how long does an 8 farad cap lasts?

How long does an 8 farad cap lasts on charging a battery on 2500 rms of power ?...The setup is in my room so thats why i'm using battery and cap...

Are you asking how much extra playing time the cap is going to give you?

well that too, but just wanna know the life time of the current in the cap when under a frequent useage

I still do not understand your question, but the cap, without the battery connected will run your amp for less than a couple seconds.

daniel gt1 wrote: How long does an 8 farad cap last....

Over 1,000 times less than a $35 NP7-12 battery (AGM 12V 7AH).

oldspark wrote: daniel gt1 wrote: How long does an 8 farad cap last.... Over 1,000 times less than a $35 NP7-12 battery (AGM 12V 7AH).

That may be true, but the instantaneous current that the cap can VERY safely supply would freakin' melt to a pool of plastic and lead the 12V7AH battery... You've hawked this battery now at least twice as the savior of voltage drops and sags, and it seems as though you believe that it will do much more thna a cap will... You're no more correct in your assumptions than anybody else believing that a cap is their solution. Here's why: The internal resistance for the AGM 12V 7AH battery you are COMPLETELY and (as far as I am concerned) inaccurately in love with is around .025 ohms, allowing for a MAXIMUM discharge current for that battery of 40A for 5 seconds. Ohm's law tells us that equal voltage sources wired in parallel will provide equal current to the load. If your battery can only provide 40A, and the primary battery will only be able to provide the same thing, then your secondary battery is as worthless as is ANY cap. An 80A drain? That's about an 800 watt Class D amplifier. Beyond that, you will be pulling more than you AGM 12V 7AH battery can safely supply, deeply discharging it, and providing an additional load for the alternator to charge - even more than a cap! Pulling that much from that battery and having an alternator under the hood (a current source of in most cases a MINIMUM of 65 to 80A) you will not be able to limit the charging current effectively to 2.1A (INTITAL, not a continuous current by ANY means) A decent 8F cap will have an internal resistance of somewhere around .002 ohms, about 10 times lower, for an order of magnitude better peak current capacity in the same time frame, i.e. four HUNDRED amps for the same 5 seconds! Now, caps simply don't work in time frames that long. They don't. They want to dump ALL of the current as fast as possible. The internal resiatance is the limiting factor! 8F times 14.4V = 115.2J of energy. 115.2J /.002 ohms internal resistance = 57,600A! Peak instantaneous current Let's say it dumps in .5 seconds. This is four THOUSAND amps! .050 seconds? Fourty thousand amps! Now, granted, the 40kA math is far fetched, but it demonstrates to you the dramatic currrent superiority of caps over AGM batteries. NEITHER will do what anybody thinks they do, in the way they are pitched!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."

I know, and I apologize. I'm having a fundamental go at a very basic level. Do you happen to know the input (power) impedance of the amplifier involved? PS - Granted, for a 200A as load described, I'd use a larger battery. My point was merely how long the battery would last compared to a capacitor. How long would the 8F last? [Noting the energy stored at 14.4V is ~830J not 115.2J (even though only ~570J of that can be used if discharged to 8V), but what is the energy in the NP7-12 from 12.7 to 30% discharge. That is my point. I wasn't considering transient response in this thread. Since you brought the issue up, I'd love to see a transient analysis - hence why I asked above for the amp's PSU impedence. (S-domain?)

I have never seen an amplifier list the spec for "Power input resistance"

And hence the plot thickens.... I wonder why not? Can someone calculate or measure it? FYI Mr I - I updated my reply during/after your reply....

ok guys..here's the question in a clearer format " how long...does the current in the 8 farad cap lasts , when charging the battery\or being used for powering the amp ?"