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For a home robotics project I want to fly a kite using a computer. For this to happen I need an electro motor up in the air (attached to the kite) which obviously needs a battery up there as well. From some smart people I understand that using a LiPo battery would be the best for this (because of the high energy to weight ratio). I also understand however, that a LiPo battery needs a special charger. My end goal is to charge this battery in the air by also hanging a small wind-generator under the kite so that the kite can fly indefinitely. If the battery cannot be charged while it is being used I can of course use two batteries, charging one while the other one is used.

So let's say I've got a 11.1V LiPo battery which uses about 500mA to 1A for the motor.

My question: is it possible to charge this battery from a wind generator, and if so, what hardware (preferably very low-budget) would I need for this to happen?

All tips are welcome!

[EDIT] Wow! I am overwelmed by the amount of in depth answers. I am a bit lost in them as well though, and I think I am after a bit more of a "beginners answer". As far as I understand, a wind generator supplies AC power in varying amount of Volts and Amps (right?), which needs to be converted to DC power with a constant amount of Amps (right?). So I guess that would mean I need the following order: Wind generator => Rectifier => Li-Polymer Battery Charge Management Controller (like e.g. this one) => LiPo battery pack.

Is this correct?

kramer65
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    Electric motor in the air and attached to the kite. For my benefit, can you explain why or link me to the question that resolved this. – Andy aka Mar 29 '13 at 13:48
  • Interesting, I wonder if the electric motor can also act as the generator. – geometrikal Mar 29 '13 at 14:05
  • Powering the 'kite' via its line(s) from the ground is not an option? – JimmyB Mar 29 '13 at 14:37
  • Sorry; I just re-read your post, there is no perpetual motion. For flight only consider an ultra capacitor, it should provide more power to weight. – Optionparty Mar 29 '13 at 14:48
  • Charging the battery is the least of your worries! You also need to answer (through testing or research) a lot of other questions: How strong must the motor be to do what you want? How heavy will that motor be? Can your kite lift that motor's weight? How long must the motor operate, and at what power? What is the weight of a battery (or ultra capacitor) that can give that power? Can the kite lift that, too? It looks like a lot of fun for you! Good luck. – Bobbi Bennett Mar 29 '13 at 15:11
  • Heh - to start with, try 5-10 amps for that motor. Possibly more. –  Mar 29 '13 at 15:18
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    Here is the best link I could find on SE on charging LiPo:http://electronics.stackexchange.com/a/25405/15779 – Bobbi Bennett Mar 29 '13 at 15:29
  • @HannoBinder - No unfortunately powering through the line from the ground is not possible. – kramer65 Mar 29 '13 at 17:01
  • @All of you - I just added some more info to the question. Would the following order be the correct one: Wind generator => Rectifier => Li-Polymer Battery Charge Management Controller => LiPo battery pack? – kramer65 Mar 29 '13 at 17:41
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    How much weight can your kite lift? – Bobbi Bennett Mar 29 '13 at 17:47
  • @BobbiBennett - It gives serious pulls to my body and at some moments can lift my body for a second, but the main part of the time it just pulls "firmly" (that's as specific as I can get now.. :S) – kramer65 Mar 29 '13 at 17:50
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    "I need the following order: Wind generator => Rectifier => Li-Polymer Battery Charge Management Controller => LiPo battery pack." - That's correct. Finding an appropriate charge controller seems a little difficult because of the varying nature of the power supply, so you may want/have to come up with something 'homebrewed'. – JimmyB Mar 29 '13 at 18:04
  • A lot of the energy harvesting stuff deals with varying power supplies, I wonder if that would work. Also I wonder if changing to NiMH battery type would make things easier. – geometrikal Mar 29 '13 at 22:19
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    Lead-acid batteries would probably make things easier - if one has some kg of lift to spare ;-) - Yet I think that LiPo isn't any more complicated to handle than those: Limit the charging current and voltage (voltage regulator + resistor for the simplest case, plus a diode maybe) and you're ready to go. Not very efficient of course, but extremely simple to build and resistant to varying supply power. – JimmyB Mar 30 '13 at 12:22
  • @kramer65 - Have you to date managed to get the self flying kite or battery charging kite system going? | LiIon (LiPoly or other) will be the easiest battery to use. Charge current can vary safelyt as long as Vcharging is less than Vmax. Vmax is usually chosen as 4.1 to 4.2V for LiIon. **Once Vmax is reached** charging is at constant voltage and declining current (determined by battery) until some I terminate is reached. Once I terminate is reached it is ESSENTIAL that charging voltage is removed. If not then battery dies quickly. If there is no charge I is 0 so < Iterminate so ... – Russell McMahon Jan 05 '14 at 10:19
  • ... some designing is needed to make system work and treat battery well. Not hard - just needs thought AND only applies when battery is almost fully charged. – Russell McMahon Jan 05 '14 at 10:20

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