how to build a recharging circuit?

Are you talking about a resistor?

Could you provide some more information.

Hello "Guest",

Yes. I know how to build what you ask, but it is more complex that you think.

The main problem with your idea, is that if the capacitor is going to charge a secondary cell, that same capacitor also needs to be constantly charged from a separate supply.

There are always losses in any physical circuit, however the circuit is made up and connected.

Thus you would need a Mains supply/Solar power/Generator set or similar to bring Voltage/Frequency to DC to charge the capacitor.

From there you could charge the secondary cell - all those various losses as percentages are multiplied, not added.

Is this what you hope for?

Reply here, with

Kind Regards....

thx very much.

if fast, i want to build a circuit to discharge the useless battery and than charge the power from the capactor into a secondary cell.

i am thinking is it possible ^^'' if i using a variable capacitor.

Q=CV, to Voltage V by changing of capacitance C.

Hello "Guest",

Variable capacitors are not by their nature able to store large amounts of Dielectric stress, which may be released later as Electrical energy.

You don't seem to explain exactly what it is you hope to do.

Remember a capacitor can only give out what went in, less the inevitable losses.

If you explain properly, it is possible you may be assisted.

At present, because I am unable to correctly translate your idea into something useful, and cannot read your mind so that I know what you intend, it is not possible to give you a proper answer.

Reply here, with

Kind Regards....

^^'' sorry about my poor on English. i can't speak in English very well.

my idea is discarge the the energy of some useless battery (i think there are some energy in a battery although it is useless) and store it by a capacitor.

Then recharge the secondary cell using the energy in capcitor.

Yes, the capacitor only output what the input. My problem is how to force the power from a battery input a capacitor. and then force the power out from this capacitor.

Hello LesterLau1984

I have just sent you a PM (Private Message) which should show up in your Mailbox.

If you are able, place your Location and Country in your Member Profile, then readers replying may understand your situation better.

Here's the reply to you:

To: LesterLau1984

Hello LesterLau1984

OK re the poor English, now I understand your idea.

The "useless" battery can only ever charge up the capacitor to very slightly under the battery terminal voltage.

Please note that a capacitor cannot ever be fully charged from any voltage source, the reason is the "charging curve" of all capacitors, which gives maximum charging current at the instant the capacitor is connected to the supply voltage, and the charging current falls exponentially as the voltage across the capacitor increases towards the battery voltage. (The actual Voltage which charges the capacitor is the DIFFERENCE between the battery/supply voltage and the Voltage actually across the capacitor plates at any time)

(The reverse is true for Inductors = coils - and that is how a capacitor + coil combination may be used for tuning radio circuits.)

Although you could charge the capacitor from the "useless" battery, the capacitor (unless extremely large in FARADS (NOT microfarads) is not really able to store a charge of any great energy storage, because the electrical energy which charges a capacitor is altered into "Dielectric stress" of the insulation between the capacitor plates, and that "dielectric stress" is actually stored as "Mechanical energy" between the capacitor plates, having to be converted back to electrical energy as the capacitor is discharged.

As you place a capacitor to charge a "good" battery, it will discharge in less that ¹/₁₀₀₀ of a second, and then you would have to reconnect the capacitor to the "useless" battery again to charge the capacitor.

In all of the connection to "useless" battery -> charge the capacitor -> conversion of electrical energy to mechanical energy ->disconnection from "useless" battery -> connection to "good" battery -> conversion of mechanical energy to electrical energy -> charge "good" battery -> disconnect from "good" battery, and repeat it all over again.....and again.....there will be considerable energy losses.

Am I right in thinking the "useless" batteries you are talking about are "dry cells" (torch/flashlight cells, which actually store electrical energy as "Chemical Energy", and that "chemical Energy" is converted to Electrical energy as the battery/cell is discharged)?

Am I correct in thinking the "secondary" battery you hope to charge is a lead-acid, Nickel-Metal Hydride (NiMH), Nickel Cadmium (NiCad) or similar secondary battery?

Nearly 60 years ago I thought to do what you hope to do, but after checking out the Mathematics and Physics of it, realized with sadness that "dry" cells (torch/flashlight types of cells = LeClanche style), once used, are only fit for the rubbish heap.

It would be possible to make a step-up inverter to run from the "useless" battery, and from that inverter increase the voltage sufficiently to be able to convert the inverter AC output to DC then charge the 'good" battery, but all that would do is discharge the "useless" battery so much faster, because of the increased current requirement from the "useless battery", apart from even greater increase in energy losses.

Please do not be upset about it, because your idea means that you are a thinking person, which is really good.

Be encouraged to learn more, help others where you can, and ensure that when you leave this part of life, you have a better place to go.

Any problems, you can always send me a Private Message (PM) just like this one, and I shall do my best to assist you as soon as possible.

Kind Regards....

If you want to use more of the energy available in a cell, you could try a "boost converter". Many electronics magazines have circuits and kits.

These converters allow (for example) a 9V radio to be run from a battery that only has a terminal voltage of 6V. Mobile phones use BC's to get the most power possible from their batteries

Putting charge into a capacitor and then changing the capacitance will still require energy to be added to the system. I doubt it would be feasible except for very small power levels. It's not a foolish idea though. the principle of changing the inductance, by explosive collapse, has been used to generate massive current pulse for rail guns etc.

try to search my thread of a very simple circuit for led in the cr4 this year.

That will satisfy with your requirement.

that will take advantage of low voltage battery for some appliance like flash light etc.

Hello cnpower

The need by LesterLau1984 is not for a circuit for an LED driver, but one to recharge a secondary cell via a capacitor.

Please read my further reply to LesterLau1984 above at: http://cr4.globalspec.com/thread/27028#comment284209

What LesterLau1984 was trying to do is not practical, and he now understands that.

Kind Regards....

yeah yeah,

I read yours and know he neednt a led lighting. but with the cirvuit, he can use a battery instead of led, the battery will be recharged. right?