charging a battery without electrical connections

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nutmegzzzz
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charging a battery without electrical connections

Post by nutmegzzzz » Fri Nov 10, 2006 8:08 am

I'm working on a charging system inside an enclosed 4 inch sphere and wanted to get some feedback on some other ideas to test out. I tried inductive charging but it did not work too well and I believe requires the two coils to be very close together like the electric toothbrushes. I have used flexible solar cells with halogen lights for charging and I am able to charge the batteries to an adequate level but it can get very hot and I have to cut the solar cells into thinner strips which probably reduces the lifetime.

The purpose is for an enclosed sphere with a camera/electronics/battery to be charged with out direct electrical connection. Our camera can see 360 degree panoramic field of view and a 270 degree vertical field of view and will self right itself when thrown into operation. I cannot have any metal connections with this system.

Thanks for any help.

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Chris Smith
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Post by Chris Smith » Fri Nov 10, 2006 8:18 am

Solar shouldnt get hot unless the voltage and system is too big for the batteries. Regulate them and solar will work fine.

nutmegzzzz
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Post by nutmegzzzz » Fri Nov 10, 2006 11:16 am

I'm using halogen lights to charge the batteries. The solar cells are at the bottom of the camera, and we are unable to point the camera upwards because it is self-righting. The best charging rate occurs when the lights are close to the solar cells so they get hot not because of the power being produced but because of the light energy by the lights.

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Post by Engineer1138 » Fri Nov 10, 2006 11:46 am

Conductive plastic? Transparent aluminum?

Inductive charging will work from great distances; it's just a matter of how much power you put into the system. Why didn't it work for you?

Can you put an IR reflective coating on the sphere to reduce the heat buildup?

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Chris Smith
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Post by Chris Smith » Fri Nov 10, 2006 12:45 pm

One or two layers of standard tinted glass between the two will cut down on the IR.

Also slant one piece of glass at 45 degrees, and most of the IR will be deflected.

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Post by haklesup » Fri Nov 10, 2006 1:11 pm

If the sphere moved a lot, you might be able to create some sort of generator (like the shake to charge flashlights) but I also think that Inductive charging is your best bet.

Consider a coil wound on the surface of self adhesive tape then stuck to the inside of your (non conductice) sphere. This could be closely coupled to an external coil of similar conforming shape.

The introduction of a dimple on the sphere surface could acomodate the insertion of the coil with powdered iron core, this would increase the efficiency of power transfer greatly. With an air core, it should still work, just not as well. The dimple could be filled with a rubber plug when not charging.

Not sure but I wonder if you could find LEDs of the right color to efficiently charge with the solar cell.

A stainless steel polished parabolic shaped salad bowl makes a good solar reflector, you could shape some metal bars to hold the ball at the focal point of this collector. Sun shines down and bounces back into the ball. Heat may still be a factor but IR blocking coated glass from a variety of sources from sunglasses to auto glass will cut much of the heat producing IR light.

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Post by nutmegzzzz » Fri Nov 10, 2006 1:24 pm

Well I tried many different ratio's and everytime I needed to put the coils very close together to get any worth while signal out. I will then need to rectify the signal to get a DC and go through another diode to prevent the batteries from charging the solar cells. So in all I will have to waste at least 1.4 V in charging potential.

With the solar cells I am able to get 6mA with up to 7V after the diode and my batteries are 6V NiMH with a charging current from 14mA to 35mA. Currently I have been able to charge the batteries to get 2.5 hours of operation from the camera but once I put in a transmitter, compass and PIC that will probably drop to 30 minutes. I just don't like having to charge the batteries for 48 hours under very hot lights to get one hour of operation.

Just got an email from the VP of the solar cell manufacturer and he said the same thing about IR causing a significant amount of heat. Found some optics on Edmund Optics web site and might try those out.

Other then solar and inductive, anything else I could try???

Thanks for everyones input too, it is very appreciated.

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Post by nutmegzzzz » Fri Nov 10, 2006 1:45 pm

I love the shaking idea, and I bet it would work great but I'm worried about the optics getting misaligned. Also our sphere is only 3.25 inches internal (with most being optics) and I'm not sure if we would have the room for that (but I might just try it out for fun).

Back to my limits on inductive charging... I can not incorporate a dimple into the ball, our camera self-rights itself by being a ball floating in water inside a ball. So when the camera is thrown it is facing upward. Which then makes me think that the shaking idea would probably not work now (also there is a MEMS compass in the ball and that may screw up our direction ability).

I wanted to do LED's for the charging light but the white led's don't really follow the light spectrum that solar cells work with, If anyone has come across LED's that might work I'd be willing to look at them.

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Post by rshayes » Fri Nov 10, 2006 2:07 pm

The peak response for your solar cells is probably in the 800 to 900 nm range. Light in this part of the spectrum will transmit the most power with the least heating of the ball. Try looking for high power red or infrared LEDs.

The white LEDs use a phosphor to convert short wavelengths (blue or UV) to longer wavelengths. It is probably more efficient to generate red light to begin with.

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Chris Smith
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Post by Chris Smith » Fri Nov 10, 2006 4:54 pm

The Glaze from an ordinary piece of glass set at 45 degrees will cut down the IR signature very well.
Ir is the most efficient form of frequency to bounce, hence the 45 degrees will guide it away.

Colored Glaze or IR coating like in most modern glass will cut it down even more.

Two pieces will reduce the signature down to almost zero.

This all beats Edmon scientific’s price list by quite a bit.

And the heat sinature wont bother the glass.

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Externet
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Post by Externet » Fri Nov 10, 2006 5:02 pm

Nice challenge...
Some water pumps use magnetic coupling impellers; such method could be applied being the internal sphere side a generator, and a spinning magnet outside.
Or spinning outside a modern magnet canibalized from a hard drive, a sustantial transfer should be achieved to a coil inside. :grin:
Miguel

rshayes
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Post by rshayes » Fri Nov 10, 2006 8:37 pm

Inductive energy transfer can probably be made to work as well. It will be more practical at intermediate frequencies, such as 100 KHz to 1 MHz. Resonating the transmitting and receiving coils at the operating frequency will also help. Using a sine wave source will reduce problems with harmonics of the operating frequency.

The transmitting and receiving coils should be parallel to each other. Better coupling will be obtained with coils whose diameter is considerably larger than their spacing. Making part of the magnetic path out of ferrite would also help. This might be more practical for the transmitting coil.

Efficiency won't be very high, but it might be adequate if you can get a reasonable amount of coupling.

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Post by Bern » Sat Nov 11, 2006 9:10 am

When you were testing using induction, what frequency were you using? If you were using 60 cycle, I would suggest you try a higher frequency; it should increase coupling, and therefore efficiency. With the inside self-righting capability, you will know where the inside coil is, so position the drive coil for maximum coupling.

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Post by rshayes » Sun Nov 12, 2006 6:02 am

A monocrystalline solar cell should be capable of about 20 percent efficiency when illuminated with solar radiation. If the radiation is limited to the area around the peak response, around 800 to 900 nm, I would expect better efficiency, possibly 40 percent. To generate 50 mw of output power would require 125 mw of optical power. I would look for a red or infrared LED that can put out as much power as possible, possibly as much as 1 watt. This would give you 400 mw of power from the solar cell. If more power is needed, several LEDs could be used.

The best LEDs seem to have efficiencies on the order of 15 percent, so the input power to the LED would be about 7 watts. This should be less than the power required by the halogen lamps that you are presently using.

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