# Is it possible to harness energy from electric fences?

Discussion in 'High Voltage' started by toozie21, Mar 6, 2014.

1. ### toozie21New Member

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Was BSing with a friend who has a hobby farm and uses electric fences for his livestock. Got to thinking about if it was possible to do any sort of energy harvesting from the fence since "electricity" is running throughout his property. My (not very) educated guess is that there isn't enough energy there to be worth the effort.

The way I see it, under ideal conditions, you could probably expect to see a 16kV pulse for 100us. Worse than that, the pulse generally decays linearly (from 16kV down to 0V over 10ous), so a safer assumption would be ~8kV for 100us. Doesn't seem very promising.

My conundrum comes in from the P(W) = E(J)/ t(s) equation. I think these units are generally about 1 Joule output, so if I substitute in 1 joule for 100us, I get 10kW, which seems a little extreme. Even if I extend that out and say that a 1J pulse occurs once a second. that comes out to be 1W. Is that really how much energy is on the line in ideal situations?

That DOES seem very promising, but I think I am missing something important here.....

2. ### picbitsWell-Known Member

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You *could* but if you take the energy away from the fence, the livestock will be at risk of escaping.

If you increase the power to the fence, you risk injuring the livestock.

Easier to bang a solar panel up and use that.

3. ### ChrisP58Well-Known Member

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Electric fence chargers are severely current limited, or they would electrocute instead of shock. I don't know the numbers but it's probably only a few milliamps. The current is most likely limited by a high value resistor. Which means that as soon as you connect any kind of load to the fence the voltage will drop. So there is very little real energy available.

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5. ### shortbus=Well-Known Member

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Even grass and weeds touching a electric fence will prevent it from working correctly.

6. ### toozie21New Member

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OK, thanks everyone. I knew leaching power could be detrimental, but I was hoping that if it was only a few milliamps, that it would be insignificant in the scheme of things. I also know that they are putting out such a small amount of current; but since they were doing it at such a high voltage, I was hoping there would be enough to spare. Guess not. Thanks though.

7. ### Little GhostmanWell-Known MemberMost Helpful Member

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you can use a pickup coil placed near the fence wire and a Linear technology LDo booster to get 3.3V out at around 50mA.
LT do a energy harvesting board and with a couple of copper plates you can power a micro board from the electric field off a neon strip light 2' or more long, I have messed with this a fair bit

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8. ### Little GhostmanWell-Known MemberMost Helpful Member

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Forgot to add though, this is not free energy. Whatever you take will cost, but it is handy for getting small power sources in strange places. You can also do it with over head power lines! And get more energy, this is totally free and dosnt cost you a thing until you get caught and fine, then it turns out to be an expensive way to get free power

9. ### toozie21New Member

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I didn't think about a pickup, that is a neat idea.

I guess my physics (which was never great), is a bit rusty. Even though the E-field is always there radiating (whether or not I am trying to tap off of it), if I grab some juice via induction, it will draw down on the available power?

10. ### Nigel GoodwinSuper ModeratorMost Helpful Member

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Yes, any power you take from it will simply be drawn from the battery - assuming you had some valid (and legal) reason for wanting to do this, just run a wire from the battery - least possible amount of loss.

11. ### toozie21New Member

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I was thinking more of the originally problem of leeching some power from the electric fence (not the overhead power lines).

Switching to the comment on the overhead power lines, I guess since it will drawn down even though it is always emitting the field, this is the main issue the power company has (since it is supplying power that it can't figure out how to charge it to).

12. ### cowboybobWell-Known MemberMost Helpful Member

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Yes. To the degree that you interrupt (inductively tap) the source field.

However, since the "primary" of your inductive tap is a single wire strand (the electric fence), the "load" on your source will be very low (probably uA or pA levels). Which, of course, means that your output will also be miniscule.

AND, only occasionally. As I recall, an electric fence only pulses every few seconds.

13. ### Little GhostmanWell-Known MemberMost Helpful Member

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They pulse around once a second for 250mS, ours is 12KV and I forget the joules but its a fair bit! They are called Bull fences and mostly used in zoo's or Bull breeding farms. They put out a kick that on full power takes me off my feet! lol, if you use the red and white nylon multi strand fence wire, you can easily light a Led from a pickup coil, we have 30 of them around the fence line so in the dark we know where the wire is .
I have bounced off the fence many times but still cant get used to it! Only thing I have ever seen that puts a bull off a cow in the next field

14. ### cowboybobWell-Known MemberMost Helpful Member

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Maybe TMI, but I inadvertently pee'ed on an electric fence once and it was an experience NOT EVER to be repeated...

15. ### steveBWell-Known MemberMost Helpful Member

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I power my electric fences with batteries charged by a solar cell. A very small solar panel is needed, which shows how little energy is required. So free power at this level is available directly with solar cell or small wind turbine.

16. ### Little GhostmanWell-Known MemberMost Helpful Member

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Ours is a mains powered unit, it would drain a battery pretty quickly. We have had loads of the battery ones that put out around 4KV and tiny amounts of power, you get used them and they dont tingle much anymore. But the Bull one always makes me feel nervous when its on lol. I saw the tips of a rabbits ear touch the stainless steel line for the bottom wire once, I had NO idea rabbits could jump that high or run that fast!

17. ### Nigel GoodwinSuper ModeratorMost Helpful Member

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I don't see as it could be a very successful fence then?, usually car batteries are used, and charged every week or two.

18. ### steveBWell-Known MemberMost Helpful Member

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It works very successfully. The charging is done continuously in daylight. The cell can charge the battery and provide enough power for the fence during the day, and then the fence runs on the battery at night. The system uses a 6 V 14 Ahr lead acid battery. Typically the battery lasts over 5 years.

By the way, there is something to be said about the "tingle" versus "jolt" of these small systems. Often people touch them and think that the tingle is nothing. They can hold their hand there and, while not comfortable, they can tolerate it. But, they forget that they are standing there with shoes on, which insulates them considerably. The animals are not insulated and have direct connection to the earth with 4 feet. I learned this lesson the hard way one day. I was leaning with my hand on a metal post that was going into the ground, and touched the fence wire with my other hand without thinking. The shock felt like a punch from Mike Tyson, and was not something I would subject myself to voluntarily, like I normally do when I test the fence with my hand, with shoes on.

19. ### ChrisP58Well-Known Member

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Well said. How much current will flow is all about the loop resistance.

20. ### RatchitWell-Known Member

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Electric fences are designed for pain, not gain.

You cannot do any energy calculations unless you specify the load for those voltages and times.

A joule is a watt-second. That will not keep you very warm in winter.
It appears to be taking candy from a baby's crib to supplement your grocery supply.

Ratch

Last edited: Apr 2, 2014