MPPT input voltage choice

706jim
706jim Solar Expert Posts: 518 ✭✭✭✭
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.
Island cottage solar system with appriximately 2500 watts of panels, 1kw facing southeast 1.3kw facing southwest 170watt ancient Arco's facing due south. All panels in parallel for a 24 volt system. Trace DR1524 MSW inverter which has performed flawlessly since 1994. Outback Flexmax 80 MPPT charge controller four 467A-h AGM batteries. Insignia 11.5 cubic foot electric fridge 1/4hp GSW piston pump. My 31st year.

Comments

  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    edited December 2019 #2
    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
    706jim Solar Expert Posts: 518 ✭✭✭✭
    Assuming I buy the Outback, I would give you a deal on the Arcos that you couldn't refuse.........Will be in the spring.
    Island cottage solar system with appriximately 2500 watts of panels, 1kw facing southeast 1.3kw facing southwest 170watt ancient Arco's facing due south. All panels in parallel for a 24 volt system. Trace DR1524 MSW inverter which has performed flawlessly since 1994. Outback Flexmax 80 MPPT charge controller four 467A-h AGM batteries. Insignia 11.5 cubic foot electric fridge 1/4hp GSW piston pump. My 31st year.
  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    There is a tradeoff between line losses and charge controller losses that you should be able to calculate (at least roughly).

    I am available for custom hardware/firmware development

  • mike_s
    mike_s Registered Users Posts: 156 ✭✭
    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
    706jim Solar Expert Posts: 518 ✭✭✭✭
    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.
    Island cottage solar system with appriximately 2500 watts of panels, 1kw facing southeast 1.3kw facing southwest 170watt ancient Arco's facing due south. All panels in parallel for a 24 volt system. Trace DR1524 MSW inverter which has performed flawlessly since 1994. Outback Flexmax 80 MPPT charge controller four 467A-h AGM batteries. Insignia 11.5 cubic foot electric fridge 1/4hp GSW piston pump. My 31st year.
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    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.

    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,