Question about Capacitors

hello guys,
i might sound silly about this a silly question maybe but it crossed my mind, what is the real difference between a rechargeable battery and a capacitor except that it's the life span, i mean both generates charges, why not talking about Amps for a capacitor rather than farad ?

I am sure 15 minutes with Google would answer your question. :wink:
.

Capacitors store charge without chemical reaction - the charge is stored on the surface of the dielectric inside (the capacitance is actually a function of area - those tiny ceramic caps are stacks of thousands of very thin layers). The discharge curve is linear (in ideal capacitor, real ones aren't quite). Which means, if you discharge half the current, the voltage is half what it was at the start (which means that energy stored goes as the square of voltage it's charged to). If you do the math out, even beefy "BFCs" common in electronics store pitiful amounts of energy (supercapacitors have much higher capacitance, but only at low voltages, and still small compared to batteries). Capacitors last for many many charge/discharge cycles, particularly tants and ceramics (electrolytics dry out over time)

Batteries store energy in chemicals inside the battery, and electrochemical reactions generate the energy while the battery is discharging. Charging reverses the process by applying a voltage to drive the electrochemical reaction the other direction. This is less efficient than charging or discharging a capacitor, and the internal structure is slowly degraded with charging and discharging, resulting in performance (capacity, etc) falling over time. The discharge curve is usually flatish, so at 50% charge, a battery has almost the same voltage it had at 90% - and then it falls off sharply near the end (when there's no more chemical to react). This makes them better power sources.

The word battery is inconsistently used, for example, a lithium-ion "battery" is really an ion capacitor. There is no chemical reaction it is just ions migrating from one plate to the other. If you start looking into the difference between a battery and a capacitor it gets a little murky near the ion battery, but generally batteries store chemical energy and capacitors store energy as an electric field (e.g. charge in or on conductive things separated by a dielectric). One other note I'll add is that chemical reactions have entropy and that takes energy to drive forward during both charging and discharging. So basically Lithium-ion is so good because it is not really a battery and does not suffer from entropy losses.

DrAzzy:
Capacitors store charge without chemical reaction - the charge is stored on the surface of the dielectric inside (the capacitance is actually a function of area - those tiny ceramic caps are stacks of thousands of very thin layers). The discharge curve is linear (in ideal capacitor, real ones aren't quite). Which means, if you discharge half the current, the voltage is half what it was at the start (which means that energy stored goes as the square of voltage it's charged to). If you do the math out, even beefy "BFCs" common in electronics store pitiful amounts of energy (supercapacitors have much higher capacitance, but only at low voltages, and still small compared to batteries). Capacitors last for many many charge/discharge cycles, particularly tants and ceramics (electrolytics dry out over time)

Batteries store energy in chemicals inside the battery, and electrochemical reactions generate the energy while the battery is discharging. Charging reverses the process by applying a voltage to drive the electrochemical reaction the other direction. This is less efficient than charging or discharging a capacitor, and the internal structure is slowly degraded with charging and discharging, resulting in performance (capacity, etc) falling over time. The discharge curve is usually flatish, so at 50% charge, a battery has almost the same voltage it had at 90% - and then it falls off sharply near the end (when there's no more chemical to react). This makes them better power sources.

ok so what i understand is that making the energy is what makes them different ? a chemical will give more power at a longer time with different discharging function where as the capacitor no matter how big it is it will not be same as a battery and doesn't have a constant voltage no matter what ?
even if it was like i don't know maybe a 10^10 farads ?

ron_sutherland:
The word battery is inconsistently used, for example, a lithium-ion "battery" is really an ion capacitor. There is no chemical reaction it is just ions migrating from one plate to the other. If you start looking into the difference between a battery and a capacitor it gets a little murky near the ion battery, but generally batteries store chemical energy and capacitors store energy as an electric field (e.g. charge in or on conductive things separated by a dielectric). One other note I'll add is that chemical reactions have entropy and that takes energy to drive forward during both charging and discharging. So basically Lithium-ion is so good because it is not really a battery and does not suffer from entropy losses.

so can we say a battery is a specific type of capacitor ? or just the li-ion ?

I would not say all batteries are a subset of capacitors, I would say that Li-ion is really a capacitor (but everyone calls it a battery anyway).

firashelou:
hello guys,
i might sound silly about this a silly question maybe but it crossed my mind, what is the real difference between a rechargeable battery and a capacitor except that it's the life span, i mean both generates charges, why not talking about Amps for a capacitor rather than farad ?

Actually, we can talk about "Amps" when discussing Capacitors ...
A 1 Farad capacitor charged with 1 volt with give you 1 amp for 1 second.
In that 1 second about 6.241 x 10^18 electrons went down the wire.

Charge that 1 Farad capacitor to 9 Volts and you get 1 amp for 9 seconds.

A 1 1/2 ounce, 9 Volt battery will generate 1 amp for about 1,080 seconds.

A 9 ounce, 9 volt Lithium battery pack will generate 1 amp for 21,600 seconds.

It is all about Energy Density ...

A lithium ion battery and a lithium ion capacitor are not the same thing (and both exist).

Well I'm no doctor but when I look at

and then at an example of its construction

and compare it to the various LIB's there seem to be many similarities.

mrsummitville:
Actually, we can talk about "Amps" when discussing Capacitors ...
A 1 Farad capacitor charged with 1 volt with give you 1 amp for 1 second.
In that 1 second about 6.241 x 10^18 electrons went down the wire.

Charge that 1 Farad capacitor to 9 Volts and you get 1 amp for 9 seconds.

A 1 1/2 ounce, 9 Volt battery will generate 1 amp for about 1,080 seconds.

A 9 ounce, 9 volt Lithium battery pack will generate 1 amp for 21,600 seconds.

It is all about Energy Density ...

ok but what if i need less than 1 amp resistors come in hand ?
and more than 1 amp ?

firashelou:
ok but what if i need less than 1 amp resistors come in hand ?
and more than 1 amp ?

I used a constant 9 Volts and constant 1 Amp to show the Energy Density ratios between:
a) Capacitor = poor
b) 9 volt battery = good
c) Li-Ion battery = best

Yes, you can use more or less than one amp, within limits, for your circuit.

mrsummitville:
I used a constant 9 Volts and constant 1 Amp to show the Energy Density ratios between:
a) Capacitor = poor
b) 9 volt battery = good
c) Li-Ion battery = best

Yes, you can use more or less than one amp, within limits, for your circuit.

ok so if my circuit needs 300mA and the cap can provide up to 1amp in a sec it will give 300mA for 3 seconds for example ?

guys i have a question about decoupling:

i am studying a bit about components and now as you see it's capacitors, but one thing which i couldn't get an answer is how to determine the decoupling capacitance needed for an IC ?

Rule of thumb is to use a .1uf ceramic (one per I.C. is best) mounted as close as possible to the chips power supply pin, VCC/VDD.

Some late night reading:

Multiple caps needed if there are multiple VCC pins.
AVCC also on microcontroller chips.

It is not hard to find articles from TI, IBM, Analog Devices, etc on caps. 0.1uF and 0.22uF are good. You will never go wrong putting a 0.1uF cap on the power of every device.

More reading:

http://www.murata.com/~/media/webrenewal/support/library/catalog/products/emc/emifil/c39e.ashx

LarryD:
Rule of thumb is to use a .1uf ceramic (one per I.C. is best) mounted as close as possible to the chips power supply pin, VCC/VDD.

yes but is this like a rule from experience they found that 0.1uF is the best ?