Which setup would work better ?

327 watt panel 54v 6amp with mppt charger or 2  155w 17v 9.1amp with pwm charger. Charging  4 6v batteries in my RV.

Comments

  • BB.BB. Super Moderators Posts: 26,837 admin
    Either will work fine. If you will be camping in hot weather (deserts), the MPPT+54v panel will work better... Panels when they get hot have Vmp fall. The "17 volt" Vmp charging a 12 volt battery bank can be a bit of an issue. In very hot weather, the Vmp of the panel could fall below ~14 volts while the batteries (assuming flooded cell) need closer to 14.7 volts or so for charging. Not a deal killer, but if you are trying to get the most out of the panels in hot desert sun--It could limit your charging/equalization current.

    Both panels are are a bit small... For a 5% rate of charge, you would need a bout 377 Watts of solar. And if you are full time off grid (muiltipe weeks of dry camping without genset charging), it would be nice to have ~10% minimum rate of charge. For weekend camping, 5% rate of charge can work OK.
    • 14.5 volts charging * 2 strings of batteries * 200 amp (Golf cart) batteries) * 1/0.77 panel+controller derating * 0.05 rate of charge = 377 Watts minimum
    • 14.5 volts charging * 2 strings of batteries * 200 amp (Golf cart) batteries) * 1/0.77 panel+controller derating * 0.10 rate of charge = 753 Watts nominal
    • 14.5 volts charging * 2 strings of batteries * 200 amp (Golf cart) batteries) * 1/0.77 panel+controller derating * 0.13 rate of charge = 979 Watt "cost effective" maximum
    You also have the option of 2x 17 volt Vmp panels in series (Vmp-array~34 volts) + MPPT charge controller.

    Otherwise, what fits best and is most cost effective for your needs. Any of the three setups are very typical configurations.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • arharh Registered Users Posts: 5 ✭✭
    I already have 1 of the 155w panels, can I combine it with a 1 of the high voltage 280 to 300watt panels with an mppt controller?
  • myocardiamyocardia Solar Expert Posts: 96 ✭✭✭
    edited November 7 #4
    arh said:
    I already have 1 of the 155w panels, can I combine it with a 1 of the high voltage 280 to 300watt panels with an mppt controller?
    You absolutely cannot*. Since you already own one, your two intelligent choices are buying a second one, and either keeping your current PMW charge controller, assuming it has the ability to handle twice the current, or buying an MPPT controller for the pair. You can only series two panels that are either identical in both voltage and current, or at the bare minimum, that are almost exactly the same in voltage, and are close in amperage/current.

    *If you did, you would lose 100% of the extra power from the larger panel, making it not at all the way to go.
  • arharh Registered Users Posts: 5 ✭✭
    Thank you
  • arharh Registered Users Posts: 5 ✭✭
    So I have decided to go with an epever 4210a, 40amp mppt charger. I found 140 watt panels that based on there dimensions I can fit 3 panels on my rv. Panel specs are VOC 42.8, VMP 33 , 4.7 amp. . What would be the best way to wire them to get the most charging power for my 4, 6v batteries wired for a 12v system. I am very mechanically inclined but don't know much about solar. A simple diagram on how to wire them would be much appreciated.
    Thanks,
    Adam
  • myocardiamyocardia Solar Expert Posts: 96 ✭✭✭
    edited November 8 #7
    Since you are planning on using a 12V nominal system, you cannot wire 3 panels together. At least you can't without buying what is called a combiner box. The combiner box will allow you to run the outputs from all 3 panels into the box, then only run a single set of larger diameter wires (one positive, one negative) out of the box, to your charge controller. You'll want to use 12 gauge wires into the box, one positive and one negative per panel, then use an 8 gauge pair out of the box, into your charge controller, right above your batteries. This is the box you'll want to use:
    https://www.solar-electric.com/mnpv3.html I would highly recommend also buying three of these DC breakers in the 12A or 15A* version, as well:
    https://www.solar-electric.com/mnepv.html

    edited the breaker sizes above, after the lady reminded me that they are supposed to be somewhat oversized

    *12A if you live in the south, where it doesn't get very cold, or 15A if you live very far above the Mason Dixon line, or live on a mountain, places where it gets considerably colder
  • Raj174Raj174 Solar Expert Posts: 461 ✭✭✭✭
    This video may help illuminate what myocardia has described,



    12 x 300W Renogy PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 195AH HI Power LiFePO4 no BMS, 4000W gen.
  • mcgivormcgivor Registered Users Posts: 1,280 ✭✭✭✭
    edited November 9 #9
    Other than illustrating some bad practice, such as placing multiple stranded conductors under a single terminal,  reference to the SPD, along with the fact two strings of equal capacity don't need a combiner, the video demstrates the importance of using hair spray to keep the head moving without the hair.
      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • arharh Registered Users Posts: 5 ✭✭
    I ordered the charge controller today but have not bought panels yet. If i wanted to use my existing 155w panel, how close in watts, volts and amps do the rest of the panels have to be?
  • myocardiamyocardia Solar Expert Posts: 96 ✭✭✭
    edited November 9 #11
    MPPT chargers check every few seconds for which voltage is going to provide the most amperage. Since the technology behind "12V" solar panels hasn't changed in a very long time, besides their efficiency, you only need to find a panel or two that have absolutely as close as possible to the same VMP as your current panel, and is/are rated for 150, 155, or 160 watts. Note that spending more money on higher wattage panels will do you no good. You will only get the wattage of your lowest producing panel, multiplied by the amount of panels you install. IE, you want a panel or two that are rated for either 150 or 155 watts, at the exact VMP of your current panel, or absolutely as close as possible. I wouldn't buy panels that were more than 0.1V more or 0.1V less, if it were me.
  • BB.BB. Super Moderators Posts: 26,837 admin
    You probably would be OK with panel in the range of Vmp~17.5 to 18.6 volts or so when paralleling panels. Going much below 17.5 volts Vmp can reduce charging current (Vmp falls when panels get hot).

    With MPPT controllers and higher voltage arrays (i.e., a 12 volt battery bank and Vmp~30 volts), you can run 10% (or ~3 volts) between the high and low panel Vmp (30 volts to 33 volts, or 30 volts to 27 volts) and be pretty much OK.

    I think +/- 0.1 volts is a bit severe of a Vmp limit here. Vmp curves are fairly flat going +/- 5% to 10% off the peak does not "kill" the output.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • myocardiamyocardia Solar Expert Posts: 96 ✭✭✭
    edited November 9 #13
    BB. said:
    I think +/- 0.1 volts is a bit severe of a Vmp limit here. Vmp curves are fairly flat going +/- 5% to 10% off the peak does not "kill" the output.

    -Bill
    I never, ever, ever argue with engineers*. Seriously, I know two of them, and I've not once won an argument with either of them! I admit, that I'm kind of a perfectionist. I would honestly do my best to exactly match any panel I was adding to an array, VMP-wise. Thanks for giving him the real scoop, though. I'm sure he appreciates it immensely, along with making his panel-matching experience much, much simpler. :D

    *Bill/BB. is an engineer.

    edit: Not that I disagree with Bill, mind you. My point was only that anytime my opinion differs from Bill's, listen to Bill every time.
  • BB.BB. Super Moderators Posts: 26,837 admin
    I am always learning--And I am certainly not correct all of the time--Ask my wife.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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