Storing energy in Capacitors

[Electro132]

Hi,

I am trying to store energy from a 6v battery into a capacitor to run my leds. So far i have tried a 1000 uf 16v electrolyctic capacitor and it's ok (the lights on my circuit dim after i switch off the power source). But i want my circuit to have "longer lasting batteries" similar to about 2 years coming from the same batteries. I have looked at li-ion rechargeable batteries and was thinking of hooking them up to the circuit along with the electrolyctic caps. I also have looked at super caps as they discharge to power up an entire computer. So really i am stuck and need some suggestions.

Furthermore, i was wondering whether the 1000 uf rating on the electrolyctic is just for charging the cap or whether it does charging and discharging? Because i know that the 1000uf rating means there is a lot more space in the cap compared to a 100uf but it doesn't necessarily mean that it discharges as quickly/slowly as the 6v battery.

Thanks

 

[Arouse1973]

What current are the LEDs drawing, how many LEDs do you have, how are they connected. So you want to remove the battery and let the LEDs run for a certain amount of time. Is this correct? If so how long do you want them to run for? You do know the capacitors won't increase the capacity of the batteries, the energy stored in the capacitor will come from the battery.

Capacitors can be used for impedance matching to a low impedance loads that are switched on and off and they can supply the very high instantaneous currents required by some loads right down to the cut off voltage of the battery.

Without the capacitors the high current needed by the load would reduce the battery voltage on a battery that's nearly discharged due to the higher impedance of the battery in this state.

I might not be quite understanding what you want to do.

Adam

 

[Gryd3]

http://ca.mouser.com/new/Taiyo-Yuden/taiyo-yuden-lithium-ion-capacitors/
May solve your capacitor from emptying too soon... but I am unsure what you want your circuit to do with the batteries..

 

[Arouse1973]

I think he has looked at super capacitors already.
Adam

 

[Laplace]

How would you calculate how long a capacitor should be expected to keep an LED on? Unlike a battery, a charged capacitor begins to lose voltage as soon as charge begins to drain off into the load. Let's suppose you have a red LED with 2.2V forward voltage, driven by a 20mA current source that uses an additional 0.8V. This is powered by a capacitor charged to 6V starting at time t=0. The LED will stay on until the capacitor discharges to 3V.

Capacitance is defined as the charge stored per volt, C=Q/V. Q=0 when V=0 so C is also the slope of the Q-V graph, C=ΔQ/ΔV. But with a constant current ΔQ=IxΔT so C=IxΔT/ΔV and ΔT=ΔVxC/I

In the case of a 1000μF (0.001F) capacitor charged to 6V (ΔV=3V) with discharge current 20mA (0.02A) the LED will remain on for ΔT=3x0.001/0.02=0.15sec

In the case of a super-capacitor of 5,000,000μF (5F), ΔT=3x5/0.02=750sec (12.5minutes).

 

[(*steve*)]

So if 5F gives you 20mA for 750 seconds, all you need is a 6V 3.8MF capacitor.

That will run for 4 years if you reduce the current to 10mA.

Easy!

 

[Arouse1973]

And here's the enclosure to put them in. RS have 15 in stock, you can have any colour as long as it's blue

 

[Electro132]

Just a question: Isn't it suppose to be 16V cap since i'm using 6v? I'm pretty sure using 16v is close to 12v being that it is the double amount of voltage required in case the power surpasses the 6v threshold. Although i could just use a 3v battery. Btw, the LED i'm using is white and has 5000 mah. It is being used to light up the hallway corridor with about 4 LEDs so i need them to be bright enough to lighten up the room.

 

[BobK]

The energy density of even super-capacitors is much less than that of batteries. I.e. for the same weight / volume, a battery will run your LEDs many times longer than a capacitor. Forget the capacitors, what you need is a rechargeable battery.

Bob

 

[(*steve*)]

and has 5000 mah.

No it doesn't. (unless it's a battery). I can't even figure out what you might be referring to. Oh hang on, you might mean 5000 mcd?