Howto: choosing a charge controller for ~250-400W system

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WYsolar
WYsolar Solar Expert Posts: 31
Thanks to the kind help of folks on the forum I am a new guy who is getting more knowledgeable about putting together my first solar system. It will be a portable backup design, with probably two 200Ah 6v or 12v batteries on a hand cart. The panels will be portable as well and placed on an angled frame and set on the ground. I will just roll the batteries to the back of the panels and plug them in. I hope to afford a controller and small (1500w) inverter too to make a portable emergency backup station to power small loads like my ham rig, dvd player, radio, etc.

I may have happened upon a few used 200w (or possibly 115W) panels that I might be able to afford. Depending on which I can obtain I may have a 230W or 400-600W initial setup. I *believe* these are all 12v panels, but have not been able to confirm.

Question: How do I choose which controller to use?

I have read all about the PWM vs MPPT types and I understand that somewhat. After you chose between these two, what is the next important thing? Do you then choose by size? If so, how do you do that? Are battery charging features (float, etc.) important?

To start I have been looking at the Morningstar Sunsaver MPPT, the Rogue 3024 , and the BlueSky 2000E. I would really like to keep the cost down to $250 or less.

I don't see myself ever building a much bigger system than 600-800W. So I do not need a grid-tie capable controller, nor one that can handle lots of panels.

Thanks again for all the forum's awesome help.

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,442 admin
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    Re: Howto: choosing a charge controller for ~250-400W system

    600-800 watt is around 42-60 amps on a 12 volt controller... So, the smaller MPPT controllers may be out of your price range anyway.

    For an MPPT controller (remember, they can "safely" limit their output current even if you have a larger array):
    • 800 watts * 1/14.5 volts * 0.77 panel+controller derating = 42 amps

    For PWM controllers--they cannot control their output current--so check the manual for their "designed" rating (Isc, Imp, x1.25 or x1.56 derating factors, etc.).

    For a portable system to power Ham, electronics, etc... I would suggest looking at a small TSW inverter like the MorningStar 300 watt (600 watt for ~10 minutes) 12 VDC inverter. It has "sleep" and remote power off modes that can really save energy and switching costs.

    If you need more power, you might get a 1,200 watt MSW inverter to run power tools, etc...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • WYsolar
    WYsolar Solar Expert Posts: 31
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    Re: Howto: choosing a charge controller for ~250-400W system

    It appears that the panel voltage will be a deciding factor.

    If I have 400W of 12v panels I have 33A. If they are instead 24v I have only 16.7A. At the 24v size I could use the BlueSky 2000E ($237 at solarblvd.com) which is a 25A controller.

    Is my thinking correct on this?
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Howto: choosing a charge controller for ~250-400W system
    WYsolar wrote: »
    It appears that the panel voltage will be a deciding factor.

    If I have 400W of 12v panels I have 33A. If they are instead 24v I have only 16.7A. At the 24v size I could use the BlueSky 2000E ($237 at solarblvd.com) which is a 25A controller.

    Is my thinking correct on this?

    no your thinking is wrong. when the pv voltage is higher an mppt controller does not just down convert the voltage to 12v, but it almost acts like an ac transformer in that it will raise the current almost proportionally too. that is why the current ratings for mppt controllers are based on the output current and not the input current. well, that and the fact it will hold the output current at its rated output even if there is enough on the input to translate to more on the output. in other words a 5a 24v pv will roughly output 10a at 12v from the mppt cc. if the mppt cc has a current limit of say 20a then you can have 2 of those pvs, but a 3rd will go over the output limit that the 20a mppt cc has and no more current will be realized than 20a.

    also note that a 30a mppt cc like the rogue mppt 3024 may work fine, but with a 33a output it could clip off any extra gains by the mppt action bringing it to a steady 30a output and a 3.33a loss. most times this much output won't be realized off of a 400w array due to efficiency factors and other circumstances, but it can happen. being that tight would not allow any wattage upgrading either so opting for a 45a or even a 60a mppt cc might be desirable.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,442 admin
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    Re: Howto: choosing a charge controller for ~250-400W system

    WYsolar,

    Yes--Sort of.

    There is the output voltage and current (MPPT can "manage" those directly).

    And there is the input voltage/current. PWM and MPPT both have a minimum input of ~17.5 to 18.6 volts (for a 12 volt bank).

    For PWM the "minimum" input voltage is also the "optimum" input voltage (i.e., the panels are most efficient at Vmp~17.5 to 18.6 volts). Neither controller will perform as well with 16 volts or less. And PWM controllers will be "more inefficient" with voltages >>18.6 volts.

    Whereas MPPT controllers can "efficiently down convert" from high voltage/low current from the solar array to low voltage/high current needed by the battery bank (typically a buck mode switching power supply in MPPT controllers for down converting.

    MPPT controller can operate from Vmp of ~17.5 to ~100 Volts input (for 150 VDC maximum input voltage power supplies) (higher Vmp-array minimum for 24/48 volt battery bank -- ~35 volts/70 volts).

    If you have Vmp "matching" the battery bank voltage (i.e., 17.5 to 12 volt bank)--Then either controller will probably do the job.

    If you have high/not "battery matched" Vmp solar panels (typically > 100 watt solar panels), then you almost are forced to MPPT type charge controllers.

    Also, if you have a long wire run from the Array to the charge controller/battery shed--The "high input voltage" Vmp-array system will allow you to use much smaller gauge wiring from the array to the battery shed (around 10-20 feet or farther with higher wattage arrays, higher voltage arrays can save lots of copper costs).

    Remember that Power=Voltage*Current ... Instead of 17.5 volts, you run 87.5 volts -- that is 5x the voltage, and 1/5 the current -- With much more headroom for voltage drop.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • WYsolar
    WYsolar Solar Expert Posts: 31
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    Re: Howto: choosing a charge controller for ~250-400W system

    Ok. Now I am understanding this part of the project a bit better.

    Cariboo mentioned this in another thread: "However what goes in to an MPPT controller isn't as important as what comes out of it. Your MidNite Classic will pick what it thinks is the best Voltage and current for the panels to output appropriate charging power to the batteries. That's why we use this formula for MPPT output: array size * efficiency / minimum charge Voltage."

    So now I understand that a mppt controller is rated on amps OUTPUT to the battery bank. I don't have to be as concerned that my panels will output more (volts or amps) than the controller can handle if I use this type. Likewise the mppt style can take a 24v panel and convert its output to the "best voltage and current" for my two 6v (wired to 12v) 220Ah batts.

    I am hoping to start out with either 2 x 200W panels (hopefully 24v) or 2 x 115W (12v). Now I just have to determine (using the rules of thumb the mods have already shared) to see if my panels will provide the current I need to charge my batteries appropriately.
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
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    Re: Howto: choosing a charge controller for ~250-400W system
    WYsolar wrote: »
    Ok. Now I am understanding this part of the project a bit better.

    Cariboo mentioned this in another thread: "However what goes in to an MPPT controller isn't as important as what comes out of it. Your MidNite Classic will pick what it thinks is the best Voltage and current for the panels to output appropriate charging power to the batteries. That's why we use this formula for MPPT output: array size * efficiency / minimum charge Voltage."

    So now I understand that a mppt controller is rated on amps OUTPUT to the battery bank. I don't have to be as concerned that my panels will output more (volts or amps) than the controller can handle if I use this type. Likewise the mppt style can take a 24v panel and convert its output to the "best voltage and current" for my two 6v (wired to 12v) 220Ah batts.

    I am hoping to start out with either 2 x 200W panels (hopefully 24v) or 2 x 115W (12v). Now I just have to determine (using the rules of thumb the mods have already shared) to see if my panels will provide the current I need to charge my batteries appropriately.

    Yes! The MPPT CC, which is rated primarily by output current, will therefore be able (given enough panel wattage) to deliver twice as much power to a 24-volt battery bank as to a 12 volt battery bank, based on its maximum output current rating.
    However, even some MPPT manufacturers set an upper limit on the PV array wattage they can handle during Bulk mode for a particular system voltage. Check your instructions.

    NOTE: See later clarification from Niel. Thanks!
    SMA SB 3000, old BP panels.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Howto: choosing a charge controller for ~250-400W system
    inetdog wrote: »
    Yes! The MPPT CC, which is rated primarily by output current, will therefore be able (given enough panel wattage) to deliver twice as much power to a 24-volt battery bank as to a 12 volt battery bank, based on its maximum output current rating.
    However, even some MPPT manufacturers set an upper limit on the PV array wattage they can handle during Bulk mode for a particular system voltage. Check your instructions.

    wysolar,
    just so you don't misunderstand what he's saying here you won't get more power from the array by justy outputting to 24v as that is pretty much set by how much wattage in pv you buy. it is the controller's power handling ability that increases due to the fixed current output at the higher voltage. from v x i = w a 60a cc at 12v is 12v x 60a = 720w. now at a 24v output at 60a you get 24v x 60a = 1440w or double the wattage handling ability. the 24v output at 60a will need twice the pvs to produce as what the same 60a at 12v produces.
  • WYsolar
    WYsolar Solar Expert Posts: 31
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    Re: Howto: choosing a charge controller for ~250-400W system

    UPDATE -

    I am slowly learning about the problems of designing one's own solar system. Lots of trade-offs involved regarding solar panels vs. voltages vs. required controllers etc.

    I have some across a fellow selling 2 x Canadian solar CS6P-200P 200W panels for $1/watt. These are new and he just wants to be rid of them. The specs are:

    Pmax = 200W
    Vmp = 28.9
    Imp = 6.93
    Voc = 36.2
    Isc = 7.68

    Again I am trying out building a small portable solar backup project which has 2 x 6v golf cart batteries at 220Ah each. I will eventually install a Morningstar 300 inverter in the future.

    Question: Am I correct that, if I buy both panels, am I restricted to buying a MPPT controller because of the higher voltages of these panels?

    If so I am considering a bluesky 25A or a Rogue 30A...opinions as to which is better?

    Thanks for being patient with a new guy. I would rather ask lots of Qs before buying. :)
  • Dill
    Dill Solar Expert Posts: 170 ✭✭
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    Re: Howto: choosing a charge controller for ~250-400W system

    get the Rogue. I own both the Blue Sky 2512ix and the Rogue, and the Rogue wins hands down.