response to a pm, what are caps good for?

I know people always ask "What are caps good for?". I thought this was well worded, even if I do say so myself, and I thought others might benefit from my response. A member of the forum here (who didn't want to ask publicly) asked me when I would use a cap in this qustion:

anonymous forum member wrote: I did not want to post this up on the public board. I know they do have a use when you have done the big 3. I just wanted to know when you would go add one to a system.

And I responded to him with this:

haemphyst wrote: There ARE times that I would add a cap, but NOT for what you have been told they are needed for. I add one to stabilize the voltage that my amplifiers are seeing... I know that's what you've been told they are for, but I do NOT beat my system hard like that. To stabilize the input voltage to the power supply CAN help to lower the THD and improve the damping on the outputs, by virtue of keeping the internal voltages more stable. I have never tested this, mind you, but with my background, I can see it as being the truth. I also do not use caps on JUST the "bass amp". Although the filtered current "shares" through the entire electrical system of the car, the most benefit is seen by connecting the device most wanted to see the benefit as close as possible to the filter.... And what is the filter? THE CAP! Ohm's law says the voltage will be distributed and maintained through all connections in a parallel circuit, but the resistance of extra wire will have some effect on that. This means that the further the device is from the filter, the less that device will benefit from that filter being present, due to the additional resistances in the wire. My cap's connections in my particular system ARE the distribution block, and ALL of my amplifiers (three PDX amps, for an "advertised" 2200 watts, but more like 2600 "birthsheeted" watts) "benefit" from the caps being present - I have two 1F caps in my trunk. WAY overkill, really, as I probably use 200 to 300 watts of my entire system when I'm really "gettin' jiggy wid it". (This is estimated, by the way, based on the amount of current I can see my alternator making. I have a digital current meter in my dash, and I rarely see even peaks of 30 to 35A even on heavy songs...) So you see, in theory (my theory, anyway) they CAN be an appropriate addition, but they can NOT provide magic power. If your supply (in the car, that's your alternator) is too small, you cannot add a cap and expect it to provide you suddenly with all the power you need to run your system correctly. That's NOT the way a cap works, but it IS the way caps are marketed. They DO filter noise, and they DO stabilize small voltage fluctuations and current drops, which are both good things to have, but they do not and can not "stiffen" a power supply. The CONTINUOUS AVERAGE power consumption of ALL your car's accessories MUST be below the continuous current capability of the power source. It just makes sense when you read it like that, right?

So... That's haemphyst's answer to "The Cap Q". Hope this helps.It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

This should be a sticky, as it is very helpful when one wants to add a cap to their system.

Great job writing this haemphyst,very insightful and easy to understand.I too vote for this to be a sticky.-A vision without a plan is just a hallucination-

the only thing missing (I think) is that you should USE AN H/O ALT WITH A BATT THEN TO CAPS. The 2nd batt would then keep the cap charged the H/O Alt would keep the 2nd batt charged.