100" watt panel, it might only put out 50 watts where in full sun it was making 70 watts. but remember watts is amps times volts. Yes the voltage drops, as the shaded cell becomes a resistor. Once the whole cell become dark it kills it all. Now shaded does not mean its dark, it means it not full sun, example is 50% of the energy is being blocked ( over the whole panel it can still produce 50% of the energy. see video below.
So yes if your battery is at 13.6 volts so almost fully charged, and its get cloudy enough that the panel can only put out 13.5 volts at say 5 amps then charging stops. Where a mppt will bump the 13.5 to the 13.7 needed to push power into the battery but it might only be at 4 amps. The other side is the higher voltage wasted by pwm chargers, they just block the extra voltage.
The reason most people say you will get 20 to 30 % out of a mppt charger is everything I wrote above, sun angle, cloudy, shading etc. During the peak direct sun they may put out with in a few % of each other, but at large angles, cloudy etc the mppt will put out more when the pwm would just stop charging. So if you tested it every day for a year, you might get 20% more over the whole year with a mppt, remember its an average, not a perfect day test.
Again if you in full sun and have plenty of spare power, like tony does pwm if fine.
I dont know your install so I cant say what charger you need. But hopefully the above info can help answer that for you.
https://youtu.be/Cwvf6lS7P9s