MPPT input voltage choice

706jim · December 2019

From my signature one can see that I'm presently using an old C40 PWM controller. It has worked faultlessly since 1996 but I'm considering an upgrade to an Outback FM80 unit. I have two banks of three 330 watt 72 cell Canadian solar panels with open circuit voltage of about 45 volts. Wiring runs are about 50' for the southwest facing panels and maybe 70' for the southeast facing panels. For wiring I use triple #14 each way for each array. Would I be better off to run each array in series to get about 135 volts to the charge controller or just leave them in parallel at about 45 volts? There would be somewhat less line loss at 135 volts but I don't know if it would be significant.
One reason to leave the panels in parallel would be to keep the C40 as a backup in case of problems with the Outback.
And unless I'm mistaken, I think now would be the time to retire my 7 old Arco's as their current would no longer be matched to the larger Canadian solar panels unless I kept the outputs at 45 volts in parallel.

BTW, the C40 has survived input amps of up to 57.5 without kicking out even though that is obviously above its rating.

icarus · December 2019

Jim,

I would certainly consider series wiring them to the highest possible voltage, assuming that there are no shading issues, simply for line loss. I understand the idea of keeping the C40 as a spare, but remember, it is old, and being pwm it would not be an ideal candidate. I have far too many “spares” that I now have spares for spares and spares for hardware long retired. A while ago I gave away a whole collection of Xplornet Sat internet hardware. I had a number of dishes (in case a tree came down and got the 1st, 2nd or third, a bunch of modems and Trias along with miles of coax. In my case, getting replacements were difficult, a matter of many days or weeks. Nowadays, most of the stuff you would really need may be on the shelf at Meir Hwd, or certainly two days at Rydens.

That said, If you want to retire the Arcos, let me know, I would be interested in them.

Tony

PS I haven’t forgotten about my previous threat to be in touch off site on another matter... still in the works,

T

706jim · December 2019

Assuming I buy the Outback, I would give you a deal on the Arcos that you couldn't refuse.........Will be in the spring.

jonr · December 2019

There is a tradeoff between line losses and charge controller losses that you should be able to calculate (at least roughly).

mike_s · December 2019

706jim said:

...Outback FM80 unit. I have two banks of three 330 watt 72 cell Canadian solar panels with open circuit voltage of about 45 volts. Wiring runs are about 50' for the southwest facing panels and maybe 70' for the southeast facing panels. For wiring I use triple #14 each way for each array...

Those put out about 9A each at max power. So that's 27 A going through an average 120' (have to count both + and -) of 3x 14 ga.

14 ga has a resistance of 2.575 ohms/1000 ft. So, the resistance of your runs is 2.575 * 120/1000 / 3 ~= 0.1 ohm. Voltage drop at 27 A would be about 2.7 V. 27 A * 2.7 V = 73 watts lost. That's a loss of about 73/990 ~= 7%.

MPPTs lose efficiency as voltage increases. Outback has a graph in their manual showing about a 2% loss with a tripling of voltage (34V to 100V). Putting the panels in series would reduce the wiring loss by 2/3 (saving 4.6%). So, with increase in wiring efficiency and the decrease in controller efficiency, you'd pick up a couple of percent.

706jim · December 2019

Mike, very well explained. Having had no experience with an MPPT controller, I would have thought that a higher input voltage would incur lower losses at the controller. In fact it goes the other way. As installation won't be until May, I'll have time to ponder both options.

mike95490 · December 2019

Losses:

In the cables, it's widely distributed and seldom causes a hot spot

With a MPPT Controller, the wider the difference from the PV voltage to the battery voltage, causes a slight decrease of efficiency. This efficiency loss is realized as Heat, the charge controller runs hotter. I see this with my classic 200 running at 170V, going to a 65V charge on my battery, over 800w of solar, the fans in the controller run quite a bit. I had no choice - could not safely fit larger wire into existing underground conduit.

MPPT input voltage choice

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