Big cap in parallel with SLA battery supply?

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I'm actually looking in the same direction. The EE store on campus sells parts at low prices and I'm going to wonder in there tomorrow. The online site says they have 10,000uF 16V caps for $2.80. Given I there aren't too many good electronics stores imeadiately nearby I was thinking I'd give them a shot. First though I'd like to know what sort of ESR I should be shooting for.

(oh yeah, I've gotten some lower valued caps there before, and they were Fu Lee brand.)
I ran into the EE store on campus, turns out the things are only $0.70 for the 10,000uF 16V electrolytics so I snagged one. It's labeled "SC" with a a little lightning bolt logo. The guy working the counter today had no clue what brand it was, or what any of the specs are, but for $0.70 I can take a chance.
ohenry said:

I was wondering the same thing regarding ESR. Is 0.02 ohms for a 20k uF cap a reasonably good value?

battery have a high ESR
any cap will do the job , but it's better to have low ESR in the audio bandwidth , you can use especially caps for audio ( Elna Silmic or Cerafine , Blackgate , Nichicon Muse ... which have vibration-softening electrolyte and foils )

Dr. Photon said:
If you want a low ESR, you might want to consider paralleling several caps, or several smaller caps. You might also consider a small film or ceramic capacitor in parallel to maintain a low ESR at high frequencies.

Thanks for the advice. I'm still a bit confused regarding the "low ESR at higher frequencies" statement. This isn't a signal cap application, but a power supply cap. I saw graphs depicting ESR slopes across frequencies, but wonder if that's a concern in this application. BTW, the cap I have is 0.02 ohms at 120Hz if that matters.

I hope you guys don't mind schooling me (old dog learning new tricks:)).

Thanks again...
ohenry said:

I'm still a bit confused regarding the "low ESR at higher frequencies" statement.
You're right to be confused here. ESR is frequency independent. Impedance isn't.

Simplifying slightly, an electrolytic cap can be modelled as a series circuit of an ideal capacitor, a resistance and an inductance. The resistance is partly in the electrolyte and partly in the aluminium foil. The capacitance is formed on one side by electrolyte, on the other side by aluminium and a thin layer of aluminium oxide serving as the dielectric.
The inductance is largely determined by the size of the cap, and corresponds roughly to a U shaped wire loop with the legs as far apart as the leads and about half as high as the can.

This shows that "low ESR ar higher frequencies" is not quite correct. You can have low impedance at higher frequencies but this is the inductance which is at play. The ESR is dominant at intermediate frequencies only.

Both for ESR and inductance, a parallel combination of many smaller caps is better than one large one.

"Audiophile" caps have a lower metal resistance compared to the electrolyte (=thicker aluminium foil) and have better damping of mechanical resonances. And of course a nice story.
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Battery ESR is a useful measurement of battery charge. Some advanced charge management systems use the ESR to control the battery charge.

The Car Audio guys add 1F and even 10F caps to "help the bass impact", But, in many cases, it really only helps the dealer's bottom line.

At high frequencies the cable inductance is liable to dominate. It would be interesting to do some real measurements of battery impedance vs. frequency. If I can scrounge an SLA I will make a stab at it.

I have done some network analyzer tests on a battery - given that the information is hidden in a loong thread, I would like to point at it
post399 again. The test setup description starts at post 390, the general meas. setup is described in post 153.
The values from the graph are deducted in post 402.

Ciao, george
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