Constant power supply in a circuit

Hi,

I would like to build something that provides a constant power supply in a circuit. I looked around the internet and it seems that capacitors can store alot of power kind of like Li-ion batteries. It would be useful if i could get some help building it as i would like to attach it to my devices (motion detector cameras, security alarms, etc) for personal use.

thanks

Lighter64 said:

Hi,

I would like to build something that provides a constant power supply in a circuit. I looked around the internet and it seems that capacitors can store alot of power kind of like Li-ion batteries. It would be useful if i could get some help building it as i would like to attach it to my devices (motion detector cameras, security alarms, etc) for personal use.

thanks

Click to expand...

Hello Lighter64

Welcome to ETO. Your first post here.

Firstly, please do not confuse Capacitors with batteries....they are NOT the same.
A battery is a battery and is NEVER a Capacitor.

Batteries and Capacitors are used differently....

Secondly, please read up a little bit about how things work here:

https://en.wikipedia.org/wiki/Battery_(electricity)
https://en.wikipedia.org/wiki/Capacitor

Saves time explaining when it has all been done before.

Regards and happy learning
tvtech

Lighter64 said:

I would like to build something that provides a constant power supply in a circuit. I looked around the internet and it seems that capacitors can store alot of power kind of like Li-ion batteries. It would be useful if i could get some help building it as i would like to attach it to my devices (motion detector cameras, security alarms, etc) for personal use.

Click to expand...

Why don't you just use ordinary batteries. Maybe rechargeable batteries that you can easily replace/recharge when they run out of power. Or did I misunderstand what you want to do?

Capacitors can't store a lot of power compared to batteries. Capacitors can charge and discharge quickly.. they can release a large power spike for short amount of time (for flashlights etc.).

misterT said:

Why don't you just use ordinary batteries. Maybe rechargeable batteries that you can easily replace/recharge when they run out of power.

Click to expand...

Then attach a small solar panel to recharge the battery. Will be as constant as it gets.

Modern super caps can store a huge amount of energy sizewise compared to a Lithium cell. However it's their discharge characteristics thet let them down as an alternative.

Li cells can deliver constant voltage for their discharge duration. Caps discharge exponentially immediately once the load is attached as their stored energy is stored differently chemically wise.

As said, Super caps are fine for a large amount of current instantaneously.

WTP Pepper said:

Modern super caps can store a huge amount of energy sizewise compared to a Lithium cell.

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Do you mean can provide a huge current pulse?, as far as I'm aware supercaps are still considerably behind battery technology for the amount of energy stored.

Yes it is what I meant.

Now what ??

A Super cap is NOT a battery. It canno't give a sustained discharge at said current combined with a reliable output Voltage too . High current for a short period of time. Gives it's all and has nothing left after that.

Super Cap or not. A Cap and a Battery are totally different. Different applications....

Batteries are designed for long term discharge.
Caps are designed for short term discharge.

Easy hey

If ever I am fortunate enough to purchase an Electric car one day..and the options are:

1. 0 to 100 KPM within say 3 Seconds. Has to be a Super Cap powered car. Does the sprint and wins hands down. Please push after the race.
2. 0 to 60 KPH within say 100 Seconds. The Battery powered one. Loses the sprint and you gracefully drive the car home and park it...maybe charge it at the same time.

I know that there is huge money being spent by Manufacturers to try and replace conventional storage Batteries with Super Caps....I believe, this is a long way off.

Regards,
tvtech

also, capacitors store charge differently than batteries. batteries store energy as a result of a chemical reaction, capacitors store energy in the form of electrostatic charge. in a battery, the chemical reaction takes time to build up or release energy, and is dependent on the chemistry of the cell. capacitors store charge based on physical attributes of the capacitor (surface area of the plates, distance between the plates, physical properties of the dielectric, etc...), and so can be charged or discharged at rates limited only by the physical construction of the capacitor.

there are battery technologies that combine battery cells with capacitors in order to get the "best of both worlds", but these technologies are still very new, and the trick is to get the correct balance between battery and capacitor (and that balance is different for each application).

Thanks unclejed for helping here.

Regards,
tvtech

Hi,

Unclejed, that's a very good point. I would vote that one of the best points made if we had a voting system for that here


As to a more direct comparison for the battery vs capacitor, we can begin with calculating the maximum energy that can be stored in an air capacitor and then compare that to alkaline cells. I took the liberty of doing most of this already so i'll show mostly the results here and the facts behind the calculation. Also note that this is still an approximation because there are various assumptions such as air pressure and temperature and moisture content, to name but a few.

Knowing that the capacitor will arc over if the voltage is too high versus the distance between plates, the maximum energy storage in air is about 17.7 Joules per cubic meter. That means that a capacitor that takes up 1 cubic meter with air as dielectric can only store at best 17.7 Joules.

Now if we look at one alkaline cell, it's pretty small, taking up a volume of about 1/120000 of a cubic meter. Yet it holds approximately 10000 Joules (or more) of energy when brand new.

So for the air capacitor, we have a ratio of at least 500 to 1, and that's even before we look at the entire volume!
Noting that we can fit about 120000 AA alkaline batteries into a space of 1 cubic meter, that means we can store:
1200 million Joules of energy in 1 cubic meter if we use alkaline cells instead of air.

Now to be fair, we have to realize that most capacitors used today have a dielectric. And that dielectric boosts the energy stored in a given volume because it is denser, and also helps to prevent the plates from arcing over. The very fact that it can store more energy per unit volume means a lot, but the added benefit means we can charge the capacitor to an even higher voltage within the same volume. So for a mica capacitor (dielectric constant around 5) we end up with a storage ability of about 2200 Joules per cubic meter.

So for alkaline we have 1,200,000,000 Joules per cubic meter,
for air we have about 17.7 Joules per cubic meter,
and for mica we have about 2200 Joules per cubic meter.

Note even with the mica we are still way behind the alkaline as the alkaline holds more than 500000 times more than mica, and about 68 million times more energy than air.

I am reviving this old thread for a good reason.
I have recently come across a 40,000Farad 2.2V super cap and have seen youtube videos of a guy using just 7 of these in series to start a small truck.

I am thinking of using theses newer super caps in a small 1kva single phase UPS that only needs to be run for less than one minute but the UPS is in
a area that the temperatures swings from 110f in the summer to under 35F in the winter and normal AGM lead acid batteries fail after one season.
where as super caps wouldn't have such problems.

I know that these caps are up to 5 times as expensive as a battery but will last many years longer than a standard battery and that will pay for the extra
cost alone.
and these caps are less expensive than other technologies like flywheels that need almost constant PM schedule where these caps would not.

What are your thoughts on this application.

If your AGMs fail after one year, your UPS is overcharging them. Have you ever measured the voltage across the AGMs?

Most of the UPS I have taken apart (after the batteries are have been dried out and wont hold a charge) were designed by idiots.

Mike, sorry that is not the issue the float voltage is perfect but the operating temps are killing the batteries. optimal temps are from 68 to 77F and the temps on the 2000 foot level of a tv tower range from 32F to 110F that is the problem.

BTW I work for GE as a UPS service engineer and have a lot of experience with these issues with customers that don't read the operating temps and that these systems are designed to be operated in controlled environments.
and they wonder why they have battery failures.

KG4MXV said:

the UPS is in
a area that the temperatures swings from 110f in the summer to under 35F in the winter and normal AGM lead acid batteries fail after one season.

Click to expand...

I've just built a micro solar power supply with a solar panel and Cyclone AGM batteries. There batteries are rated from -55 to +80C. It's only been a week or so, but so far they worked well at -35C. The charger has a temperature compensation, so the charge voltage is much higher when it's cold, but it'll be much lower when it's hot. If you don't have temperature compensation, batteries will be undercharged when cold and overcharged when hot.

I maintained mountain-top repeater sites in the Intermountain West, and have dealt with battery issues there and elsewhere all my life. My homebuilder buddies are running AGMs mounted in the 140deg F engine compartment of their aircraft, and they routinely get more than five years.

110degF is harder on lead-acid batteries (sealed or flooded) than 32degF temps, but they should still last five or more years. I still think that the charging profile is wrong if you are only getting a year.

Hi,

Without more information another idea i can throw out there is that the original lead acid battery (or batteries) do not have the required ratings for operating the load properly. If the load requires higher current than the battery should be putting out then that could limit it's life right there, and if it does not have enough storage capacity then it may be being discharged too low each run even though it only has to run for a minute or so.

So another idea, again without more information on hand about this application, is that the original batteries are simply under rated for the job, which means that it is possible that a better grade battery or a higher capacity battery would last a much longer time before needing replacement again.

If you have a link to the actual batteries you are using now we can take a look at the spec's and render a better answer. If you have a link to the super caps you are thinking of purchasing we can take a look at those spec's too.

Thanks guys i'll look into it, cheers