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Originally Posted by thewayneo
Hi--in your recent post you said: "40 feet per cable = one "extension cable" at 80 ft. That's an awfully long run - switching to MPPT (at twice the voltage, half the current) will be beneficial."
I've installed panels on previous RV's but due to weather, trees, deep valleys, ended up with a Honda 2000 (actually a pair). The gen saved my hide a couple times when in deep woods with a dead truck batt, so I'll always carry one no matter what. I remain interested in solar, but I really want 1 single panel around 200 watts, which are usually 24v not 12. In what I quoted from your post, are you saying that you are going to use 2 12's in series and the MPPT converts that down to 12v for the batteries? I can't find a description of a MPPT that clearly (to me anyway) says they do that, but maybe it's just a well known feature.
thanks for the info,
wayne
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That's exactly right. A "Maximum Power Point" controller will take
all of the input power at higher voltage, and convert it into more AMPS at lower voltage for the batteries (assuming that the batteries can accept all the current). They're somewhat more than 90% efficient at doing this job.
Example 1: A single "24V" panel, nominally 240 watts at maximum power, consisting of 31 Volts * 7.75 Amps. With MPPT, this will charge your battery at 14.4V, or maybe 13.7 or 13.8 (whatever you set for the different charge stages), but it will crank out as much as 16A on the battery charging circuit. (BTW, These are all real numbers for Renogy panels.)
Example 2: A single "12V" panel, nominally 100 watts at maximum power, typically running at about 18.9V * 5.3 Amps. With MPPT, if the entire 18.9 volts actually got to the controller, it would send about 7.3A on the battery circuit (at 13.6 volts).
A PWM controller can only cut the voltage, and send 13.6 Volts * 5.3 Amps (just 72 Watts, at "peak power" from the panels and a charging voltage of 13.6V). The rest is wasted. "PWM" rapidly turns on and off the Solar load, and runs it into capacitors, creating an "average" of 13.6V. During the short time periods when it disconnects from the Solar panel, the "excess" power is not being collected. (For 120VAC at home, an incandescent chandelier dimmer switch works the same way, except that you do not "pay" for utility power which you don't use.)
So even with a "12V" panel or parallel array, an MPPT controller can accept and convert more power from Solar into useful Volts * Amps ( = Watts) than a PWM controller can. It's because all of those "12V" panels run at much more than 13.6, or even 14.4 Volts. My 18.9V panels are a bit high, but not greatly out of line with others.
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But PWM controllers cost about $20 -- while MPPT controllers start at a couple of hundred. Still, I am going to buy MPPT: I intend to buy the new Rogue, after the designer finishes shaking out design bugs and begins to deliver in quantity. Until then, my "200 Watts" is really,
really no more than 140 watts -
before consideration of any power losses along the Solar to Controller cable, and those will be greatly reduced when I put my panels in Series.
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Two "12V" panels in Series is a lot like one "24V" panel alone, except that they cost more - they've got duplicate junction boxes, MC4 connector cords, aluminum frames, and backing sheets. My Maximum Power Voltage will be 37.8V instead of 18.9V, and my amperage will be the same as one panel alone (5.3A). I-squared-R power losses will go down - by a square root factor. (That's why power plants run AC wires at extremely high voltages, e.g., 512KV.) But, if the fridge, or air cleaner, and/or batteries can make use of the additional 60 watts, it's like getting more than half of another panel -- and without new wiring or increased weight on the roof.
It will "unleash" my current panels.