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Do I Really need a CAP ???
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<blockquote data-quote="helotaxi" data-source="post: 3699875" data-attributes="member: 550915"><p>I will get all tech on you then. It seems you still have the old mindset that was brought about by the old school idea of what a cap does. It was a decent idea it's just there just wasn't any real physics behind it.</p><p></p><p>What does a cap do? It stores power. Nothing more. Unlike a battery that stores it in chemical form, the cap stores it as a differential charge between to conductor plates. In theory, this should be easy to tap. In reality, the effective series resistance (ESR) of the cap keeps this power from coming out freely. The cap only charges to the highest voltage in the system. If the car is off, the cap is totally useless since the battery is the highest voltage and stores much more power than the cap and will maintain voltage better than a cap ever could.</p><p></p><p>With the car running, the cap charges to the voltage of the alt. Let's give the alt some credit and say it runs at 14.4v. Your battery sits at 12.6v with the car off but it also charges to the voltage of the alt with the car running, and until you put some load on it and draw off the skin charge, it will discharge at that voltage, too. SO knowing all that, where does the cap do anything? In theory, it would begin to discharge as soon as the alt capacity was exceeded and would discharge down to the voltage of the battery. How much power could it provide in that gap? I showed the math in a previous thread and the basic result was the theoretical usable power stored in a cap between 14.4v and 12.6v was on the order of 0.5 amp-seconds/Farad. So if you asked 10A of a 1F cap it would give you power for a whole 0.05 seconds before the battery took over. The whole time that it is giving that power the voltage of the system is dropping as well.</p><p></p><p>Now let's toss in the reality of that ESR mentioned above. I don't have the article in front of me, but Richard Clark (the guy who put the first cap bank in a car remember) ran some numbers with a low ESR cap that basically showed that once you get above a certain amp draw on the cap (I don't remember what the number was, but it was very low) the cap can't provide any power because the difference between the alt voltage and the battery voltage was lost to the ESR of the cap. Now you go from what is, in theory, very little help to what is, in reality, zero help.</p><p></p><p>So why did Richard Clark put those caps in the car? Simple answer: noise filter. The whole point of that capacitor bank is in the Grand National was to totally filter out the alt ripple from the amp power supply. Nothing more. As a noise filter, a cap works great, it doesn't need to really provide any current to speak of, just do some very minor voltage stabilization. The marketing departments took the "voltage stabilization" statement and turned it into a catchphrase and then extrapolated it to mean helping the charging system. Something that it just can't do.</p><p></p><p>Better than know-nothings though don't you think?</p></blockquote><p></p>
[QUOTE="helotaxi, post: 3699875, member: 550915"] I will get all tech on you then. It seems you still have the old mindset that was brought about by the old school idea of what a cap does. It was a decent idea it's just there just wasn't any real physics behind it. What does a cap do? It stores power. Nothing more. Unlike a battery that stores it in chemical form, the cap stores it as a differential charge between to conductor plates. In theory, this should be easy to tap. In reality, the effective series resistance (ESR) of the cap keeps this power from coming out freely. The cap only charges to the highest voltage in the system. If the car is off, the cap is totally useless since the battery is the highest voltage and stores much more power than the cap and will maintain voltage better than a cap ever could. With the car running, the cap charges to the voltage of the alt. Let's give the alt some credit and say it runs at 14.4v. Your battery sits at 12.6v with the car off but it also charges to the voltage of the alt with the car running, and until you put some load on it and draw off the skin charge, it will discharge at that voltage, too. SO knowing all that, where does the cap do anything? In theory, it would begin to discharge as soon as the alt capacity was exceeded and would discharge down to the voltage of the battery. How much power could it provide in that gap? I showed the math in a previous thread and the basic result was the theoretical usable power stored in a cap between 14.4v and 12.6v was on the order of 0.5 amp-seconds/Farad. So if you asked 10A of a 1F cap it would give you power for a whole 0.05 seconds before the battery took over. The whole time that it is giving that power the voltage of the system is dropping as well. Now let's toss in the reality of that ESR mentioned above. I don't have the article in front of me, but Richard Clark (the guy who put the first cap bank in a car remember) ran some numbers with a low ESR cap that basically showed that once you get above a certain amp draw on the cap (I don't remember what the number was, but it was very low) the cap can't provide any power because the difference between the alt voltage and the battery voltage was lost to the ESR of the cap. Now you go from what is, in theory, very little help to what is, in reality, zero help. So why did Richard Clark put those caps in the car? Simple answer: noise filter. The whole point of that capacitor bank is in the Grand National was to totally filter out the alt ripple from the amp power supply. Nothing more. As a noise filter, a cap works great, it doesn't need to really provide any current to speak of, just do some very minor voltage stabilization. The marketing departments took the "voltage stabilization" statement and turned it into a catchphrase and then extrapolated it to mean helping the charging system. Something that it just can't do. Better than know-nothings though don't you think? [/QUOTE]
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