Recharge Your Battery ... With Crystals!

I’ll bet you thought this was going to be some wacky New Age thread, eh? Nope.

Background: A few years ago, Northeastern North America had a rather bad ice storm. I went for over seven days without power, with the temperature in my apartment hovering around 0 degrees Celsius (32 degree Fahrenheit). Since everybody was buying up every battery that wasn’t nailed down, I dug into my electronics drawer and built a little crystal radio.

My Observation: All my life, I’ve always found crystal radios somehow magical, even though I know precisely how they work. The diode rectifies the AM radio signal and drives a little earphone. So I got to thinking: hey, this thing is giving me free electricity!

Question # 1: Since ni-cad batteries are “trickle charged”, could I possibly cover one wall of my living room with a serpentine wire (giving me about 1500 feet of wire), rectify it, and use that to recharge a single AAA nicad battery?

Possible Objection # 1: I think that the “serpentine wire” approach might not actually give me 1500 feet of useful gathering “surface” because AM broadcasts have very long wavelengths. However, that’s not really the point. The air is full of energy at many frequencies. What is the power density to which we are exposed? Is there enough of it that I could juice up a little battery to keep my pocket flashlight topped up (assuming I only use it for, say, 20 minutes a week)?

Possible Objection # 2: Maybe the voltage or amperage isn’t high enough. Would a tiny transformer help? Yes? No? Well, could I possibly build up the charge in a capacitor and press a button from time to time to give the ni-cad battery another jolt? I’m not so much interested in sheer practicality as in the idea of getting electricity for FREE! (Anybody who suggests that I just use solar cells has obviously never lived in Montreal during the winter.)

Question # 2: Am I nuts?

Question #2: “Am I nuts?”

Yes, because if you figure out how to make this work, Consolidated Edison will have to send assassins over to your house, and your wife will be left a widow and your children fatherless.

Seriously, though, wouldn’t the very low current just dissipate in all that wire, hardly any of it surviving to come out the other end and recharge the battery? You’d have to have solid gold wire or something, for better conductivity, wouldn’t you? And if you can afford 1500 feet of solid gold wire, why do you care about getting free electricity?

Don’t know if you are nuts, but try this link, maybe the two of you could work on something together…

Hard Drive outta Bran Muffins

Solid gold is kind of impractical and completely unnecessary because of the skin effect. The “habit” of electricity to “Travel” on the skin or outersurface of wire.
More than a Quarter mile of wire Huh.
Good Luck!!

Ideally, you’d have an electrically large antenna (made with a very long wire (or wires) in a big pattern). Unfortunately, for maximum power transfer, the electrical impedance (resistance+inductance/capacitance) at each frequency must much the impedance of the circuit it’s hooked to (your battery charger). Normally, you’re only receiving one frequency at a time, and the antenna is tuned to that frequency (or a single band of frequencies). If you want to get all the radio waves passing by the antenna, you have to tune all the frequencies at once- not easy. In addition, as others have mentioned, the wire losses, and losses in any tuning components, would probably eat up a lot of power. Your best bet is to tune the antenna to the strongest radio station in your area- make a highly directional antenna, and point it at the transmitter tower. Unless you’re close to a high-power AM station though, you almost certainly won’t get enough energy to even overcome a nicad battery’s self-discharge, much less trickle charge it.


Duck Duck Goose: There’s no need for gold wire. My crystal radio pumps the power to a 99 cent microphone through some ultra-cheapo braided wire. The antenna? I attach the radio using bargain wire and an alligator clip to a length of old telephone wire. The ground wire is connected to a cold water pipe.

Ajuna34: You don’t sound like you’re quite sure. In any case, there are several high-powered AM broadcast stations here in Montreal – and I’ve got a cell-phone antenna about 200 feet from my window.

Okay, so say I have to tune in each candidate frequency with a tuned circuit. Can’t I just hand-wrap some coils with bell wire (which is what I did on the first crystal radio I ever built)?

Before you object that cell signals aren’t AM (maybe they’re FM, or FSK or whatever), it doesn’t matter because I’m not interested in the content – just the power. I don’t even care if I get all of the power (which would be the case no matter what modulation scheme, if we’re using simple, passive tuned circuits) just so long as it can contribute something to the total current flowing into the final stage of the device.

What would convince me that I’m wrong about what I say is if somebody could show me that the power density in my living room is negligible. But I don’t think it is.

I keep coming back to the fact that teeny little radio waves are making enough power in my crystal radio to make the diaphragm of my earphone move. It’s not just moving a few dust particles; it’s flexing a foil membrane.

Remember that I’m not talking about driving a motor. I’m asking if you could EVENTUALLY charge up a AAA ni-cad with the tiny trickle of “stolen” power. I don’t care if it takes two weeks; it’s the principle of the thing.

shiiiaat, if you can make a hard drive out of bran muffins, why the heck not.
does the composition of the crystal make a difference?

In theory this could work, but there are several practical limitations, as others have already pointed out. However, rather than just speculate, a simple experiment could be done. You already have the crystal set, so if you have a simple multimeter you should be able to measure the rectified voltage. Assuming that you are using a high impedance earphone (piezo or similar) there should be little loading on the circuit and the voltage drop across the diode should be fairly small. You should be able to get a good reading, hopelly several hundred millivolts if you have a strong local AM station. Anything less than about 100mV makes it very unlikely that you will ever get this to work.
If there is enough signal available you could make a charge pump to get an incremental voltage over the ni-cad, but the charging current will load the circuit so available power may be too small. As someone else said, the rate of charge may be less than the natural discharge rate, so you would not actually gain anything.
Its definately worth a try, however.


Have you considered using a reflector. A satellite dish is used for exactly what you are wanting. There may even be a free source of power you haven’t thought of. It’s called the SUN.You would have to track it. A couple of photo cells,and two satellite jacks and you could probably do it.I know that twice a year ths sun has enough spurrious radiation to wreck havok with satellite programming.
Just a thought.