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havro, what is the fuse rating on your amp? judging what size cap or battery is needed to optimally power a system depends more on the current draw than the stated watts output of the amp. most amps are overrated these days and if you go by what the watts say, you're overestimating the actual current draw on your system.
 

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my amp is not overrated....
when u buy cheap bestbuy/circuitcity/fleamarket amps are overrated!
im pushing 750 watts at 2 ohms

i really dont understand this FUSE on the amp deal....
the amp has a 80 maxi-fuse!
one of them fuses that the cars use in the fuse box under the hood...the big fat ones

the fuse rating doesnt have anything to do with current draw!
didnt i post the formula on how to calculate how much the alt can handle?

southpaw....i understand ur a electric genius and all but i know ALOT about car stereo equipment, ive been doing this shit for a long time.
i may not understand all the crazy stuff you know but i know what i know!

and south....u knowing so much about all this....why would you suggest a cap???
save the money and get ur alternator beefed up if u are that worried!
in most big cities there are shops that re wind alternators to make them bigger amps
 

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woah, no offense intended havro. not claiming that your equipment is overrated or that you don't know what you're talking about-- i agree with you on the alternator issue (see bottom of this msg). but my point is this: a lot of people do have cheap equipment that is for sure overrated, and they may think they are overburdening their alternators when they really aren't. for example, lots of amps generate a huge amount of heat, sometimes over half of the power they consume is converted to heat and not speaker current!!! for example, i have an old a/d/s pq10 amp that churns out a meager 50Wx4. it also has a tendency to run hot. if i used the calculation W=V*A, (or A=W/V), i would calculate a maximum load of 14 amps on my battery. but the fact is, the amp draws up to twice that current, because it also produces heat. that's something most people forget. that's also why it has a 30A fuse and not a 15A fuse. lots of energy is wasted in heat production.

let's take another example to further the point. i also have an old soundstream amp that puts out 35x4 watts + 100x1. total rated power is 240W. it is also one of the most efficiently designed amps out there, whereby virtually all the power consumed is converted to speaker power, not heat. it puts out more power than my a/d/s amp (240W versus 200W) yet it draws less current from the battery. it has 4 3A fuses + 1 5A fuse, for a total of 17A.

so in the end, if you are concerned about the load on your battery/alternator, it is more important to know how much current your amp(s) pull rather than how much power is going to the speakers, which doesn't take into account the efficiency of the amp, how much is being lost as heat.

you are right that the cap works essentially as a spare battery, but the difference is that it can get repeatedly charged/discharged for thousands of cycles as opposed to a battery, which can only survive a couple of charges/discharges. they are designed to be discharged and charged rapidly also. caps WORK BEST WHEN your system DOES NOT routinely draw more current than your alternator can supply but is capable of drawing much more under sudden peak conditions such as heavy bass hits. these are very short and transient, and that's where a cap really shines. but if you are consistently pulling more than your alternator can supply then you are right, you should upgrade your alternator.
 

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I would also take account of the total amperage output from the battery when making calculations in this issue. A weak or small battery will drain easier (which also charges the cap) than a heavy duty one. Our batteries are rated at 550 CA and might not be heavy enough to run everything plus the sound equipment. A higher CA would keep up with the demands of the equipment draw without draining to much so the alternator would have to work as hard.

My friend has an old school American Pro 600 watt amp (probably peak power) and has no problem with his charging system which currently has a 40 amp alternator and a 800 CA battery (however, I recommended he put the 63 amp alt in).
 
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