The correspondence of capacity between accumulators and capacitors

[Kirill V.]

Hello every one!
I have idea about power supply with accumulators battery instead capacitors after rectifier.
I'd like to know how correlate the capacity of the battery and the capacitor?
For example: battery of 1A*h replaces X Farads of capacitors
battery of 2A*h replaces XX Farads etc.
Thanks.

[soldar]

They are different things, they work differently and you can't just replace one with the other. You need a better understanding of how each of them works.

[xani]

Capacitors in power supply are used to smoothen/filter the voltage, not as long term storage devices. Major differences are much higher current capability, basically infinite cycles and no charging requirements aside from "dont overvoltage it", while on other side batteries need special charging circuit, straight up irreversably die if discharged to zero.

Also if you can't convert the battery capacity to capacitor capactity at given voltage you shouldn't probably be going around trying to replace one with another for no good reason.

[Kirill V.]

Thank you all.
My idea: rectifier - battery - voltage stabilizer - output. This is for low noise and ripple. Rectifier can be used for standby charge of battery and in this case battery able to smooth out ripples?
For ultimate performance AC part can be turned off and in this case battery used as storage

[MrAl]

Hello there,

There is a close equivalence between a battery and a capacitor but when we compare we must take into account the voltage drop.  Let me explain.

We will start with one of the definitions associated with a capacitor:
dv/dt=i/C

where
dv is the change in voltage over the time dt,
dt is the time period of the discharge in seconds.
i is the current, considered constant for this simple explanation,
C is the capacitance in Farads.

Ok lets see what we can do with this.
First lets solve
dv/dt=i/C
for C because we would like to know that.
Doing that, we get:
C=i*dt/dv

Now say we have a 12v battery that reads 12.5v when charged and 11.5v when completely discharged, and say it is a 1 ampere hour battery.
We now can plug in the values.
dv=1 because 12.5-11.5 equals 1 volt.
i=1 amp for the test to make it simple.
dt=1 hour because 1 amp for one hour gives us a 1 ampere hour 'battery' but the time must be in seconds so we use 3600 for one hour.
Plugging in the values into:
C=i*dt/dv
we do that:
C=1*3600/1=3600 (note there are 3600 seconds in one hour)
and so we get finally:
C=3600 Farads.

We can do another one.
This time we have a 20 ampere hour battery that is charged to 5v when charged and when discharged is 3v.
This means that now dv=5-3 which equals 2 volts.
We keep the current 'i' at 1 amp, and if we have a 20 ampere hour battery then it must take 20 hours to discharge at one amp so dt=20*3600=72000 seconds.
Plug in the values we know now:
C=i*dt/dv
C=1*72000/2
so:
C=72000/2=36000
so
C is 36000 Farads.

Note how high C has to be to emulate a decent size battery.

One thing we leave out is the factor that relates discharge current to capacity.  We assume above that is 1 although most real batteries show a decrease in capacity as the discharge current increases.  We also ignore the total charge of the capacitor for simplicity.

One more point is that if you look at good spice models for batteries you will see the main component is a large capacitor although there are other components too including a 2nd capacitor.

Try one see if you can calculate the capacitance.

[David Hess]

Batteries are sometimes used like that.  Capacity is selected based on expected operating life.

Capacitor values are selected based on allowable ripple voltage at a given current and for 120 Hz from a full wave rectifier, comes out to about 8200 microfarads-volt/amp.

[Circlotron]

Ripple current in lead acid batteries causes internal heating that can shorten battery life. http://www.aeroelectric.com/Reference_Docs/Battery/Effects%20of%20AC%20Ripple%20Current%20on%20VRLA%20Battery%20Life.pdf

[MrAl]

Super caps are sometimes used to power LED's too these days.  Things have changed since i first started.
So the super cap is compared to some type of small battery.

[Kirill V.]

Wow, thank for all!
Thanks, MrAl! In fact, you expressed my own thoughts.
In my opinion, this idea is better for battery

When the battery is not fully charged its internal resistance is very low and in this case it can smooth the voltage ripple as if "closing" them to the ground
I am correct?

[MrAl]

Yes that sounds right but you'd have to double check the internal resistance to make sure it is low enough.

[digsys]

What you're describing is a common DC UPS, I've designed dozens of them over the years, mostly 24VDC. Yes, they will do the job you ask BUT you need to consider a couple things. Depending on the chemistry, they do NOT filter out HF ripple very well, it'll just pass straight through, so you'll still need the usual HF / MF decoupling caps at the battery, and ideally a common mode choke / filter after the battery and before the regulator / load. IF you are drawing high surge currents, then a large low ESR cap is STILL recommended. Different regulator types also vary on how well they attenuate HF noise etc, and if you need more filtering after the regulator.
What is you load / current profile ? and what are you trying to filter out ?

[Kirill V.]

What you're describing is a common DC UPS

You're almost right but not quite. General purpose it is reducing of ripples with frequences is multiple to power grid absolutely
Load will be low current only, such as op-amps or similar. This is laboratory power supply
I understood general thing: I should add quality charger with caps as filter for long battery life. For excellent performance this part should be turned-off.
Regulator after battery it is precision op-amp with power amplifier. Not LMxxx or similar ICs.

[edavid]

It's been tried many times, and it's a bad idea.  Batteries are expensive, they wear out, and they leak and damage the equipment.  There's no filtering you can do with them that you can't do better and cheaper with normal methods.

[digsys]

You're almost right but not quite ....

If it has any sort of direct connected battery ie not a RTC type, then it must have battery charging / monitoring of some sort, or as others say, you can get in trouble.
IF it continues to supply load when the Input has drop-outs / or fails, even for a short time, then it is technically a DC-UPS. Irrespective of what your main aim is after that. Just clarifying nomenclature :-)
Now, given the conditions you supplied, a series of caps - LF, low ESR, HF, bulk energy etc is FAR more practical. Batteries are really not good filters as such.

[Kirill V.]

Ok, If I will make this device now, I will use battery as source only, with charge-discharge cycle.
I lean toward capacitors or supercapacitors now.
My second idea:

I can use one cap with large capacity or many caps in parallel. What can you say about this?

[digsys]

ok going back to your original requirements -

For ultimate performance AC part can be turned off and in this case battery used as storage

Set your first regulator to 13.8V (trickle charge) and direct on to a lead-acid type battery. It can be SLA, AGM etc whatever you like. Lead-acid don't need fancy charge / energy profiles. They can stay on trickle charge voltage for ever. So that part is fixed

rectifier - battery - voltage stabilizer - output. This is for low noise and ripple. Rectifier can be used for standby charge of battery and in this case battery able to smooth out ripples?

As I mentioned, batteries are no good for HF type ripple, so you'll need HF / low ESR type capacitors after the battery to fix that. By using the battery as well, you won't need huge bulk energy caps for storage, just filtering. Several smaller / low ESR are best for your application, as well as the usual types to cover your requirements (which we don't really know :-) )

[Kirill V.]

Thanks, digsys!

(which we don't really know :-) )

The brand-name power supplies have noise no more than several millivolts. But high quality supplies have noise in microvolt range. I want to get as close as possible to this result. It's very interesting technical problem.
I want use battery of CSB brand. What can you say about this product?

[digsys]

.. I want use battery of CSB brand. What can you say about this product?

Looks like a nice series .. I may check these out myself :-)
One option I didn't mention - IF HF type ripple / noise is important, have you looked at linear P/Supplies? These are often specified in critical applications, where S/Mode is not allowed or dangerous. I have been designing these for years for mines / hospitals / intrinsic areas. Literally zero EMI / noise. Just some 50Hz ripple, which is easy to fix.
The only possible downsides are size / weight and heat. The ones I design have far better efficiencies than most S/Modes, so the heat part can be fixed.

[Kirill V.]

My supply will be linear only. I sayed already that:

Regulator after battery it is precision op-amp with power amplifier. Not LMxxx or similar ICs.

Simplified diagram:

I choose electrolytic capacitors for bypassing. I can buy this:
https://www.tdk-electronics.tdk.com/inf/20/30/db/aec/B41858.pdf
Or this:
https://www.tdk-electronics.tdk.com/inf/20/30/db/aec/B41859.pdf
I found some types: high ripple current, low impedance, very low impedance, low ESR,
In my opinion, for my application low/ultralow impedance or ESR is more suitable, I am correct?

[digsys]

My supply will be linear only. I sayed already that ...

Yes, but some people use a S/Mode DC Input (from mains), at a slightly higher voltage to derive a stable fixed DC source. That was the part that wasn't clear. IF you are using a transformer, then is it way less of an issue - correct.

  ... Or this:
https://www.tdk-electronics.tdk.com/inf/20/30/db/aec/B41859.pdf
In my opinion, for my application low/ultralow impedance or ESR is more suitable, I am correct?

Yep, those are nice. You will STILL need to add - say 0.1uF, 1000pF, 100pF HF caps as well, and maybe even a common mode / choke. That will depend on exactly how clean you are looking for. No one can advise you on that - When I come across that situation, and I did last month, I left room on the PCB for various combinations of filtering etc, then tested the results in the "real world". Theory is fine, but real-world always like to mess with you. Even happens to me, after 40 yrs of the stuff :-)