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Is it possible to charge some batteries in this case:

  1. May 9, 2015 #1
    Hi, I would like to know if it is possible to charge some batteries in this case: 4W output power, 5 V voltage DC in system that is shown in this video:

    Which is minimal power needed to charge batteries? Thanks
  2. jcsd
  3. May 9, 2015 #2


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    Staff: Mentor

    Welcome to PF!

    As long as the output voltage is higher than the battery voltage, it should be possible to charge batteries. The wattage just determines how fast they charge.
  4. May 11, 2015 #3


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    Even if the voltage is lower you can still charge a battery - it just makes the electronics you need more complicated and less efficient.

    If we assume that the water comes out of the tap with zero pressure and falls 1m onto the water wheel the flow rate you need for 4W is at least...

    Energy = mgh
    Power = mgh/t
    m/t = P/gh
    = 4/(9.8 * 1)
    = 0.4 kg/S = 0.4 L/S

    A well designed waterwheel is about 75% efficient I think. The generator is probably about the same? So you can probably double that flow rate at least. Call it 1L/Second?

    So perhaps budget 60L a min? 3600 L an hour?

    Would be less if you utilised the pressure of the mains.

    If you have to pay for water I'll let you work out which is cheaper water or electricity.

    Years ago you could buy a water powered washing machine. Very wasteful of water. It may even be illegal to extract power from the water mains in some countries?
  5. May 11, 2015 #4


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    PS Those stick up lights in the video are usually very low power. It's hard to find data quickly because most adverts don't mention the power. The first one I found was 0.5W. Some sellers say they will work for 100 Hours on three AAA batteries. If those AAA cells have a capacity of 1.2AH then that works out at just 54mW (but don't believe that).
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