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PerspectivePure2169 t1_j5demap wrote

Others covered chemical vs field storage, but there's also differences in practical useage. Capacitors work better for shorter term, rapid cycles, and large fast energy discharges. They can also correct power quality in a way batteries can't.

Batteries work better for long term storage, shallower discharge, and sustained energy conversion over a long time.

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dbx999 t1_j5kfdp4 wrote

I was told that batteries do work well to correct power quality.

I was told that a car battery helped regulate the delivery of a constant 12V current inside the car by buffering and regulating the electricity produced by the alternator.

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MiffedMouse t1_j5m18e7 wrote

It is a question of timescales. Capacitors are typically used to “smooth out” power over timescales of nanoseconds to seconds (super capacitors can sometimes go up to minutes or hours). Batteries can smooth out power over timescales of minutes to years.

Batteries are also better at “clamping” to a certain voltage, while capacitor voltage is typically more variable.

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PerspectivePure2169 t1_j5mb8id wrote

Yes batteries can do this for a DC system like in your car. But that is mainly a voltage regulation function.

Power quality correction is a consideration in large industrial AC power networks. It helps with the problems that arise from running large amounts of electric motors where the power factor gets out of balance between inductive and reactive currents.

Since electricity essentially sees the coils within a motor as an inductor, it affects multi-phase power in a way that's detrimental to its ability to transfer power

So capacitor Bank systems are designed to correct the issue so that the overall AC power quality isn't effected for downstream users.

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hatsune_aru t1_j5nxahk wrote

The actual regulation in a car is done by the ECU or some circuit inside the alternator that limits the output voltage to a certain amount.

The battery is mostly there so that when there is a load transient, the regulator mentioned above can have a relaxed response time--the battery takes up the slack when there's a sudden load increase or decrease.

If a control system has to react quickly, it's more liable to instability like oscillation and divergence. Better to keep it safe by making it slow to react, especially if you already have a big ass battery that can smooth things out that you need to start the car.

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PromptCritical725 t1_j5qtyxu wrote

This is a reason people with bigass car stereos add capacitors. Large sudden power draws like amps driving large bass hits can cause equally large drops in system voltage. The capacitor act to "stiffen" the system by providing reserve of current that can be drawn and replenished much more rapidly than a battery can. This keeps the voltage more consistent.

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TheDefected t1_j5cbxqz wrote

You could divide it by batteries store power as chemical energy, and capacitors store electric charge on plates, with no change in chemistry.
Both (have types that) can be charged and discharged, both can be built and need charging first.

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PromptCritical725 t1_j5qu3uh wrote

Lead-acid batteries are charged as soon as they are filled with sulfuric acid.

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JonJackjon t1_j5ccwaz wrote

Yes, they both store energy:

The capacitor stores the energy as an electric field.

The battery provides energy from a chemical reaction.

With today's technology a battery can generate more energy than the capacitor can store.

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ZZ9ZA t1_j5ecx4b wrote

… but provided both are capable of holding a given amount of charge.. the capacitor can discharge much more quickly, often almost instantaneously.

With supercapacitors actually starting to be a thing, there isn’t the huge gulf of capability there used to be.

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JonJackjon t1_j5g1nf3 wrote

I've never measured any delay in a battery. I do agree that batteries have very different discharge characteristics than capacitors.

The capability gap is actually much larger even with super capacitors. Flow batteries make the possibility for huge increases in battery capacity.

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PromptCritical725 t1_j5quftc wrote

Batteries have basically always been able to store more energy than caps. That's what makes supercaps compelling. They store more than caps, but are faster than batteries.

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yak-broker t1_j5dx705 wrote

Other than the physics (chemical storage vs. electric-field storage) one huge difference is they have different charge-vs-voltage curves.

The voltage across a capacitor is proportional to how much charge has flowed through it (it's the integral of the current). This simple mathematical relationship is vital for all sorts of analogue circuitry (and all circuitry is analogue eventually).

A battery, on the other hand, has a relatively constant voltage for most of its lifetime. A common 1.5v alkaline cell will only gradually droop to 1v or so before suddenly dropping off. NiMH batteries are very flat around 1.2v for most of their discharge. The voltage is determined by their chemistry, and is strongly affected by all sorts of real-world stuff like temperature and diffusion rates. For rechargeable batteries the relationship between state-of-charge and terminal voltage can be quite complex.

Of course real-world capacitors do have a lot of non-ideal behavior, like leakage, ESR, dielectric absorption, and microphonics, but even with all that they're a lot closer to a simple "voltage × capacitance = summed charge" relationship than a battery is.

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Fit_Solution3312 t1_j5e6qeq wrote

Capacitors store energy in an electric field, resulting in a linear dependence of stored energy to cell voltage. You can also see this from the capacitor equation for stored energy Emax [Ws] = 0.5 * C [F or As/V] * U^2 [V^2]

Capacitors also do not age by cycling but by holding them at high voltages at high temperatures.

A should can see, capacitors are rated in Farad or Ampere-seconds per volt: it means that a 1 F capacitor will take 1 second at 1 A to charge to 1 V.

Batteries on the other hand depend on a chemical reaction which happens at one specific voltage - as long as there is enough chemicals still available to react, the voltage barely changes.

Batteries are rated in Ampere-hours, as their voltage is stable, meaning that at their given voltage, you can draw 1 Ampere for 1 hour from a battery rated at 1 Ah.

Batteries age by cycling, as the reactions are not 100% reversible.

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Buford12 OP t1_j5el5xl wrote

Thanks , One more question, which device stores or produces the most energy per unit of mass.

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SonofaBartfast t1_j5ffst9 wrote

Capacitors store energy like static electricity. Imagine two metal plates one positively charged and one negatively charged, really close together but not touching. Until something does connect them and a surge of electrons move to cancel out the charge.

Same kind of potential energy you have built up whenever you touch a metal door knob and it shocks you. Lightning is also a form of static electricity discharge.

..

Batteries how ever work chemically. They exploit a type of chemical reaction often called a redox reaction, in which, two dissimilar materials (like lead and lead oxide in a car battery) give or take electrons from one another.

This same kind of chemical reaction is also how your body produces electricity to live. There are lots of details removed, but the both reactions are redox reactions.

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hatsune_aru t1_j5nx4v7 wrote

Capacitors are devices that store energy by moving charge carriers (usually electrons) in places it really doesn't want to be in--which means they have to be forced in that position--which takes energy that you can get back later, quite easily. Charge carriers like electrons can move around efficiently and quickly in conductors like metal, so capacitors generally can be charged and discharged rapidly.

Batteries are devices that store energy by making ions stay in energetically excited conditions that it doesn't really want to be in. Ions are generally floating charged particles in a liquid solution, so they take much longer to float around and do their chemistry to get your energy back. The upside is that you get to store a lot more energy into the structure, but the downside is that the rate at which you can charge and discharge is beholden to the chemistry and the speed at which ions move, so it's quite slow. They are also inefficient since ions aren't efficiently transported, as much as electrons are in metal.

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