Thursday, Dec 04, 2014 at 16:35
Kelpie,
There are some crap MPPT regulators out there. Some are not proper MPPT at all.
So providing you don't have one of those, and instead have a good, functioning one you will get the best from your panels in all conditions until the panel voltage drops below the regulator's minimum tracking voltage.
Panels work on light intensity, mostly visible light. So it can be cloudy but still enough light to generate useable output. What happens in cloudy conditions is that it's not the voltage that drops, it's the ability to produce power. Power is the product of volts times amps.
Simplistically the MPPT reg will adjust the load it applies to the panels to keep the product of volts time amps at a maximum. Some, particularly the more sophisticated ones, are better at it than others.
In monocrystalline panels the Vmp (maximum power voltage) is typically about 3/4 the open circuit voltage (Voc), but that's an approximation - it can vary with brand and prevailing conditions.
The simplest budget systems assume that rule. They
test the Voc and then load the panels to hold the voltage at about 3/4 of the Voc. The power produced is then fed to a circuit that changes it from 40 volts at 3 amps (using your figures) to, say, 12 volts at 10 amps at the battery.
What happens then is further complicated by the multi-stage charging algorithm usually built into these regulators, but let's just stick to the solar part for the moment.
When a cloud comes the regulator would adjust the load on the panels so that the volts stayed at 40. Because of the reduced light, fewer amps can be produced, say 1.5. So power produced is 1.5 amps times 40 volts, = 60 watts. That is then converted to, say 12V and 5 amps at the battery.
There are more sophisticated methods in more advanced controllers, where the controller tries a series of voltage/current combinations until it finds one that gives the maximum product. There are a number of ways of doing this, no need to discuss them here, but the advantage of those is that they get the absolute best out of the panels in the prevailing conditions.
You won't always get the stated wattage out of your panels. The stated wattage is at a standard set of conditions which don't happen much in real life. Cell temperature in particular affects the output of panels, and being dark in colour they do get hot in the sun. After all losses and factors are accounted for, you will be doing ok to get, on average, about 70% of your stated 120 watts as useable output. So your sunny 40 volts and 3 amps could be 40 volts and 2.1 amps and your cloudy 1.5 amps could be only 1.05. But on a good day - cold clear
bright sunshine on a mountain top :-) - you could get 100% and maybe more.
Cheers
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