How to wire my 18v Panel as TWO 9v panels?

hamlet_joneshamlet_jones Registered Users Posts: 12
I want 9v from my BP275 panel. Each panel consists of two series strings, with a bridge diode(s) for bypass protection. There isn't any specific details regarding a 9v option in the BP literature that came with my panel.

I've measured inside the junction box, and see either 9v or 18v. Unfortunately, I don't have access to the fourth wire, so the "common" center terminals voltage is either positive/negative, depending on how you measure it. If I added two diodes to the center junction point, with cathode/anode arraignment, then I could "see" 9v from each string, and parallel them, for more current.

Does that make any sense? Is this how it's done?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,747 admin
    Re: How to wire my 18v Panel as TWO 9v panels?

    Really, you need to back up a minute and say exactly what you want vs what the panels can deliver...

    Very roughly, the range from full current on a hot day (Vmp-hot) to open circuit voltage on a sub freezing day (Voc-cold) can be almost 1:2 ratio...

    If you need 9.00 Volts for a summer in Death Valley, the open circuit voltage will be nearly 18 volts.

    So--This means your load/charging circuit needs to handle a fairly wide range of voltage. And that you will run into efficiency issues if this is done with a PWM controller, and thermal issues if this is done with an Analog Pass Transistor (or 3-T regulator) design.

    Solar panels are very temperature and load dependent (because the output is really much closer to a current source than like a "solar battery").

    If the loads are small (less than a watt of power)--A 3-T regulator or switching PWM type circuit will work pretty well (you can over design/heat sink, and a 2x larger panel than you really need--because it is all still pretty small and cheap).

    If your loads are larger--The matching of solar panels vs using a standard 12 volt battery and charge controller with a 12 volt to 9 volt down converter (like a 12 volt car adapter from Radio Shack, etc.) starts to look better.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • hamlet_joneshamlet_jones Registered Users Posts: 12
    Re: How to wire my 18v Panel as TWO 9v panels?

    Thanks.

    I think I'll just try it and find out. I don't live in death valley (4k feet, in Hawaii. Perfect temps.)

    Shouldn't blow anything up charging a 6v electric fence battery, or tacking the 9v legs to adjacent panels to charge an 18v tool battery, quickly :p

    Should I retain the bypass diodes, or yank 'em?
  • AntronXAntronX Solar Expert Posts: 462 ✭✭
    Re: How to wire my 18v Panel as TWO 9v panels?

    How are you going to keep your 6V battery from getting overcharged?
  • hamlet_joneshamlet_jones Registered Users Posts: 12
    Re: How to wire my 18v Panel as TWO 9v panels?
    AntronX wrote: »
    How are you going to keep your 6V battery from getting overcharged?


    That's a good question. A voltage regulator from an old Volkswagen maybe?
    I can probably find a circuit online somewhere, but that's not my point, and
    I'm not trying to throw out silly questions.

    A 12v nominal panel should be able to be broken into it's two series strings.
    Why BP only gives three wires out is beyond me. The only way I can see to
    do it is by attaching to the center-post/common terminal two diodes:

    One diode has it's anode side to the common, and the second diode has it's cathode side to the common. From the panels negative terminal and the free cathode side of the appropriate diode, a voltage of 6v nominal may be measured. From the positive terminal of the panel and the free anode side of the remaining diode, the second string is isolated, and it measures 6v nominal.

    Now we have a panel we can work with! If I want a nominal 6v/9A panel, we just parallel the two strings. If an 18 nominal volts are needed, or 30v, this can be achieved...


    Has anybody else used this approach by using two diodes to isolate each string in a panel?
  • AntronXAntronX Solar Expert Posts: 462 ✭✭
    Re: How to wire my 18v Panel as TWO 9v panels?

    You get a short circuit. You will have to make careful cuts to the back sheet of the panel and cut the bus wire that connecting two 18 cell strings together. Then you will solder two wires to each end of that cut bus wire. Count how many cells your panel has, and draw circuit diagram of how they are connected.
  • hamlet_joneshamlet_jones Registered Users Posts: 12
    Re: How to wire my 18v Panel as TWO 9v panels?
    AntronX wrote: »
    You get a short circuit. You will have to make careful cuts to the back sheet of the panel and cut the bus wire that connecting two 18 cell strings together. Then you will solder two wires to each end of that cut bus wire. Count how many cells your panel has, and draw circuit diagram of how they are connected.


    I was afraid of that. Thank you.
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