9v solar panel and 12v battery?

Discussion in 'Power Electronics' started by jho659, Jan 10, 2015.

  1. jho659

    Thread Starter New Member

    Jan 10, 2015
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    0
    Hi,
    I have a fairly simple question. Will a 9v (0.5w) solar panel charge a 12v (7ah) battery? If so, will there be any defects? Will the battery only charge to 9v? Thank you for any help at all!
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,151
    3,058
    Short answer is no. You need to exceed the battery voltage to drive energy back into it. And a 12V battery should never be discharged to as low as 9V.

    The longer answer is that you could use a DC-DC boost converter to get the voltage up high enough.
     
  3. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    It will if it provides enough voltage & current to do so. Research this: How much voltage and current are required to amply charge the specific battery you want to charge. Manufacturers data sheets of the pertinent components in question are truly your friend to help YOU find the answer to your question.
     
  4. jho659

    Thread Starter New Member

    Jan 10, 2015
    3
    0
    What if I used them together with a charge controller? So that the battery would not discharge into the solar panel. Would that affect its ability to charge?
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,050
    3,244
    You either need:
    A solar panel with at least a 15V output
    or
    A DC-DC converter to step up the 9V to 15V (but those have losses which will reduce the power from the panel).

    A charge controller only regulates the charging current, it won't generate the higher voltage you need.
     
  6. jho659

    Thread Starter New Member

    Jan 10, 2015
    3
    0
    So with a DC-DC converter, would it just make it charge slower? And if I were to go ahead and use the solar panel and charge controller, what would be the effects?

    I've also included a picture of the charge controller itself.
     
  7. #12

    Expert

    Nov 30, 2010
    16,343
    6,827
    Yes, a DC to DC converter will make the charging process very slow. You have already been answered, "What will happen if I use an inadequate solar panel and a charge controller". It won't charge the battery. If you don't believe this, harness a bunny rabbit to your car and tell him to pull you to the grocery store.
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    I think its more like the bunny rabbit on a treadmill with up-gearing to the road wheels.
     
  9. Steve R in FL

    New Member

    Jan 9, 2015
    21
    1
    You could charge two 6V deep cycle golf cart batteries in parallel (with a regulator) and then discharge them in series for 12V. Or you could use two solar panels in series for 18 V and use a regulator to charge 12V
     
  10. bwilliams60

    Active Member

    Nov 18, 2012
    725
    90
    Either way, you are going to be waiting a long time for all this to take place. You might as well throw a coat of paint on the wall while you are at it and watch it dry.
     
  11. Steve R in FL

    New Member

    Jan 9, 2015
    21
    1
    Don't most people wait 130 hrs between paint coats?

    I failed to notice they were only .5 w panels!
     
  12. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    What does the charge controller say the minimum input voltage is?
     
  13. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    You really need a panel which will make an open-circuit voltage of ~18V.

    With the existing panel, and a charge-controller that contains a step-up switching converter (most do not; they require that the panel voltage be ~ 1.5times the battery voltage), follow the following calculation:

    A step-up converter is at best about 80% efficient. A charge controller might be 90% efficient while initially charging the battery.

    o.5W stepped up would yield 0.5*0.8 =0.4W at the charge controller.
    0.4W at the charge controller would get 0.4*0.9 = 0.36W into the battery.

    At 12V, the charging current into the battery would be I=P/E = 0.36/12 = 0.03A

    To get 7Ah into the battery, it would take 7Ah/0.03A = 233 h of bright sunlight.

    You will get at most 8h of direct sunlight on the panel per day (unless you track the sun), so it would take 233/8 = 29 days of sunlight to charge your battery.

    Clearly, you have not selected the right panel for the job...
     
  14. MrChips

    Moderator

    Oct 2, 2009
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  15. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I have some of these. The open-circuit voltage of the HF panel is about 20V, which is just right for a 12V battery.

    However, at 1.5W, it will still take days to charge a 7Ah battery. I = P/E = 1.5/12 = 0.125A

    7Ah/0.125A = 56h of direct sunlight.

    56 h of direct sunlight/8 h of sunlight per day = 7days.

    A 15W 12V panel would be more like it... (or 10 of the HF ones in parallel ;))
     
  16. MrChips

    Moderator

    Oct 2, 2009
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