Solar amateur needs a little help.

HarkmeisterHarkmeister Registered Users Posts: 4
I have some old Photowatt PW 750 panels that I bout in 2001 that were never installed.  I have long since lost the installation instructions.  I have been looking on the web for how to connect the panels, but nothing I find has the same terminals as mine.  Mine appear to be 12V only, 80W.  These panels only have terminals 0, 2, 3, and 6.  The other positions are empty.  I measure 10V from terminal 0 to 2, and 10V from 2 to 6, and 20V 0 to 6.  I don't get anything at terminal 3.
Do I need a jumper or can I just use 0 and 6 as my power?  I am trying to power a DC water pump.

Comments

  • littleharbor2littleharbor2 Solar Expert Posts: 635 ✭✭✭✭
    0 and 6.  You can see a "+ and -" alongside the numbers. Those junction boxes were also used for their 12/24 volt 72 cell panels and contained diodes and jumpers which were set up differently depending on which voltage was chosen.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • mike_smike_s Registered Users Posts: 39 ✭✭
    They bring out the middle of the array so you can add bypass diodes. If losing power due to shading isn't a concern, no need to add them. Just use 0 and 6.

    This shows a panel divided into 3 - it appears your's is divided into 2.


  • HarkmeisterHarkmeister Registered Users Posts: 4
    edited July 15 #4
    Thank you littleharbor2 and mike_s.  
    Can I run a small pump directly off the panel or do I need a charge controller and battery?  I have been experimenting with a small 30W panel connected directly to 30W pump. It runs for a second or two then stops and a while later repeats.  I have full strong sun right now. 
    I wonder if the 30W panel is too small, but I was afraid to connect the 30W pump to the Photwatt 80W panel.  The panel says it has a minimum power of 75W. 
  • littleharbor2littleharbor2 Solar Expert Posts: 635 ✭✭✭✭
    I'm going to say yes. The pump will draw what it needs from the panel. As long as you have good sun on the panel the voltage shouldn't collapse.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • littleharbor2littleharbor2 Solar Expert Posts: 635 ✭✭✭✭




    Just cuz I have a photo of the 12/24 volt version I thought I'd add it. It was a 72 cell version. Are yours 36 or 72 cell?

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • BB.BB. Super Moderators Posts: 26,758 admin
    Motors have starting surge current... With even a small battery, your 3 amp running motor may take 6-9+ amps to start. Batteries are (more or less) constant voltage devices. They simply provide more current and hold the voltage to supply the surge requirements.

    With solar panels, they are (more or less) constant current power supplies. If you have a 4.6 amp panel (full sun, pointed at sun), the motor will try to draw more current to start (assuming brush type "universal motor").

    Remember the equation for power is:
    • Power = Voltage * Current
    If you pump tries to draw 6 amps, the only thing that the panel can do is supply 4.6 amps (in full sun) and the pump will draw the voltage down from ~12-17 volts to ~6 volts or less. Your pumps wants (for example):
    • 12 volts * 3 amps = 36 Watts (running power)
    • 12 volts * 6 amps = 72 Watts (starting power)
    • 6 volts * 4.6 amps = 27.6 Watts (actual starting power from solar panel)
    More or less, your options are to use a solar panel with 2-3x more current than your pump needs to run (may need more current, depends on the pump's surge current requirements).

    Or, you can get an LCB (Linear Current Booster). These are electronic power supplies that behave (a bit) like the DC equivalent of a Variable Transformer (VARIAC). Since P=V*I , if the electronic supply will draw the maximum power from the array:
    • 17.5 volts * 4.6 amps = 81 Watts (from panel)
    And attempt to supply maximum power to the motor--At 12 volts, it could supply:
    • 81 Watts / 12 volts = 6.75 Amps (to motor at 12 volts running/starting requirements)
    • 81 Watts / 6 volts = 13.5 Amps (to motor at 6 volts surge current)
    While an LCB is a really neat device--They tend to be very expensive and you would be better off with more solar panels (in general). That will also allow the pump to start/run earlier in the morning and later into the afternoon.

    https://www.solar-electric.com/catalogsearch/result/?q=linear+current+booster

    Hmm, prices are dropping a bit--May be worth it for even a smaller pump installation.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • HarkmeisterHarkmeister Registered Users Posts: 4
    Mine are 36.  Pumps not working now with either the 30W or 80W panel.  It feels warm to the touch even though it is submerged in cold water.  Maybe I fried it.  It was/is a $12 pump...
  • HarkmeisterHarkmeister Registered Users Posts: 4
    edited July 16 #9
    Thanks BB.  Very interesting.  How does the LCB work?  Is it a capacitor?  I have a small 12V battery and charge controller that I could hook up, which might solve the problem too.  I didn't go that route because I didn't want it running at night.  
    Here's a pic of my home made pool heater.  A thermometer said the temp inside the box was 145F this afternoon.  Unfortunately, I couldn't get any water to run though it!
  • BB.BB. Super Moderators Posts: 26,758 admin
    A bit more detail on how to use an LCB:

    http://www.solarseller.com/solar_converters__inc__linear_current_boosters___faq_.htm

    I do not know how they work--I suspect it is similar to how a MPPT charge controller works. Some version of a (typically) buck mode switching power supply that modulates current flow from the solar array to keep the array voltage near Vmp (i.e., Pmaxpower=Voltagemaxpower*Currentmp; Pmp=Vmp*Imp).

    The output is regulated two different ways, One is to keep the voltage maximum at rated motor input (i.e., 12 volts, 24 volts, etc.) and a second mode (like bulk charging for an MPPT controller) where the LCB generates as much current as possible (letting the output voltage float at or below "rated output voltage"--I.e., feeding as much current to start the motor, and run better under low sun conditions).

    -Bill

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