Am I on a reasonable track?

Montanaclan
Montanaclan Registered Users Posts: 9 ✭✭
Hi all - I just joined after finding this site and seeing how incredibly helpful this community is, and how detailed and informative many posts were.   So I've already benefited - thanks.

Like many, I'm setting up a new system on a small RV and looking to be frugal.   I can get my hands on a pair of Suniva ArtiSun 60's - 275w each at 38.5v open circuit.  (this model in the 275w build; specs at 8.8A, though 275w/38.5v=7.1A; I've been using the ful 275w in my calcs to be safe, but given panel age (5y), they might be less..  I can get these for $150 each.

I expect to either 2 deep cycle 12v batteries in parallel, or 2 6v golf cart batteries in series, for about 100 useable AH.  My rough calculations for needs are about 110AH in the cold months (owing to the furnace fan) and about 80-85 in the warm months.    My CPAP is by far the biggest draw - nearly 50 AH per night when I use the full features that make it comfortable.  We're a family of five who will do primarily off-grid usage in 3 seasons here in western Montana.  

Based on what I've read, I'd go with an MPPT controller and roof mounted panels.  The roof is pre-wired with MCF connectors to an unused access panel made for a charge controller.  

With this arrangement and a nod to Mr. Ohm, I'd figure that - from each panel - I could get 12v out from the MMPT controller to the battery at as much as 23 amps (38v*7.1A input = 275w = 12v*23A output to the batteries).  Or likely less, when charging at above 12v.

Seems to me that I should run two parallel panels to have access to the 110 AH I need in the colder months?  What am I missing?   Am I correct that charge controllers are rated by output Amps, rather than input?    Would I go with a 60A controller to be safe?  Would it be advisable to go with a larger battery bank or to ignore the sticker price and go with a lithium battery selection?    An I'm encouraged that I could have enough power to even wire in an inverter to add access to some other creature comforts. 

All feedback welcome - I'm concerned that I've done just enough research to have missed something important, or to have made a silly assumption!


Comments

  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    With this arrangement and a nod to Mr. Ohm, I'd figure that - from each panel - I could get 12v out from the MMPT controller to the battery at as much as 23 amps (38v*7.1A input = 275w = 12v*23A output to the batteries).  Or likely less, when charging at above 12v.
    Un fortunately, solar panels only put out about 75% of their 'name plate' on a normal basis. Unfortunately Suniva doesn't print the Normal Operating Cell Temperature values (NOCT) but just gives the temperature coefficients.

    Here are a couple examples of printed NOCT values, I've looked at enough that 75% is a pretty good number.

    I don't know if you also understand that charging lead acid batteries is not 100% efficient, Flooded lead acid batteries charge at around 85% efficiency. Also inverters take a chunk, running at 85-95% efficient. Usually figure about 50% system losses for lead acid based systems using AC output.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    You have enough un-accounted for losses, that 12V 200ah of storage is going to be borderline, if your medical device consumes 50ah overnight.  Either larger battery bank, higher voltage bank, or maybe work the batteries hard and replace them every 6 months, or run with 12v 200ah and see if you need to adjust that when the batteries fail and then decide if you use it enough to justify more expense for Li batteries.
    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 ,

  • Montanaclan
    Montanaclan Registered Users Posts: 9 ✭✭
    Thanks to you both.  New and helpful things to think through before committing.  Much appreciated.