Electrolytic capacitor

I just noticed an advert for a 2.7V 500F capacitor, what would that size be used for?

A reasonable substitute for a battery in some applications

MichaelG.

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https://en.wikipedia.org/wiki/Supercapacitor

Edited By Michael Gilligan on 15/03/2020 20:39:48

Correct, supercapacitors have come a long way since the 1980s when they could barely run a real time clock IC or memory backup at a few microamps. Last year I was involved in a project where the client wanted the single board computer (SBC) to shut down in an orderly fashion after mains failure. Given the boxes were up lamp posts they weren't keen on batteries. We ended up with two supercapacitors in series, which kept the SBC going for the minute or so that it took to shut down while drawing several hundred milliamps. Analog Devices do a rather nice control IC with two switchers, one for charging and one for supplying the load, while also looking after all the housekeeping.

Andrew

Andrew,

Shouldn't the capacitors have been "in parallel" rather than "in series"?

Peter G. Shaw

They were probably in series as the working voltage of supercapacitors is quite low. (Less than 6 volts from memory.) So the series configuration was probably to get the required voltage.

Les

I've just been reading, but not necessarily understanding, the Wikipedia entry for supercapacitors.

Suffice to say that I don't really know what I'm talking about in this respect!

Peter G. Shaw

Edited By Peter G. Shaw on 17/03/2020 12:04:43

That particular size is often found in computers where they are used to maintain a Real Time Clock when the computer is switched off. (Computers use time-stamps for multiple purposes and are time-sensitive. For example, if a computer's  clock is slow by more than a day, network security certificates are assumed to be invalid.)

In older computers the Real Time Clock is powered by a Lithium Cell, usually a CR2032, as in Digital Calipers. Trouble with batteries is they eventually fail and have to be replaced, which can be a right pain on a computer - opening the box, finding & replacing the battery, and then resetting the clock with the BIOS.

As RTC's only consume nano-amps a big capacitor is better for standby power than a relatively unreliable chemical battery. Capacitors recharge almost instantly and should last at least as long as the computers other electronics. They eliminate the need for owners to mess around with the insides of their computers.

Super-capacitors aren't a good substitute for CR2032 cells in Digital Calipers because they draw a lot more current and there's no convenient way of recharging them. Horses for courses again.

Dave

Edited By SillyOldDuffer on 17/03/2020 12:32:14

Supercaps are generally 2.7v max, so higher voltage requires series connection and balancing of the voltage across each cap.

Very useful devices - not only for real time clock backup, etc, but are being trialed starting internal combustion engines in the stop-go mode in city driving. Live much longer than the rechargeable cells, charge very rapidly - 10 to 100's of seconds, etc.

I build Wildlife tracking collars ( Lions..) which are powered from a 3.6v lithium cell - a D cell, 17AH type which is tailored for long life at typical max 150mA draw, due to a non-trivial internal resistance.. The Iridium Sat modem in the collar draws tween 300 and 400mA intermittently during a 30 second period in which it transmits the collar position via satellite to our server. A 5Farad supercap in parallel with the battery makes this possible - it supplies the large currents thanks to its very low internal resistance, and is replenished slowly by the battery. 4 hours later, when its time for another position transmission, the supercap is long fully charged again.

It also allows wringing the last bit of energy from the battery as well.

A neat device!

The green device to the right of the PCB's top center through bottom center - a 5F supercap

Interesting stuff and great explanation - ta!

My physics teacher insisted that you'd never see a 1F capacitor that was small enough to be of practical use!

> The Iridium Sat modem

So it's YOUR satellites ruining all my astrophotos, Joe!

Neil

Sorry....

Send you some Lionphotos instead?..

Definitely in series, for the reason given by Les.

I designed an Iridium system for oceanography some years back. Must have been 2G as it took 2A pulses for a few hundred microseconds. Sorted that by using a fast buck converter with several hundred microfarads of low ESR tants on the output. A supercapacitor wouldn't have worked as the PCB needed to fit in an existing 'phone.

Andrew

Several(?) guys have replaced car batteries with about 7Ah accumulators and use supercaps for starting the engine. Been going for at least a couple years and working satisfactorily. No good for a diesel engine with heater plugs in a cold climate, I would think.🙂. Also, might not be good if the car were stalled, in traffic, by a learner driver.🙂

Back in the 1970s you used to be able to get a "battery eliminator" that was just a large capacitor to fit to Triumphs and Nortons etc with Lucas alternators and coil ignition. KIckstart only of course. Seemed like you always had to kick the bike over two or three times to charge the capacitor up before it would fire the bike into life though. Then at the traffic lights at night, if you did not keep revving the engine up, the headlight would go awful dim.