charge controller for a small system?

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levsmith
levsmith Solar Expert Posts: 56 ✭✭
in Off Grid Solar & Battery Systems #1
Alright guys, I'm not quite ready to buy panels and a charge controller yet but am shopping around. The plan is to do a fairly small system, about 500 watts. I have been looking at the grape solar 250 watt panels. I like the idea that I can buy them at the big hardware store so I can buy one at a time and not have to worry about the crazy shipping prices of the big panels when purchased online.

now my only problem is that I dont know what charge controller to use. I'm not sure I need an mppt with a small system but I dont know of any pwm type cc's that can be used with high voltage panels. The grape solar panel shows 30.7 volts and 8.15 amps. Would a 24v cc work with that?

Or does anyone have a better way of going about this? I appreciate your thoughts!
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: charge controller for a small system?

    Good thing you haven't bought anything yet.

    To start with, what size and Voltage is the battery bank? You need to know that to know what you need for current and Wattage. It will also tell you which panels you can use and with what controller.

    For example the panels you cite have a Vmp of 30.7. This will not work on any system Voltage without significant waste of power potential: the proper Vmp for a 12 Volt system is around 17.5 and the proper Vmp for a 24 Volt system is around 35. To make these "GT panels" work with a battery system an MPPT type controller in order to maximize the power.
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  • levsmith
    levsmith Solar Expert Posts: 56 ✭✭
    #3
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    Re: charge controller for a small system?
    Cariboocoot wrote: »
    Good thing you haven't bought anything yet.

    To start with, what size and Voltage is the battery bank? You need to know that to know what you need for current and Wattage. It will also tell you which panels you can use and with what controller.

    For example the panels you cite have a Vmp of 30.7. This will not work on any system Voltage without significant waste of power potential: the proper Vmp for a 12 Volt system is around 17.5 and the proper Vmp for a 24 Volt system is around 35. To make these "GT panels" work with a battery system an MPPT type controller in order to maximize the power.

    Ah yes, I forgot about the battery bank. At the moment I have two 6v golf cart batteries, but plan to upgrade that to six batteries here soon and keep it at 12v. I forgot that the input needs to match output on pwm controllers, so it looks like an mppt might be in my future. Do you know of a decent mppt cc thats not too expensive and would match up well with 2 of those panels?

    I suppose the price of an mppt with smaller wire at a higher voltage would help offset the cost of 12v panels with thicker wiring and a pwm cc. The run would be about 150ft.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
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    Re: charge controller for a small system?
    levsmith wrote: »
    Ah yes, I forgot about the battery bank. At the moment I have two 6v golf cart batteries, but plan to upgrade that to six batteries here soon and keep it at 12v. I forgot that the input needs to match output on pwm controllers, so it looks like an mppt might be in my future. Do you know of a decent mppt cc thats not too expensive and would match up well with 2 of those panels?

    I suppose the price of an mppt with smaller wire at a higher voltage would help offset the cost of 12v panels with thicker wiring and a pwm cc. The run would be about 150ft.

    Well I see a couple of problems here.

    The first is six 6 Volt batteries at 12 Volts. That would mean three parallel strings of two, with the potential current sharing troubles that can be had with multiple parallel battery connections.

    The second is that MPPT controllers aren't cheap. A pair of 250 Watt panels would output about 32 Amps for 12 Volts. So they would more than fully load something like the Rogue 3048. At that point you'd want to go up to a 45 Amp Morningstar MPPT which is about $400. At that point you have to think about future expansion. For example three parallel 220 Amp hour batteries becomes 660 Amp hours and your charge current should go up to 60+ Amps. Then you need at least a 60 Amp controller, and preferably an 80 Amp like the MidNite Classic. Now you're looking at a $600 controller.

    This comes back to the issue of how much stored power do you need? If it truly is 7920 Watt hours (660 Amp hours * 12 Volts) you'd be better off increasing the system Voltage and reducing the current requirements.

    The 150 foot wire run will require some high array Voltage to avoid the need of extremely large wire size.
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #5
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    Re: charge controller for a small system?

    If you are not in a rush, there is the new MidNite product that is MPPT called the 'KID" and it is expected to be hitting the market soon , ~early 2014? Price? to compete with another well known brand.

    http://forum.solar-electric.com/showthread.php?20484-Midnite-s-quot-The-Kid-quot&highlight=KID+Classic
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
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  • levsmith
    levsmith Solar Expert Posts: 56 ✭✭
    #6
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    Re: charge controller for a small system?

    Thanks for the replies. Almost sounds like I should step it up to a 24v system. I believe I am going to sams today so I can probably pick up a couple more batteries. That should alow me to use some of the smaller amperage mppt cc's right? How did you figure 32 amps at 12 volts? Just curious how to do that calculation
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  • jcheil
    jcheil Solar Expert Posts: 722 ✭✭✭
    #7
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    Re: charge controller for a small system?

    In the long run, better off starting with 24v like they said and just spend the $600 to get an FM 80 controller. That will be your foundation.

    The $200 difference in price will be priceless in the future when you expand. And I know right now you may not be thinking that you will expand much but trust me, solar can get kinda addicting. Once you see what you can do with it, it will be like a drug that you keep wanting to get more of.

    Having the "foundation" to be able to expand ALWAYS ends up being cheaper in the long run. With an FM 80 and the rest of your foundation in place you can LITERALLY just plug in extra panels in the future with nothing else to buy. And just the money you'll save in being able to use a reduced WIRE SIZE can be close to $200 sometimes.

    I know it will be hard to believe right now, but someday you will look back and be glad you did.
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #8
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    Re: charge controller for a small system?

    For an MPPT charge controller, the typical maximum current for a solar array can be estimated at:
    • 500 watts * 0.77 panel+controller deratings * 1/14.5 volts charging = 26.6 amps

    Unless you are at a high/dry elevation and/or very cold region, your charge controller will not spend too to many hours a year in "current limit".

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
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    Re: charge controller for a small system?
    levsmith wrote: »
    Thanks for the replies. Almost sounds like I should step it up to a 24v system. I believe I am going to sams today so I can probably pick up a couple more batteries. That should alow me to use some of the smaller amperage mppt cc's right? How did you figure 32 amps at 12 volts? Just curious how to do that calculation

    Standard formula:
    Watts of array * 0.77 (typical efficiency factor of array & controller) / nominal system Voltage (which is as low as you want to go and should be the point where the most current is required, barring loads).

    So: 500 Watts * 0.77 = 385 / 12 = 32

    The same array on 24 Volts would produce 16 Amps.
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