Series or Parallel Question

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JustinAdams
JustinAdams Registered Users Posts: 5
I have a project that i'm doing which is building a few 60watt 3.5a solar panels. I am trying to figure out if I should hook them up in series or parallel.

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  • ehorn
    ehorn Registered Users Posts: 6
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    Re: Series or Parallel Question

    Alot of good questions and the folks here are very knowledgeable and very giving of their time. Perhaps you could expand on your project idea and that would help with the discussion. I will try to help out a bit with your questions.

    "I read that you need to produce more volts than what you are charging. (why?)"

    Technically speaking, the DC source (solar panels) must always have a higher operating voltage than the battery bank in order for current to flow from one to the other. A handy way to remember this fact is the statement, “Voltage flows downhill.”


    "How does increasing my amps affect the charging? Also, when do I want to increase my voltage?"

    Watts is the value we concern ourselves with.

    Watts = Volts x Amps

    In this case, we assume (3) 60 watt panels @ 18V (nominal) and 3.5Amps, so we have:

    3.5 Amps * 18 volts = 63 Watts each * 3 panels = 189 Watts total panel output

    Regardless of how the panels get wired (series or parrallel) they will still only produce 189 watts (nominal). So it is the job of the charge controller to take the incoming wattage from the panels and use that wattage to properly and efficiently charge the batteries.


    "If this is correct then I should not need to hook them up in series right?"

    This depends on various factors which will affect a design. Some of which are:

    What is your system voltage (i.e. batteries, inverter, charge controller)?
    What is your charge controller capability (voltage, amps)?

    Some other design considerations:

    Temperature affects panel voltages and must be considered in a design which approaches min/max of charge controllers range.
    What is acceptable line losses (voltage drops from PV to charge controller)?
    What is your intent for future expansion?
    How many "strings" do you wish to combine (fusing, breakers, etc...)? This will be based on your system components and how they match up to the panels?


    "Also, when do I want to increase my voltage?"

    Generally speaking, the higher the voltage (system), the more efficient a system becomes. Also higher voltage systmes can support greater loads with less wiring (and wiring protection) costs.

    Here is a link to further reading that may help.

    A successful system is designed around a particular need. Once that is established, components can be selected and sized to best meet that need. Perhaps you could expand on your project goals and the helpful folks here could make some more targeted suggestions/recommendations.

    Best wishes
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: Series or Parallel Question

    First, remember the basic equation for power;

    Power = Current * Voltage = I * V

    So, from a very simple point of view--whether you choose to run 12 volts at 10 amps or 120 volts at 1 amp:

    P=12v*10a=120v*1a=120 watt

    But, notice, that that when the voltage is increase by 10x, the current is reduce to 1/10th...

    And, from the Power equations:

    P=I*V=V^2/R=I*R

    The last one, P=I^2 * R -- means that if resistance of the wire is the same, reducing the current by 1/10, actually reduces wire heating losses to 1/100th... So, within the capability of the equipment, there are some nice reasons to increase voltage/reduce current.

    Now--it is more complex to describe the issues with the solar panels...

    First, there are two major types of solar charge controllers:

    1. PWM (Pulse Wave Modulation--basically On/Off/On/Off--The simple (less costly, less capable, typically used on smaller systems)... This requires your panels to be ~2 volts or so above the battery charging voltage. So--charge the batteries to 15.5 volts (for equalization), and the solar panels have to be ~17.5 volts or higher. If you put two 18 volt panels in series, you will get 36 volts output--which might ruin the PWM controller, and will be wasted on the battery (which will clamp the voltage to 15.5 volts).

    2. MPPT (Maximum Power Point Tracking)--a switch mode power supply--which almost behaves like the DC equivalent of a variable transformer... If the Panel voltage is high (and current low), the MPPT charge controller converts the power to battery voltage (lower voltage, higher current)--and the MPPT controller can do it pretty efficiently.

    So, depending on the type of controller (PWM--use 18 volts for a 12 volt battery bank -- or -- MPPT--use any panel voltage from 18 volts to ~98 volts for a "standard" large MPPT controller--60-80 amp size).

    The typical battery itself--they start around 12 volts (near dead) and is "bulk charged" to ~14.2-15 volts, then the controller will cut back to around 14.2 volts for a few more hours, then back to ~13.8 volts when the battery is fully charged (bulk, absorb, then float stages).

    So, with an MPPT controller, why choose 18 volts vs 96 volts...

    If the panels are close and or subject to some shading, then placing the panels in parallel will be a) slightly more efficient at lower voltages; and b) if you have shading problems, parallel panels may drop less power than a series string when slightly shaded. Be aware, solar panels need full sun to generate power. Shading (morning/afternoon, or partial) dramatically kills their output.

    Choose the higher voltage (typically ~96-98 volts max) when there is no shading problems, and the panels are a long distance from the battery shed + charge controller. Because the voltage is ~5x more, the current in the wires from the array to the charge controller will be 1/5 as much--so you can use dramatically smaller wire (and cheaper) for the long runs (and/or use heavier wires to reduce voltage drop and improve efficiency).

    There are more details--but that is the basics. Each charge controller has its own specifications--so while the basics are similar--the details will differ and you need to get them right or risk damaging your equipment.

    Questions? Confusion?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • JustinAdams
    JustinAdams Registered Users Posts: 5
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    Re: Series or Parallel Question

    When choosing an MMPT do I have to calculate how much current is being inputed or how much is being created vs the controllers rating?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: Series or Parallel Question

    You have to read the specifications of the specific controller... But, basically the rating would be based on the output current at the battery bank voltage.

    For example, a 60 amp controller charging a 12 volt battery bank at 15 volts:

    60a*15v=900 watts maximum solar panel rating (rough numbers)

    For the same 60 amp controller charging a 48 volt battery bank at 60 VDC:

    60a*60v=3,600 watts maximum solar panel rating

    To see what the panel/string current is... Look at the Vmp/Imp of the panels...

    Vmp/Voc adds for panels in series (current remains the same).

    Imp/Isc adds for panels in parallel (voltage remains the same).

    Or, remember the P=I*V equation...

    For example 3,600 watts of panels, with Vmp = 18 volts, assuming 48 volt battery bank, so need 5 or more panels in series (5x18v=90volts):

    P=I*V > I=P/V=3,600 watts / 90 volts = 40 amps for controller input current...

    Of course, the above numbers are approximate. Vmp changes with temperature, Vbatt changes with state of charge, temperature, and amount of current going into/out of battery. Solar panels tend to output less than "rated" current on hot days, hazy days, etc. There is a ~5-10% energy loss through the controllers and wiring...

    There are some, not obvious, issues--so make sure you have done all of the calculations and checks (max power, Voc Max, Vmp min, Isc, etc.) before you start buying components.

    For example, there is a problem with a 48 volt battery bank and using solar panels with Vmp>48 volts... One panel's Vmp is not high enough voltage (under 60 volts + 2 volt drop)... And two or more panels in series Voc is higher than the Vmax controller's rating on cold days.

    Lower Vmp/Voc panels are fine (and even the 48 volt panels are good panels)--it is just the combination that can cause problems.

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