billtex wrote:
our intention is to have a single panel. We don't use a lot of juice. Is there any reason we would need a MPPT or will a PWM controller be sufficient for a small system?
I have a pretty big solar system on trailer I tow with my TC, and did a big write up on mppt vs. pwm chargers on a thread that was running
here back in May.
Here's the write up:
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DWeikert wrote:
If the panel you select has a peak voltage closer to 14v, then you lose the advantages of MPPT and may as well go with PWM. On the other hand, as the panel voltage increases the greater the advantages of an MPPT controller.
100% Correct.
Let me see if I can help explain a little better. As a lot of you know, I'm an Electrical Engineer by day, TC'er by weekend.
First, you have to understand what "Watts" actually is. It is a simple math equation, not a direct measurement -
Watts = Voltage x Amperage. or the age old P=IxE.
Also worth noting, this conversion can be used for running your microwave off a 12v inverter. (There's other formulas involving watts, but lets use this one for now). If you buy a watt-meter, like a Kill-O-Watt, it's measuring volts and amps, then displaying the two multiplied.
Let's do a real -world example--
You want 200 watts solar. You found a great deal on a 200w panel, it's voltage is 20 volts, it's output current is 10amps, hence 200 watts.
**Here's the problem**
Sure, it's 200 watts, but it's output current is 10 amps. A PWM controller will NEVER give you more than those 10 amps - and that's in a perfect lab-controlled world. Real world, probably 8.5-9amps, but for now, we'll say 10.
Pulse Width Modulation is nothing more than a fancy high speed on-off switch - that's how it regulates charge - on or off pulses, at a very high speed, varying in duty cycle. Dead battery - no pulses, full on duty cycle, 10 amps. Closer to full battery, the charge pulses become shorter in time, until finally you go into float mode with just very short pulses.
**Here's where MPPT comes in**
Your 200 watt panel puts out 10 amps @ 20v. MPPT does a voltage step down/amperage step up conversion. It drops 20v to 14.4, and by doing so,
amperage gets converted UP to 13.8amps (in a mathematically perfect controller anyway.)
As the battery voltage comes up, the charge is of course regulated like your 12v converter until it finally goes into trickle charge mode.
**So - the moral of the story**
If you're going to go PWM - you need a LOW VOLTAGE panel.
Advantage to low-voltage panel--
* can use a much cheaper PWN controller
Disadvantages -
* Needs larger wire to handle full current of panel and minimize loss to due to wire resistance.
* Needs much more sunlight before it attains charge because you must get the panel voltage over 14 before it will actually charge.
**If you're going to go MPPT, you need high voltage panels, or panels wired in series.
Advantage to high voltage panels--
*Charging will start at a much lower light level - if you use 46volt panels in series, 92vmax, it takes very little light to make 92v of solar to cross over 14v and begin charging.
*Much smaller wire needed - high voltage panels are low in amperage, high in voltage, less loss due to wire resistance.
Disadvantage -
* needs a more expensive MPPT controller
* Be careful installing them, install them inside a garage, not outdoors. 46Vdc can bite you, ask me how I know.
