Discussion about capacitors, and charging systems

[ANeonRider 3]

Thought I would post this little bit of theory that I discussed with Mattdog from Neons.org tonight. I would like to see if anyone has anything to add/discuss further?

I understand the theory about the cap needing to be charged when the voltage is already low... I assume with a decent wire gauge the resistance of the wire between the battery and the amp is also probably less than the caps esr also making it useless. However is the actual esr of the cap lower than the battery?* If it were, would it actually be benificial to have a cap up front or is it just seen as a load on the system?

*I don't know the esr numbers on the cap but if you take a 12V battery that can dump say 600amps thats 0.02ohms of internal resistance right. I guess caps are useless

Typical 4ga stranded copper wire has approximately 0.000292/ft of resistance, so for a typical 20ft run of wire to the trunk, that equals 0.00564 ohms. Pretty much negilible, the fuse will have more resistance .

A battery like the Optima Yellow Top runs approximately 0.0028 ohms of resistance. Battery resistance changes with temperature, but in the case of a gel-cell deep cycle, that battery temperature will be more stable than the typical lead-acid during use. Deep cycle batteries also have less resistance than the common lead-acid, but even the lead-acid batteries out there have less than 0.01 ohms. Resistance will also increase with age, and how well they were maintained/kept charged. All variables, but again, pretty much neglible unless the battery is very old and was allowed to be discharged regularly.

A battery will discharge at a very fast rate, and for a reasonable amount of time, for example, the Yellow Top will discharge 750A for 30 seconds at 32F, with no charging available, before dropping in voltage below 7.2V. Yikes huh.

Now, the esr of a good cap, such as the original stiffening capacitor will have a resistance as low as 0.0001 ohms @ 120Hz (typical power supply hum). But, there are some caps out there running up to even 0.1 ohms @ 120Hz. With a capacitor like that in the electrical path, it is a considerable amount of resistance in comparison to everything else in the path.

In cases where there is a slight hum in the voltage path, a capacitor such as one with a very low ESR and ESL will provide the filtering needed to remove the hum in the line, but, in alot of cases, this isn't needed. Only way to know is to monitor the dc level. Problem with determining which capacitor to use though, is that next to nobody will release specs on their capacitor's ESR & ESL values. :roll:

I have also read some individuals that state that batteries do not provide current as fast as a capacitor does. I do not know where the idea that a battery is slow to respond to instanteous current demand comes from, nor why people say that a capacitor will provide current faster than a battery will. They will provide current no fast than the other, at the speed of electron flow. Sounds alot like some people talking about skin effect on dc lines

A capacitor is as simple as a filter, no more, no less.

From what I understand about the alternator is that it's like a 3phase regulator... and the output is pretty darn close to DC. This would make the cap even more useless...

All depends on how good the diode bridge is within the alternator, and how well the regulator controls the voltage. Some are better than others, personally, something like the Neon runs, where the PCM has an internal regulator is a better design, and less prone to electromagnetic noise than an external regulator on the alternator housing.

Edited November 15, 2005 by ANeonRider

[Supa_c 1]

Agreed.

So when i go to radio shack i should ask for " quick discharge" batteries rather than " normal" batteries....no

[TechSys 3]

I would never put a cap in a system. I wouldn't even let my worse enemy install one.. That's just me though....

I have to post this image

[ANeonRider 3]

Hmm, I thought there would be a little more interest