Filter Cap for a battery pack ?

I need to select a capacitor for a battery pack.  I see a lot of circuits that use Alkaline batteries and then have a ceramic capacitor across the leads of the series battery pack to act as a filter.  

My question is how do you size the cap ?  I hate just looking at what someone else has done and just guess or copy.

I have 4x 1.5v alkaline batteries in series powering a amplifier circuit.

Any help is appreciated.

Thanks

On Tuesday, December 14, 2021 at 8:30:07 AM UTC-8, sidw...@gmail.com wrote:
> I need to select a capacitor for a battery pack. I see a lot of circuits that use Alkaline batteries and then have a ceramic capacitor across the leads of the series battery pack to act as a filter. 
> 
> My question is how do you size the cap ? I hate just looking at what someone else has done and just guess or copy. 
> 
> I have 4x 1.5v alkaline batteries in series powering a amplifier circuit. 

If there's RF in the vicinity, battery wiring can be an antenna; that's all the capacitor is good for.
At frequencies in audio, battery wiring is much shorter than a transmission line, and those
batteries are very capacitive.   So, how RF-sensitive is the power rail?  And, is the battery wiring
paired, so it makes a transmission line, or unpaired, so it makes a loop?

On 15/12/21 3:30 am, Sid 03 wrote:
> I need to select a capacitor for a battery pack.  I see a lot of circuits that use Alkaline batteries and then have a ceramic capacitor across the leads of the series battery pack to act as a filter.
> 
> My question is how do you size the cap ?  I hate just looking at what someone else has done and just guess or copy.
> 
> I have 4x 1.5v alkaline batteries in series powering a amplifier circuit.


There is no need to use a capacitor across the battery.

Put capacitors across the power supply on the amplifier board, 
especially near the output devices.

CH

Thye Witkleds Fuckwit trolls again" whit3rd wrote:

============================================

> > I need to select a capacitor for a battery pack. 
> I see a lot of circuits that use Alkaline batteries and then
>  have a ceramic capacitor across the leads of the series battery 
> pack to act as a filter. 

**  Post one. 
>  
> > I have 4x 1.5v alkaline batteries in series powering a amplifier circuit.

**  Post it. 


> If there's RF in the vicinity, battery wiring can be an antenna; that's all the capacitor is good for. 

**100% wrong. 

> At frequencies in audio, battery wiring is much shorter than a transmission line, and those 
> batteries are very capacitive. 


** 110 % wrong. 

( ship even more vomitous drivel) 


.....  Phil 

On Tuesday, December 14, 2021 at 1:57:19 PM UTC-8, palli...@gmail.com wrote:
>  whit3rd wrote: 

> > At frequencies in audio, battery wiring is much shorter than a transmission line, and those 
> > batteries are very capacitive.

> ** 110 % wrong. 

Not so.

Learn some electronics, and tell me what the capacitance computes at, for a voltage source
like a battery.   I'm talking about  I = C * dV/dt when you pull current from a
battery, and dV/dt is close to zero (after all, batteries are sized, usually for ten hours or more of load).

On Tue, 14 Dec 2021 17:58:50 -0800 (PST), whit3rd <whit3rd@gmail.com>
wrote:

>On Tuesday, December 14, 2021 at 1:57:19 PM UTC-8, palli...@gmail.com wrote:
>>  whit3rd wrote: 
>
>> > At frequencies in audio, battery wiring is much shorter than a transmission line, and those 
>> > batteries are very capacitive.
>
>> ** 110 % wrong. 
>
>Not so.
>
>Learn some electronics, and tell me what the capacitance computes at, for a voltage source
>like a battery.   I'm talking about  I = C * dV/dt when you pull current from a
>battery, and dV/dt is close to zero (after all, batteries are sized, usually for ten hours or more of load).

It's easier to use an ESR (equivalent series resistance) meter.  For
example, the ESR of an Energizer E91 AA at 100KHz is about 0.15 ohms
at 25C:
<https://data.energizer.com/pdfs/batteryir.pdf>
That's as good or better than any common filter capacitor.  At RF
frequencies (>1MHz) the ESR (impedance) goes up mostly due to lead
inductance.  Instead of absurdly short battery wires, moving the
filter capacitor near the electronics works better.


-- 
Jeff Liebermann                 jeffl@cruzio.com
PO Box 272      http://www.LearnByDestroying.com
Ben Lomond CA 95005-0272
Skype: JeffLiebermann      AE6KS    831-336-2558