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How to charge a battery?

  1. Dec 1, 2012 #1

    New here, sorry if this in the wrong thread.

    I just finished a college general physics 2 course and in it we had an experiment where we passed a magnet through a coil of copper and a computer read what the induced emf was. This reminded me of a flashlight I have which uses the motion of a magnet going back and fourth to recharge two round 1.5 volts batteries, connected in series.

    So, I created a similar setup using a coil of copper wrapped around a pvc pipe. I use a strong cylindrical magnet to go through this. It is attached to a led which lights sporadically as I shake the pvc pipe. I read I can attach a capacitor to the circuit to smooth out the current. And to use diodes to make the current DC.

    Instead of light an LED I want to charge 2 1.5v rechargeable batteries I have in series. I think though that if I attach the two batteries to my coil of wire the current from the battery will create a magnet out of the coil and drain the battery, no? How would I be able to charge the batteries using the emf induced by the magnet going back and fourth through a coil of copper magnet wire?

  2. jcsd
  3. Dec 1, 2012 #2


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    Yes, it would. You need a diode to keep the current from flowing back through the charger.
  4. Dec 1, 2012 #3


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    You also need to be producing enough volts with your system (for a reasonable proportion of the time of each cycle of movement) to exceed the voltage of the battery (plus the 0.6 to 0.7V drop across the diode) so that current will actually flow to charge the battery. If you have a DVM, you can see what you get with an initial coil size and then scale it up to give you the volts you need (the Volts out is roughly proportional to the number of turns on the coil). You will need a lot of turns for this as it is not a very efficient setup, compared with a rotary generator.
  5. Dec 1, 2012 #4
    Is it absolutely necessary to have 3+ voltage going to the 3v battery to have it charge? Or will the charge just take longer if the voltage from the system is less than 3v?
  6. Dec 1, 2012 #5


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    No charge will flow into the battery unless the supply voltage is higher than the battery voltage. You could always charge the batteries separately or in parallel if you can only get, say, 2.5V.
    This sort of exercise is basically suck it and see.
  7. Dec 1, 2012 #6
    Thanks sophiecentaur and phinds for the responses. I'll read up more on how batteries work. I think I have enough information now to continue with my experiments though. Eventually I do plan on creating a rotary generator, then a wind turbine which uses that.
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