Panel strings

mountainmanmountainman Posts: 49Registered Users ✭✭
I now have a 40 amp  mppt charge controller 6 100 watt 18.9 22.5 volt 5.29 5.75 amp panels. 8 gauge 125 foot wire. What is the best way to configure for the long distance to minimize voltage drop Controller specs 135 volts at 77f max volts 150. Max pv amps 50.  4 36 cell panels per string max. 

Comments

  • EstragonEstragon Posts: 2,060Registered Users ✭✭✭✭
    2 strings of 3 panels should work, voltage drop ~3% at 125' 8ga. What is the system (battery bank) voltage?
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • mountainmanmountainman Posts: 49Registered Users ✭✭
    12 volt rv
  • mountainmanmountainman Posts: 49Registered Users ✭✭
    To get voltage drop do you use voc 22.5 x 3=67.5 and isc 5.75 per string x 2 =11.5 amps??
  • EstragonEstragon Posts: 2,060Registered Users ✭✭✭✭
    I used max power numbers (Vmp & Imp) for voltage drop. Voltage drop doesn't really matter much at Voc & Isc IMHO, as the panels aren't producing much at those values. I use Voc (corrected for record cold low temp) to ensure it's under controller limit though, as there doesn't need to be much current to overvoltage the controller. In a string of 3 though, you're well under so even really cold temps wouldn't get close to limit. A single string of 6 would be getting close enough that it would be worth checking, but you mentioned max string size of 4, so presumably the panels are the Voc limitation, not the controller.

    For a 12v bank, string Vmp at ~57v (~55v with drop at the controller) is fine. If it was a 48v bank, it would be too low. 600w panels ÷ 12v = 50a (10a over controller output), but in most climates realistic output is likely to be ~400-450w (possibly less on an RV without proper tilt and/or orientation) so should be okay.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • mountainmanmountainman Posts: 49Registered Users ✭✭
     controller specs confused me.   It reads   for 12 volt 520 watts 2 36 cell panels is best  4 is max?? 50 150 volts max or 138 at 25c. Pv 50 amps max Controller will be damaged if pv array is 3x over sized. Can't wrap my head around it  So  vmp for 6 panels is 113 volts  I'm not getting it. I would think you could just put them all in series. But ill take your advice and put it in 2 strings of 3.  by the way Panels are not on rv  pole mounted adj. 3500 elevation subject to sub freezing and frequent snow.
  • littleharbor2littleharbor2 Posts: 820Solar Expert ✭✭✭✭
    Where you have to be careful with the max voltage is the Voc. and especially factoring in the coldest low temperature for your area. The STC. Voc. for six panels in series is 135 volts @ 25degrees Celsius. On a cold winter morning you're going to get well below 25 c. in most parts of the country. When the sun first hits your panels you will have an open circuit voltage well over 150 volts. Only takes once to fry your controller.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • mike95490mike95490 Posts: 7,564Solar Expert ✭✭✭✭
    I'll vote for 3 panels in series, paralleled with another 3S

    You want to not get too close to the kill voltage of the controller.   And the further away from the battery voltage, the less efficient the MPPT becomes, struggling to convert 90V to 12V.  50V would generally be a good balance, if your wire run & conduit can accept it.
    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 ,

  • mountainmanmountainman Posts: 49Registered Users ✭✭
    Thanks all for your help 
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