Simple battery charger from wind power

Discussion in 'Power Electronics' started by Nooffswitch, Aug 22, 2016.

  1. Nooffswitch

    Thread Starter New Member

    Aug 19, 2016
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    Hello, I am looking for a simple way to charge batteries from a wind turbine to power a few LED's. I'm not very good at electronics, but I assembled a circuit for NEMA 17 stepper motor that consists of a rectifier to convert the AC to DC and a regulator with a pot to regulate the voltage output. It seems to work as I can light up some LED's by spinning the stepper with my fingers, so the next step would be to store a charge in some batteries.

    I have a broken iPad my son threw in a bucket of water that has some LIPO's I might be able to use, but I think it would be easier and safer to stick with some NiMH batteries and build a simple charger like this. What I'm not sure about is how this will work if the energy flow to charge the batteries is not consistent (no wind, no energy). My goal is to have some batteries charging whenever there's some energy generated by the wind, and have the LED's lit up at all times (unless the batteries run dry).

    If anyone can help I'd appreciate it, but please keep in mind I'm no electrician.
     
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Can you post some generator output values, V vs current @ different loads & different wind speeds ?
    Are LEDs just indicators or for illumination ?
    Any information will be a start.
     
  3. wayneh

    Expert

    Sep 9, 2010
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    Your scheme of a spinning motor–bridge rectifier–battery–load, is fine. The rectifier will prevent any reverse current into the motor when the wind is not blowing. You'll need a fairly high RPM to in the motor to overcome the battery voltage and provide enough power to charge the battery while also lighting the LED.

    If you want this to be more than an occasional toy, and to properly maintain the battery, things get more complicated. The solution is a dedicated IC for the chemistry of your battery. Some chemistries, particularly NiCad, are easier for a DIY solution and you may not need the chip. Others like Li-ion are darn finicky, and the chip is the easier solution.

    A final refinement would be a DC-DC converter that could 1) scavenge low voltage from the motor and step it up to charge the battery, or 2) step up the battery voltage to light the LED(s). I have a hunch the latter would be the better choice for overall efficiency.
     
  4. Nooffswitch

    Thread Starter New Member

    Aug 19, 2016
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    Bernard, the LED's will be used for illumination and are 3.2 forward voltage/20mA forward current. This project is mainly for fun but if I could have it light a couple of LED's all night so it's easier to find our driveway it would be great. There's no street lights in my area so it can get pretty dark. I don't have any actual results from this as I haven't built the actual wind turbine yet. I bought a 3D printer and plan to use it to create one once I have the electronics figured out. For testing I just spin the stepper with a power drill, and at full speed I was getting 28V after the rectifier and before the regulator. Not sure about the current but I can test it tonight with the drill and see.

    Wayneh, if I could even keep one bright LED lit most of the time from this generator I'll consider it a success. I haven't looked at the DC-DC converter but it sounds interesting and I'll have to google it. Is this similar to a "Joule theif" or a step-up/step-down sort of thing?
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Glad to hear you have reasonable expectations. So many projects like this are unrealistic.

    You may want to start simple and learn as you go. Just hack a solar landscape light and use your generator in place of (or in addition to) to the solar cell. It literally would require no extra parts. The solar cell will have a diode to prevent reverse current. You would tap your generator in after that diode (so that the generator charges the battery without pumping current into the solar panel). This assumes your generator output is rectified to DC through a bridge rectifier.

    I can envision a lot of upgrade ideas afterwards, but all of those would depend on how much juice you're actually getting out of your turbine. For instance if it's overcharging your battery, you might add more battery capacity and a dedicated charge controller. If it captures enough power to light 10 LEDs instead of just 1, you'll need to abandon the landscape light or maybe chain several together. So many options!

    But again, your range of options depends on what power you get in reasonable wind. I'm quite sure the drill is giving a much higher RPM and larger output than your turbine will.
     
  6. Nooffswitch

    Thread Starter New Member

    Aug 19, 2016
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    Ok, what if I wanted to charge a battery during the day with the wind turbine, and then have it light up the LED only at night due to a night activated circuit such as this. Is there a simple self sustaining charging circuit I can use that won't damage the battery from overcharging that I won't have to babysit?

    Something like what those solar landscape lights use that you mentioned Wayneh (which I don't have to play with ATM). Most of the circuits I see online are overly complicated and there aren't any good tutorials to follow on how to achieve this.
     
  7. wayneh

    Expert

    Sep 9, 2010
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    Yes, sparing energy by not lighting the LED during the day makes perfect sense and is easily accomplished with a very simple circuit. The garden lights use a single IC that switches the LED on at night, and runs an oscillator for the voltage boost circuit.

    I highly recommend getting yourself some cheap solar lights to play with. If you keep your eyes open, you can often find them for less than $1 apiece.
     
  8. Nooffswitch

    Thread Starter New Member

    Aug 19, 2016
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    Ok, stupid question time. How do I check the amount of current the stepper is generating as it's turning?

    I can check the current an LED is taking when I tap into the circuit after the regulator (which is set to 3.4V, the LED's forward voltage), which is about 20mA, and if I connect another LED I get about 40mA. This is correct based on the LED's specs, but how do I check the total current available so I know how many LED's I can power at say the top speed my drill is turning the stepper motor? Should I connect my multi meter directly to the steppers cables before the rectifier circuit or something?
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Your measurements of load current are fine but the regulator will be dissipating some power, so it would be better to measure power upstream of the regulator for that reason. You need to know both current and voltage to calculate power produced by your generator, so you might want two meters with one in voltage mode and the other in current mode.

    Another approach is to measure only voltage, but drive current directly into a known load resistance such as a power resistor. By "power" I mean one rated to maybe double the expected power it needs to dissipate. Light bulbs make great dummy loads but their resistance is not constant.

    You can only estimate the power capacity of the generator (which will be directly proportional to RPM) by steadily increasing the load at one RPM to see how the voltage drops. For solar panels, you just short the panel through the ammeter. You measure current where the voltage is pulled down to essentially zero. I don't know about stepper motors and I can't recommend doing that until you know it won't hurt it. But anyway the peak power production is probably where the voltage is pulled down to a portion of the no-load voltage. For instance for a solar panel, peak power is when the load draws the voltage down to about 80% of the open-circuit voltage.
     
  10. mtonge

    New Member

    Apr 19, 2016
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    That's a nice little charger in the picture. Is there a schematic available for that?
     
  11. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Just thinking out loud: When loaded with 15 ohms, a Clifton type 23, 1.8 deg./ step. 5.2 V stepper produced
    about 60 mW with hand twisting, est. 240 RPM using only one coil pair so maybe 120 mW total.
    How much wind is available ?
    Power = V cubed X D squared; W, MPH, Ft.
    From Solar Wind Co,
    2 ft, dia. 5 MPH = .6 W; 10 MPH =5 W; 15 MPH =16 W
    4 ft. dia. 5 MPH. = 2 W; 10 MPH = 19 W. 15 MPH = 64 W.
    For a small system use about 30 % of above figures.
    A 1W wind gen. should do well with a 2 Ah battery ( 4 AA Ni mH cells ).
     
  12. Nooffswitch

    Thread Starter New Member

    Aug 19, 2016
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    Ok thanks Wayneh, I'll try tapping in just after the rectifier this weekend and see what I get in current.

    mtonge, I don't think there's a schematic for it, but I found it from some forum thread you can contact the guy who made it from his site here.

    Bernard, yeah I was thinking 4 AA Nimh batteries or 1 9V would be more then enough to power a couple of LED's, I just have to figure out how to make a smart battery charging circuit for it. I could probably do it with an Arduino and it would be a lot easier since I have experience with that, but I'd prefer to stay away from doing any software work on this particular project.
     
  13. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    I connected a variable speed drill to the stepper, measured AC V on one coil with various loads. At 22 ohms,
    300 RPM, produced 3 V. At 15 ohms V =2.5 V, so it seems the stepper acts much like a solar panel.
    Battery management gets really sticky, so solar lights use Ni-Cd batteries which will tolerate full discharge & over charge but not so with Li. I'll throw in my version of battery management used on lighthouse. Can be seen on you tube, Jack Bernard lighthouse. LH Battery Manage. 00000.jpg
     
  14. Nooffswitch

    Thread Starter New Member

    Aug 19, 2016
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    Ok I tapped into my circuit to check the current, but I don't have much experience with a multi meter when it comes to current. Can someone please tell me what this means? My guess is 700mA but I'm not sure. In case it's hard to see in the photo, I have the red wire in the 10Amax unfused slot and the the black in COM, and the meter is set to 10 in the Amp section.

    [​IMG]
     
  15. wayneh

    Expert

    Sep 9, 2010
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    I agree with your guess. Does that value make sense for what you are measuring? In other words, where are the probes?
     
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