What is the functional difference between a battery and a capacitor?
In: Technology
A capacitor stores energy in an electric field built between two conductive plates. It’s basically just cramming a lot of electrons into the same space until the door is hard to close on them. Kinda like a water tower, actually.
A battery stores energy in the form of chemical bonds. Storing and extracting that energy is done via chemical reactions where ions move around and combine in the battery.
Actual functional difference:
* a capacitor is almost always much smaller in terms of stored energy than an equal sized battery, but also cheaper.
* Except for specialty super high capacity ones, they do not need a charging circuitry, they work just by being connected to a source of electricity. A battery needs a lot of babying in comparison (charge/discharge controller, safety switches)
* A capacitor can theoretically have an insanely high peak current (you see those used for coilguns – charge the high voltage capacitors slowly and then fully discharge all their energy in milliseconds), though most garden variety cheap capacitors are limited in current input/output by their non-ideal characteristics (resistance, inductance).
* A capacitor pretty much always follows a [well defined voltage curve](https://www.researchgate.net/profile/Yi_Luo82/publication/264980786/figure/fig12/AS:515122731126797@1499826237954/Capacitor-discharge-and-charge-curve.png) when charging/discharging, while batteries vary from type to type and even with the amount of current drawn, [but they usually ahve a large fairly flat part on the discharge curve](https://siliconlightworks.com/li-ion-voltage). This means that unlike capacitors, whose voltage pretty much falls off a cliff and you have to account for that in your circuit design, a battery is a bit closer to a constant voltage source – you still need to account for the variance but it’s much easier to handle a 4.2->3.5V journey than 5V->0V.
* Capacitors pass AC current through. The higher the frequency and larger the capacitance, the more a capacitor moves from acting like a break in the wire to being like a solid wire connection. This behavior is specific to a given frequency component in a complex waveform, so you can use a capacitor to filter out parts of a complex electric wave – it will act like no connection to DC/low frequency and like a simple wire to high frequency parts. If you try that with a battery, it says “bruh” and maybe explodes.
Batteries store electricity in chemicals, capacitors store electricity as an electric field between two plates
Since the voltage of a battery is determined by the chemical reaction that’s taking place they have a stable voltage for a long period of time and can provide a medium level of constant power for a decently long time.
As you pull energy out of a capacitor the voltage drops fairly quickly so you can’t use charged capacitors for applications where you’re going to be pulling decent power for hours or days, only seconds at most before you need to recharge it generally
Batteries are for the long term delivery of power, capacitors for very short term.
Because a device can have many different capacitors of many different sizes, it means different parts of a device can receive a different size of charge at a given time, as required. Meanwhile a battery just trucks along, providing a (theoretically) stable, flat, single voltage and current.
Thank you all for the excellent explanations. I get it now, as I’m not a total idiot, I’m just 5.
Capacitors store energy through charge separation, which creates an electric field. This means they can release energy very quickly, but they don’t store as much energy as a battery. They’re best suited to high power applications.
Batteries store energy in chemical bonds. This makes it harder for them to release energy quickly, but it also allows them to store a lot more energy. For this reason, batteries are better suited to high energy applications.
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