Why do capacitors charge faster than batteries? - Page 1
Forum section: electronics / general

author: Technoguy3
author_rank: Contributor
timestamp: July 31, 2012, 05:47:24 pm
content: I know that both are based on electrochemical reactions, but does anyone know why? Why are there no AA batteries that charge in less than a second?

author: tom66
author_rank: Super Contributor
timestamp: July 31, 2012, 05:48:44 pm
content: Capacitors store far less energy.

author: Technoguy3
author_rank: Contributor
timestamp: July 31, 2012, 05:52:50 pm
content: Too true. And if there were a way to cause all of the chemicals to react simultaneously, it would produce too much heat.... Let's solve this problem and live off lightning strikes.

author: grumpydoc
author_rank: Super Contributor
timestamp: July 31, 2012, 06:01:22 pm
content: Capacitors do not rely on electrochemical reactions to store charge, though electrolytics do so to maintain the insulating layer between the plates.

author: olsenn
author_rank: Frequent Contributor
timestamp: July 31, 2012, 06:01:37 pm
content: That's a good question actually; capacitors store FAR LESS charge than batteries do, and so they finish storing their maximum amount of charge almost instantly; however, if a standard electrolytic cap did store as much charge as a 2000mAh Li-Ion battery (bare with me here) how long (approximately) would it take to reach a full charge? I suppose what I am asking, is how many coloumbs per second do caps charge at?

author: DavidJRobertson
author_rank: Regular Contributor
timestamp: July 31, 2012, 06:02:18 pm
content: Quote from: Technoguy3 on July 31, 2012, 05:47:24 pmBatteries store energy as chemical energy, whereas capacitors store it as electrical energy. That is to say there is no electrochemical reaction happening in a capacitor.

author: DavidJRobertson
author_rank: Regular Contributor
timestamp: July 31, 2012, 06:06:46 pm
content: Quote from: olsenn on July 31, 2012, 06:01:37 pmYou can get so-called super-caps, which can store massive amounts of energy, and release it all very quickly due to having a very low ESR compared to batteries.

author: G7PSK
author_rank: Super Contributor
timestamp: July 31, 2012, 08:05:47 pm
content: Quote from: olsenn on July 31, 2012, 06:01:37 pmThat would depend on the supply rating, a capacitor will take a charge as fast as you can put it in.

author: IanB
author_rank: Super Contributor
timestamp: July 31, 2012, 08:18:26 pm
content: Quote from: olsenn on July 31, 2012, 06:01:37 pmThis is very easy to answer: you have already answered the question yourself.You could charge it with 2000 mA (2 A) for one hour; or 8 A for 15 minutes; or 120 A for one minute; or 720 A for 10 seconds. Take your pick.

author: KTP
author_rank: Frequent Contributor
timestamp: July 31, 2012, 08:38:18 pm
content: Take a look at the new nanotech 90C LiPo batteries (the ones with like .0001 ohm internal resistance)A battery the size of a deck of cards that can output 500amps at 12VI think they can even be charged something ridiculous like 10CI bet the guy in the supercap video wouldn't be shorting 4 of those with just rubber gloves on.

author: T4P
author_rank: Super Contributor
timestamp: August 01, 2012, 01:13:13 am
content: LiPo as we know can take at least 12C charge rating!Quote from: KTP on July 31, 2012, 08:38:18 pmThat's just old, compared to modern LiPos

author: Psi
author_rank: Super Contributor
timestamp: August 01, 2012, 01:50:06 am
content: I sometimes play around shorting out my 21.5V super capacitor array with some metal rods on the end of thick wire.It produces a nice shower of sparks but isn't that dangerous.I selected the wire length and width specially, it limits the current to 600A, since this is the maximum those supercaps are rated for. 600A * 21.5V = 12900WYou just have to watch that the rods don't fuse together as they get real hot real fast.And of course keep the sparks away from your eyes, safety glasses is a good idea.

author: ee851
author_rank: Regular Contributor
timestamp: August 01, 2012, 02:47:30 pm
content: DavidJRobertson is correct. Capacitors charge/discharge very quickly, at an appreciable fraction of lightspeed. However, we know from damage caused by lightning that it isn't safe to have very large amounts of electrical energy flowing very quickly. So to be safe, we must use a large resistor to check the flow of current to a safe level. Doing so also slows down the charge/discharge process. This introduces the so-called RC time constant behavior of a capacitor. This is a good thing. It keeps components in the current path from blowing up.

author: nitro2k01
author_rank: Frequent Contributor
timestamp: August 01, 2012, 03:19:47 pm
content: ee851: The damage will happen where the power is output, i.e. the part of the circuit with the highest resistance. In a ligthning bolt, both the ionized air and any humans or trees that happen to be in the way, will have a resistance much higher than 0 ohms. However, if you use low ESR capacitors and heavy duty wiring, you can even build a large capacitor bank which you can discharge the capacitors near the speed of light, and the capacitors won't as much as blink an eye in the process. However, the outlook of whatever you connect in series isn't as optimistic...

author: Technoguy3
author_rank: Contributor
timestamp: August 02, 2012, 02:53:30 pm
content: What is stopping us living off the energy from lightning strike? Could you perhaps theoretically build an array of capacitors that would charge when lightning struck, and then discharge their energy slowly into a battery that you could power your house with?

author: KTP
author_rank: Frequent Contributor
timestamp: August 02, 2012, 03:10:48 pm
content: Quote from: Technoguy3 on August 02, 2012, 02:53:30 pmWell...the energy in a lightning strike can be in the billions of joules.This would technically power a 60 watt light bulb for a year. (it needs about 5 million joules a day)Capturing that energy all at once would be a feat...IIRC, Blaine the Mono did this for its (his?) engines, so Stevie already had this idea.

author: Technoguy3
author_rank: Contributor
timestamp: August 02, 2012, 05:15:47 pm
content: But what's stopping us if we have capacitors that can take on a charge in a fraction of a second? Seems like cleaner energy than coal!

author: BravoV
author_rank: Super Contributor
timestamp: August 02, 2012, 05:20:34 pm
content: Quote from: Technoguy3 on August 02, 2012, 05:15:47 pmBasic logic flaw here, c'mon re-think again.

author: KTP
author_rank: Frequent Contributor
timestamp: August 02, 2012, 05:21:42 pm
content: Quote from: Technoguy3 on August 02, 2012, 05:15:47 pmWe have cells that can convert sunlight into electricity at 20% efficiency. Sunlight hits the earth at an average of 800 watts per square meter. What is stopping us from putting cells over 0.1% of the earth's surface and totally living off the sun?(Probably the same thing that is stopping us from having millions of capacitor banks hooked up to golf clubs waiting for a storm)

author: nitro2k01
author_rank: Frequent Contributor
timestamp: August 02, 2012, 05:43:15 pm
content: Quote from: Technoguy3 on August 02, 2012, 02:53:30 pmTwo problems:1) The capacitor needs to have a dielectric (the insulation between the electrodes) which can withstand the extremely high coltage without breaking down.2) You need to find a way to isolate the area between the capacitor and lightning rod, and the ground around it. The lightning bolt has travelled several kilometers through ionized air, from the clouds down to ground level. What is there to stop the bolt lightning bolt from ionizing the air around your capacitor setup and complete the circuit to the ground, meaning you'll lose most of the energy that way?

author: JackOfVA
author_rank: Unknown
timestamp: August 02, 2012, 10:55:12 pm
content: A bit of math should make this clear. I'll neglect some second order issues to keep the math simple.Charge is measured in Coulombs. Current is the change in charge with time, i.e., dQ/dt and 1 C/sec is 1 Ampere.Consider a capacitor used in a 5V power supply, say 10,000 uF, or 0.01F. Q=CV, so the total charge in coulombs in a 10,000 uF capacitor charged to 5V is 0.05 coulombs. So the charging circuit must deliver 0.05 coulombs of charge to the capacitor.Now consider a 5V battery, with a rating of say 1 ampere hour. To charge the battery requires 1 amp for 3600 seconds. 1 ampere/sec = 1 coulomb, so the battery's charge is 3600 coulombs.It should be apparent that it will take longer to supply 3600 coulombs to the battery than to supply 0.05 coulombs to the capacitor, all else being equal.Or, if working with charge makes you uncomfortable, consider the energy stored in the two. I'll leave that as an exercise for the interested reader.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 03, 2012, 01:36:04 am
content: Quote from: Technoguy3 on August 02, 2012, 02:53:30 pmMaybe try a different approach? Maybe make a huge cannon that fires a slug of water uphill, after which the energy is easy to extract. I read that it's possible to use a laser to get lightning on demand.