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active| newest | oldestI see all of these comments that seem to be people having potshots at the intelligence of the other people making comments... Not only that, but seem to be trying to inflate their own ego...

So my question, what is the reason for not being able to connect a cap to a power meter, measure from the maximum voltage of your circuit to the minimum voltage of your circuit, and see what the rated mAh is... Seems so dang simple to me, but perhaps I am overlooking something...

Well first of all, what is a power meter? A multimeter can measure voltage and current.. Secondly, the power at the beginning is not maintained.

You could connect your charged capacitor to a small load say (10K ohm) and measure both the current and voltage periodically, until the capacitor is drained to 0V. Then you could work out the power it delivers at each point in time by multiplying voltage and current (P=IV). But what we see is that power decreases over the duration of the experiment. You see, the voltage falls as the capacitor loses charge, so the current and power also fall.

My power meter is able to display/log current, voltage, Ah, Wh and surely anything else needed with some basic conversion/math... As such, by connecting the Capacitor to a load, surely I could obtain the mAh, along with any other applicable metrics...

On top of that, all portable power sources (that is to say, batteries) that I am aware of have a voltage that varies with their capacity, or rather, their state of charge, so why is measuring the capacity of these in any way different from measuring a Capacitor? Again, unless there is some significant difference that I am completely unaware of, I don't see the problem with finding the mAh of any Capacitor, and as mentioned, all you would need to do is run the Capacitor from the nominal voltage, down to the minimum voltage of your circuit, that way you are not measuring the energy storage that is of no use to your circuitry.

Again, I am not being sarcastic, or rude, or anything of the sort, I am just sharing information as I understand it, and given my, admittedly, relatively limited (by comparison to an engineer or such) knowledge, I do fully accept that there may be something that completely negates everything that I have just stated... :-)

Thank you for the communications, and be well!

~R

Yeah this is a doozy of question, but I think it is the result of a slight misunderstanding: basically, although the farad and mAh units both describe capacitance, you can’t charge a capacitor with 1 Farad, but you can charge it with 1mAh! Also, capacitors and batteries behave differently. Read on if you are curious for more.

mAhours are the number of milliamperes that can be sustained for 1 hour, and this unit tends to be used to describe batteries, which have a fairly flat discharge curve, meaning they don’t lose much voltage until they are exhausted.

Farads describe capacitors, which increase their voltage linearly as you charge them up and also decrease their voltage linearly as they release charge (assuming constant amperage!). 1 Farad describes a capacitor that gets to 1V when a constant charge flow of 1amp is pushed into it for a period of 1 second. If you did it for 2 seconds you would have stored 2 ampSeconds of charge but the voltage would be at 2V - and you could keep pushing AmpSeconds in until you hit the capacitor’s maximum voltage rating (at which point the dielectric would break down). As you discharge at 1 amp, each second results in a drop of 1V (for a 1F capacitor).

Now, if your capacitor is losing voltage, it’s also losing power rapidly (P=V^2/R) and current, so it’s hard to compare capacitors with batteries when supplying circuits with power.

Unless you have some electronics to regulate the voltage, you won’t be able to supply your milliamps at a constant rate with a capacitor. If you do use a regulator, you will be fine, except you are wasting a lot of power at the beginning of the discharge (Imagine a capacitor charged to 10V with a 3V regulator: that’s initially 7V across your regulator and 70% of your power wasted on the regulator). This probably needs to be taken into consideration if you are thinking of replacing a battery with a cap.

I’ll try to use water as an analogy: “1 Farad = how many mAh?” is a bit like saying “if I have a tank that fills to a height of 1m when I put in 1L of water, how many mL per second will I be able to leak and maintain this for exactly 1 hour. (You can answer it, but the hole in the bottom of the tank actually leaks more if the height of the water [voltage] is higher and leaks less as the tank gets empty, complicating things)

I’m sure what you want to know is “how big a capacitor do I need to replace a certain battery?” Well that depends on a lot of things: the circuit, the type of regulator, the voltage you are charging your capacitor to (limited by its rating). I will say that in practice, it is usually surprising how big a capacitor you need for even the smallest applications.

Try this simulation and let it run for 2 min then experiment by changing the values. https://circuits.io/circuits/5256559-capacitor-supplying-regulator-with-load

In my opinion, different battery with the same mAh (coulomb) have different farad(capacitance) assuming chemical reaction charge/discharge rate is involved or not. It also depends on both + - plates and their surrounding medium inside the battery. There are a lot factors involved so there is no simple answer to that but it's interesting to figure it out.

The answer is very simple...

A

1-farad(1,000 mF or 1,000,000uF) capacitor can storeonecoulomb (coo-lomb) of charge at1volt. A coulomb is 6.25e18 (6.25 * 10^18, or 6.25 billion billion) electrons.Oneamp (1,000 mA) represents a rate of electron flow of1coulomb of electrons per second, so a1-faradcapacitor can hold1amp-second of electrons at1volt.So yes. you need very very high capacity if you want to replace a battery, search for Supercapacitors.

But you will also need a voltage regulator.

I think what the questioner is really asking here, (and this is something I have pondered myself). Not: does mAh have an equivalent value in farads, which, if I dare be so bold to venture, I would say yes, I think an estimate could be calculated. Surely, mAh represents the total amount of power in any given battery, (or cell). Isn't the Farad scale a similar kind of measure? Cannot both be converted into Joules, and then a direct comparison made, or am I missing something more fundamental?

Laughing out loud, basically what you need is a completely separate circuit that would include voltage regulation, and the required energy storage... simply said, if you need a specific voltage and draw, and you need it to last for a specific time length (i.e. 3.7v 2000mAh), you would have to design a special circuit. A capacitor will not function as a direct replacement for a battery because the voltage changes too much from empty to full (i.e. 0v-2.4v 10F).

thx for the help

If I need a 20AH at 36 Volts, how large capasitor do I need to replace?I need to be more precise:

* theoretical battery of 1000mAh and 1,2V: at the beginning has 1,2V at output, and keeps that 1,2V at output for as long as you do not reach 1000mAh (for instance discharging it with 1000mA in 1 hour, or 1mA in 1000 hours). After that it immediately dies and the output is 0V.

* theoretical condenser starts with 2V and immediately after the current starts to flow out the voltage drops linearly and reaches 0V after whole charge had been drained.

In practice: the battery is not ideal, the voltage starts at 1,4V or similiar, drops a bit during lifetime of the battery, also the circuit is never ideal and usually how many mA it drains depends on input voltage. Also the number of mAh the battery has depends on the current drained out. Usually batteries have more mAh when you drain small current and less mAh when you drain higher current .... up to some point when tendency reverses and with very small currents the mAh of the battery will be lower than rated.

So the real question is: how large capacitor do I need so that my circuit works the same time as it would when having 1000mAh battery? This makes perfectly sense and my equation gives quite good approximation for small currents.

The 0,2777 coefficient is taken directly from the capacitor's current to voltage ratio:

C=Q/V. See the 'capacitor' in Wikipedia - all equations are there.

Remember: V drops with time (condenser discharges)

In electronics there is nothing like ideal theoretical circuit and some shortcuts in thinking are perfectly justifieble and ...

...long story short: the above question makes sense.

There are many theoretically oriented people who want to be more exact than necessary. The person asking wants simply to replace the battery with charged capacitor. The answer is:

1F=0,2777mAh/V

so: if you have 1,2V battery with 0,27mAh capacity, than you can replace it with the capacitor of 1F charged to let's say 2V. Your circuit should cope with it. After consuming 0,27mAh from the capacitor it will get discharged down to 1V which should be also ok for your circuit.

So:

* check the minimum and maximum voltage for your circuit. Take the difference between those two,

* multiply it by 0,2777 per each needed mAh

and you will get needed capacity in F.

And remember to start with fully charged capacitor up to the maximum possible voltage.

It would be nice to know how you calculated the 0.2777 constant. :)

sorry one correction:

*check the minimum and maximum voltage, take the difference

* divde 0,277 (not multiply) by the min/max voltage difference

you will get number of F.

Hi,

I'm trying to replace a 2.4v 500mah nicd battery with an equivalent or higher energy dense capacitor. What rating of voltage and capacitance in farad do it need?

Thanks

How many kilderkins in a MeV?

So why complicate everything so much. Just get a Capacitor of said Fards conected to a synchronous boost converter pull needed voltage from device and see how long it lasts.

if you use a resister to lower the voltage out of the capacitor will it have a higher f or amp hour?

2 Kilderkins = 1 Barrel

1 1/2 Barrels = 1 Hogshead

If we say that we do have perfect electronics, 100% efficient, then since Q=CV , C=Q/V and we have 2.6 C/ 1.2 V = 2F

Steve

You might ask the question, "How many donkeys are equal to one school bus... approximately?", and then wonder why no one knows what the monkey you're talking about.

There is some thing unstated, some thing donkeys and school buses have in common, some thing which will make the question make sense. Is it that donkeys and school buses are both heavy? Is it that donkeys and school buses can both be used to transport people? Knowing the quality which is common to both donkeys and school busses, the thing

that you care about, is important.Regarding milliampere*hours and farads, I suspect the thing you are interested in is energy storage.

For this you should know batteries and capacitors store energy in somewhat different ways. For a battery, the energy stored is approximately its voltage multiplied by its current*time capacity, e.g. volts times ampere*hours.

http://en.wikipedia.org/wiki/Battery_%28electricity%29#Battery_capacity_and_discharging

For a capacitor, stored energy is equal to one half the square of the voltage multiplied by the capacitance of the capacitor, in farads.

http://en.wikipedia.org/wiki/Capacitor#Energy_storage

Anyway you should meditate on these facts and formulas, and then come back when you've got a question that makes sense.

If you say, that the charged capacitor C, with voltage V has a charge on it, Q=CV, you know how many Couloumbs of charge are stored. Now, if 1 Couloub flows past a point in a second, we say a current of 1A is flowing. Likewise 0.001 C/S = 1mA.

Steve

If we say that we do have perfect electronics, 100% efficient, then since Q=CV , C=Q/V and we have 2.6 C/ 1.2 V = 2F

Steve

charge(coulombs). Farads are a unit ofcapacitance, which is charge per voltage. Get it right.L