Newbie question about MPPT controller

Arck
Arck Registered Users Posts: 1
Hello everyone, and thanks in advance for any advice you can give me about the solar panel I'm trying to set up :)
I have two solar panels, one generates 250W at 24V, and the other is 130W at 12V. I want to use one of them for my RV (I wasn't really planning on connecting both of them, because they have different amperage and volts, so I guess I'll just sell the one I don't use), and I'd rather use the 250W one. I also have two 12V batteries with 95AH each, and my plan is to connect them in parallel in order to have a total of 190AH at 12V.
Now, the problem is that, as far as I understand, it seems like I'll definately need an MPPT controller if I want to use the 24V, 250W panel, with my 12V batteries. I've been looking at MPPT controllers, but they're all pretty expensive, and the only ones I can afford go up to 15A, and the description says that the max wattage of the solar panels is 200W for a 12V system, and 400W for a 24V system.
So, that's where my question comes in: does that 12V and 24V "system" refer to the batteries or the solar panel? Because the solar panel is 24V, but the batteries are 12V... and, depending on that, I might be able to use that controller or not.
And, if it's the batteries they're talking about, meaning I'd have to buy a more expensive controller (20A would work, probably?), is there any way you can think of to make my system work with the 24V solar panel without having to spend so much money on a controller? I'd really hate to have to install the smaller panel when I have the bigger one, but I might not have any other choice because of the price of 20A MPPT controllers...

Thanks once more, I'll be waiting for your answer! :)

Comments

  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: Newbie question about MPPT controller
    Arck wrote: »
    So, that's where my question comes in: does that 12V and 24V "system" refer to the batteries or the solar panel? Because the solar panel is 24V, but the batteries are 12V... and, depending on that, I might be able to use that controller or not.

    Welcome to the forum.

    Regarding the MPPT 12 volt system refers to the battery. There are some "lower" priced MPPT controllers that perhaps you have overlooked. Rogue makes one with a good reputation. Have you looked at Steca or Phocos controllers? Read this thread about Genasun:
    http://forum.solar-electric.com/showthread.php?16470

    You can use the larger panel with a cheaper PWM controller, but you will never get the full wattage. HOWEVER, on those cloudy days where the panels are only at half production you will benefit from the larger panel.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Newbie question about MPPT controller

    Welcome to the forum.

    You're looking at the Morningstar 15 Amp MPPT controller. The "200 Watt" limit for a 12 Volt system is the maximum recommended panel, however 200 Watts of panel will not actually supply 15 Amps @ 12 Volts (more like 13 Amps). It is in fact okay to put a bit more panel on the controller; the 'extra' power will be clipped. A 250 Watt panel could be expected to supply the full 15 Amps and for longer (drop in direct insolation due to sun angle over time) because its full current capacity is about 16. This little bit extra won't hurt anything.

    On a 24 Volt system the MS's panel limit is double because the current remains the same but the system Voltage is 2X a 12 Volt system.
  • Coach Dad
    Coach Dad Solar Expert Posts: 154 ✭✭
    Re: Newbie question about MPPT controller

    Arck- Take a look at this whitepaper from Morning Star
    http://www.morningstarcorp.com/en/support/library/MPPT-Primer2.pdf
    They specifically talk about being able to exceed the input wattage rating. Just don't exceed the input voltage rating.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Newbie question about MPPT controller

    In some cases, folks will use a MPPT charge controller to down convert from a 48 or 24 volt battery bank to a 12 volt battery bank... Efficient, reliable, lots of paramaters to "play" with.

    And, from what little I remember from here over the years, at least some of the Morningstar MPPT controllers were designed for exactly that function (one even had a minor bug fix when the LEDs did not display correctly because the Vpanel input never "went away" at night).

    So--There you have a voltage source with 100's to 1,000's of Amperes available from the battery bank--And just a standard fuse to protect the battery to MPPT controller wiring. No problem with "over wattage" solar arrays... Just need to make sure wire and fusing is appropriate, and input voltage range is correct (over temperature) for the MPPT charge controller.

    Do not try this with a PWM controller. It will be nearly the equivalent of direct connecting a 24/48 volt battery bank to a 12 volt battery bank--And nothing good would come of that.

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