12v panel to keep 48v bank floating

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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #32
    The only load I have now is the controllers. The security/monitoring loads, if any, are a consideration for future. I may even run them off some old golf carts to be sure they don't drain the big bank if Murphy strikes. Maybe your simple and cheap design would work for that.
    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
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  • t00ls
    t00ls Solar Expert Posts: 250 ✭✭✭
    #33
    Estragon said:
     depending on the weather they can get covered by sticky snow early in winter that won't melt until spring.



    Any thoughts?

    you could use a dc voltage heat tape that ran off a separate panel and battery, that way it didnt take away from the other system
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  • mvas
    mvas Registered Users Posts: 395 ✭✭✭
    edited January 2017 #34
    Estragon
    You need to figure out WHY you cannot maintain 100% SOC and implement a solution to fix that problem first.
    That is issue #1.

    Per Trojan ... it is imperative that your valuable batteries are at 100% SOC before long term storage and during cold weather.
    1) 100% SOC prevents sulfate crystals from forming and hardening during long term storage.
    2) 100% SOC gives you freeze protection down to  -70° F / -56° C.

    PSOC ( Partial State of Charge ) must be prevented, especially since the battery is dormant for many months.
    Just adding heat does prevent the freezing issue but it also increases the rate of self-discharge.
    Increasing the rate of self-discharge makes the PSOC problem even worse, which creates even more sulfate crystals.

    Resolve the 100% SOC issue first, then add a "heat solution", secondarily.
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    edited January 2017 #35
    @t00ls - using heat tape to clear even one string for each array would certainly help, but when I estimate the amount of power needed to do the job it doesn't seem to make sense. Likely better to use extra panels to keep banks charged. Might make sense to use heat tape to make it easier to clear panels by hand though.

    @mvas - the heat idea was for panels, not for batteries. I know it's important to keep banks charged. I'm just trying to figure out how best to do so, and in particular whether using a boost controller for the 48v bank made sense.

    At this point I'm leaning towards just buying enough extra panel to charge with small pwm buck controller. Maybe I'll use that to run a pump for solar thermal water heating in summer or something. Having extra panel might also allow me to increase bank size at some point if I want.
    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
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #36
    To save looking back on the thread, the objective is to provide a float charge for a 12v bank and a 48v bank over the winter when main arrays are prone to getting and staying snow covered.

    In the end I bought two 150w 12v panels and two Morningstar 45a PWM charge controllers.  This solves the problem for the 12v bank, but I'm still pondering options for the 48v bank. 

    One option is to just get enough large panels to hook up to the other CC, but there are a couple of reasons I'm still thinking of alternatives.  One is I may want to expand the 48v bank when this bank dies (hopefully some years from now), and will likely want to expand array at that time.  This will involve clearing (lots of) trees, building/buying racking, etc. so I would prefer to hold off on buying panels until I know needs better when the time comes.  Even making room in a guaranteed sunny spot in winter for a few extra panels now will be a challenge, and I'm concerned that getting more matching panels in future may be a problem.

    Anyway, assuming I do get panels now, what is a realistic minimum Voc for 48v PWM controller, would 2x ~30Voc panels do it?  Panels would be cold, but batteries would be cool too.  Could do 3, but 90-100Voc seems a waste with PWM controller.

    Another option I'm thinking about is putting a step-up transformer in front of the 48v controller and power off the 12v panel(s).  With PWM, as I understand it, there's no issue of controller MPPT sweeps confusing each other.  The controller just burps out whatever voltage it gets in pulses until voltage reaches Vabsorb, then slows the burps to float after absorb.  There would just be maybe 20a current available at ~18v supplying two loads; 1) the 12v transformer and on to a 48v PWM controller and 2) the 12v PWM controller.  So a couple of questions:
    1.  Would this work?
    2.  If so, is there anything in particular to look for (or to avoid) in the transformer?
    3.  Would putting the panels in parallel be okay, or would it be better to use 1 panel per load?

    Thanks
    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
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