If electricity is so fast, how it doesn’t immediately charge up capacitors and batteries?

49 views

I’m pretty aware that this is a “dumb” question, but my basic understanding of electricity can’t figure it out. I know the basic concept of resistance, currency and voltage, but I can’t comprehend how it takes so long to store charge in a battery

In: 11

While electricity travels fast, batteries have to undergo a chemical reaction to regain their charge which takes time.

Capacitors should charge quickly, usually in devices, there is some resistance to prevent capacitors from charging ir discharging too quickly due to safety reasons.

Well, you can make a very high speed flow of water in a straw. Realistically speaking, no matter how fast you make the water flow in a straw, it will take some time to fill a large tank.

The speed of electricity is not the issue, the question is how fast can a battery or capacity store ENERGY.

Chemical batteries can only accept a certain amount of charge per second (chemical reactions are not very fast in a battery). Therefore it is not relevant how fast electricity “moves” but how much charge can be accepted.

When you push on one end of a stick, that push is carried to the far end of the stick almost instantly. The stick itself has barely moved.

Electricity moves like the force on the stick. In fact, it moves way faster. The actual electrons move very slowly.

Capacitors charge almost instantly. If not for some other component limiting them, they *would* charge instantly.

Batteries are limited by the chemicals in them. These chemicals rearrange themselves to store the charge, and if you force this too fast the battery just destroys itself. This is not a limitation with electricity, but with battery chemistry.

Electricity “spreads” fast, and individual electrons *can* be fast.

But to fill a capacitor you need a lot of electrons, and the more there are already the harder it gets to add more. And batteries are also limited by the reaction speed of the chemicals inside.

So as an example if you connect a capacitor through a long wire to a 5V source. The 5V will reach the input of the capacitor at the speed of light, but that’s just “electrical pressure”, no electrons moving yet. This pressure then starts moving electrons through the wire depending on it’s resistance. A voltage accelerates an electron until it hits an atom in the wire and is slowed down again, so they mostly just bounce around with a general trend to move forward.

The classic analogy is a waterhose. If it’s already filled with water opening the tap will (almost) instantly push some water out of the front. But only because that happens fast doesn’t mean you can fill an olympic pool in a second. Equally you can make the individual water particles very fast by using a thin hose, but that doesn’t help you filling the pool either, it gets faster but there is less of it.

This is not literally how it works obv but just as an analogy, think of electricity running through a wire as a hose. A battery is a water tank in the air. Takes a bit for the hose to fill up the bucket and depends also on the velocity/thickness of the hose (volt/amperage). The speed of the water (volt) times the width of the hose (amp) gives the total water moved (wattage). Bigger bucket (battery) takes longer to fill and more water (wattage) is faster to fill.

Once the water is suspended, you can poke a hole in the tank and the stored “potential energy” is released. Of course this is a chemical process and electricity is a field and all that but just as a way to conceptualize.

Cause charges pushing through isolated gapds can’t just hop to the other side, it is probabilistic, the more “tension” the easiest.
Also, chemical batteries like car batteries relies on redox processes that takes time to oxidate different metals, electricity is fast, but ions in an electrolytic solution arent.

To charge a battery, you create a voltage difference between its two ends. The creation of that difference is the super-fast part.

But once you’ve got that difference in place, that’s when the work of charging the battery begins. In the case of a lithium ion battery, you’re actually moving lithium ions from one end of the battery to the other, through some gooey stuff in between. That’s the slow part. It’s like if you put a turtle in a corridor and opened a gate at the far end with turtle food. It only takes a moment to open the gate, but that doesn’t put the turtle at the food immediately.

I don’t want to over-use the turtle analogy, so we’ll go back to lithium ions. The battery manufacturer wants to pack a lot of battery life in there which means they pack a lot of ions in there. Which means it takes time to move them all.

There’s something called a RC time constant. R is resistance, C is capacitance. The time is what you get when you multiply them, and is how long it takes to charge(most of the way, the rate of charge slows down the more full it gets, because the difference in voltage gets smaller) Even capacitors have some series resistance and inductance in the leads of the component, so there’s always some amount of time it takes to charge. For small ceramic capacitors it can be pretty quick though.

When you’re charging a battery, you could put current in faster, but it would change what you’re doing chemically inside the battery. It takes time for the needed chemicals to get to where the desired reaction takes place, and if you push it too hard instead of the chemical reaction you want you might get something completely different, like decomposing the electrolyte, or you might overheat the cell to the point it self discharges and catches on fire.

Thanks you all for all the replys!! I am reading them all and, besides my initial curiosity, it is more interesting that I expected!!

Fwiw. At this point, we are being bottlenecked by the chargers in most cases.

Yes, batteries have a max charge rate to manage the heat.

But right now, many batteries *could* be charged faster if they had the juice. But the DC inverters, cables, and charging connectors become a weak point in the chain. There are some physical constraints on how thick it can be and remain flexible, and how big the charging port is in the car.

Keeping in mind, most charging is done at home with relatively low amps. So do we care to spend and sacrifice for peak charging speed we may never use.

Because it’s not about how fast the electricity moves, but how much of it moves at a time. The limiting factor here is wire size. It’s like comparing a garden hose and a fire hose. By increasing the size you allow more water to flow at the same pressure.

Internal resistance in the capacitors and wires limits the amount of current that can flow, thus increasing the amount of time it takes to charge.

Voltage is roughly the same as pressure in a water system and you can use the same math to model both an electrical and water system. Imagine a capacitor as a barrier block the flow of water. It needs to build up enough pressure to break through the barrier to allow the flow to continue past it. In a capacitor, the barrier is thin layer of an insulator that prevents the flow of electrons until enough voltage has been built up to push through the insulator. You have seen this principle in action before, at an extremely large scale. In a thunder storm, voltage builds up in the clouds until the massive amount required to overcome all the air acting as an insulator. Once it can do that, all that built up voltage flows to the ground as a lightning bolt.

Batteries are something entirely different. In a battery, you have two “containers”, each connected to one of the terminals. Each contains specially prepared chemicals, one with an excess of electrons and one with a lack of them. Under normal operation, the electrons will want to flows to the container without them in order to create a balance of charges. It will keep doing this until all of the electrons have moved to the other container. In order to recharge a battery, you need to apply energy to break the chemical bonds keeping the electrons were they are and force them to move in the opposite direction. Electrons really don’t want to do that so it will take a lot of time and energy to make them move that way. Even then, you won’t get 100% of the electrons back into their container. To use the water system metaphor again, a battery is like a water tower. You need to use a pump to move the water up into it in order for there to be water to flow down from it into the system as a whole.

There’s two separate ‘speeds’ of electricity. One is how fast a voltage can travel, and another is how fast the individual electrons travel. While voltages travel at basically the speed of light, individual electrons take a lot longer to travel. Not only that, but you need a looooooot of electrons to completely fill up a battery or capacitor. The speed of an individual electron only tells you how fast it takes for the first electron to arrive at the destination, it doesn’t tell you how fast you can charge a battery or capacitor that has a capacity of billions of electrons.

To find that out, you need to know how many electrons you can send at once, which is dependent on the maximum current your charging cable is capable of.

Think of an analogy where you want to fill a giant bucket full of baseballs. You hire a pitcher to throw one baseball at a time into the bucket. While each individual baseball moves very quickly, you can see how it would take a very long time for a single pitcher to fill up a giant bucket. To increase the filling rate, you need to spend more money to hire more pitchers, or hire more expensive pitchers that can throw more balls each hour. But even then you have a limited budget and a really big bucket so it still takes a long time.

Batteries are one of many ways of storing energy. They happen to be more efficient in specific use cases. Especially in compact, mobile applications.

For example, another method to store energy is to use electricity to pump water up a hill. When you want the energy back, you release the water and use it to spin a turbine. It takes time to pump the water the hill. If you try to pump the water too fast, the pipes will burst or you’ll burn out the pump, etc. It’s the same with the turbine.

Another method includes spinning a flywheel in a vacuum, which could twist into pieces if we attempted to accelerate or decelerate it too quickly. Etc. You cannot just give it maximum torque as the electric motor has thermal limits, the drive shaft could twist, the flywheel could exceed its maximum RPM, etc. You could also use an engine and transmission to spin up the flywheel, then use the transmission to link the flywheel’s output to a generator to convert that energy into electricity.

In the case of modern batteries, the way energy is stored is to use electricity to rearrange lithium ions in each cell. When you want the energy back, you reconfigure the connections to each cell, which begins the process of causing ions to migrate back to their original distributions. The side effect of this migration is electricity.

Just as it takes time to pump water or spin up a flywheel, It takes time for the ions to be optimally rearranged throughout each cell. And like spinning a turbine, charging or discharging a battery too quickly can cause it to catch on fire, etc.

Battery chemistry is a key factor in how quickly a battery can charge, in addition to the voltage of the system. The specific chemistry and internal structure of the battery determines how quickly the potential can be built up and released.

Capacitors can charge very quickly because they discharge very quickly. A more controlled, sustained discharge is required for electric cars, phones, etc. Some applications use capacitors to provide a boost to electric drivetrains, not unlike a shot of nitros oxide for an internal combustion engine.

IOW, storing energy in a battery uses electricity to build a chemical potential, which can be converted back into electricity again. Building and releasing that potential isn’t instantaneous.