Picked up a 130amp 3g alt for my car on eBay for $100US shipped.

 

You may find that your stock alternator is sufficient for what you are looking for, what type of car is this in? Many SUVs and north american cars have 100+ amp alternators stock, and do quite well in this regard.

Also, what model of amplifier do you have?

It won't hurt to try it out with your stock electical system first. Ensure that you have a healthy battery installed, as that type of power will certainly put a lot of strain on it. Although alternators are not that expensive, labour costs can be expensive [img]graemlins/deal4u.gif[/img] </font>[/QUOTE]

 

Arg my reply didnt even stick... well my car is 1998 nissan sentra and my amp model is Sony XM-DS1600P5


And those are very decent prices for new alternator.

 

The very last thing you should add to your electrical system is a cap, IMO. Big three -&gt; battery -&gt; alternator, in that order.

 

Im pretty sure my car has an 80 amp alternator...

is that good bad average?? does it NEEEEED to be replaced or can it hang for awhile?

 

Better ground. Do that first.

 

Thx for all this feedback guys i really appreciate it...

 

Unless you run the system a lot with the car off or the alternator is way overtasked, the battery will hardly ever come into play. The alternator is the primary source of current for the car when it is running. Any demand beyond what the alt can supply is handled by the battery. A cap will simply act as a buffer between the two. It will not fix a charging system problem and if your alt is already out of its league with the power you are running (I doubt it with that Sony amp) it would actually become a hinderance.

The only real benefit of the Yellow Top Optima is that it has the current charatceristics of a starting battery (high CCA and lower ESR) and can be deep-cycled and recharged without killing it. If you have a big enough alternator, all the battery will have to do is start the car and its deep-cycle properties won't matter.

 

I'd disagree wholeheartedly with this..
But I like to proceed in the order of "bang for the buck".

It's not a simple issue of "supply", as so many people confuse it as.
Each of these items plays a role, IMO none should be missing.

Chances are you have a battery, and an alternator, and wiring... including the "big three" (which are present, albeit in stock form).
Chances are you don't have a cap.
Chances are you are putting a more dynamic load on your electrical system than it was designed to handle.

There's two issues:
1) the "supply" aspect - in terms of quantity - how many amperes can the thing supply? It's really a complicated thing to examine, particularly when you dissect all the elements that boil down essentially into the absolute, or average draw that's on your alternator/battery combo.
2) the "dynamic" aspect - which is related to how well your electrical system can respond to dynamic bursts and fast transients.
That's where a capacitor comes into play.

It's not about how much, but how fast.

Consider that any time that you are operating at 14.4v, essentially all current is being drawn from the alternator.
When something (like your amplifier) draws enough current to exceed that which your alternator can provide, the voltage begins to fall.
Once the voltage reaches the level of the battery (12v), the battery can begin supplying current to augment the alternator, to provide that current above and beyond what it's capable of handling. Both are working together - at 12v.

You see your lights dim simply because the voltage level has fallen to 12v, and light bulbs are brighter on 14.4v than they are on 12v.

There's a rise-time though, how long it takes the battery to respond... the transient response of the battery.
A battery charges relatively slowly... and likewise, discharges relatively slowly.
Things get worse when you are talking about deep-cycle batteries (yellow-tops are deep cycle batteries), because the same thicker plates that make them more durable in surviving abuse and deep-discharges, make them slower in transient response - the response time it takes them to "wake up" and begin supplying current.

A capacitor fits in nicely here, because it charges to the highest level of the electrical system - generally 14.4v if wiring is sufficient.
It's noteworthy that it begins discharging as soon as the battery drops below this peak charge level.

Capacitors, in sharp contrast to batteries, have virtually instantanious charge and discharge properties...
...which is why capacitors are shipped with little resistors to charge them initially. Without that, the capacitor would try to suck so much current, so fast, that it could discolor or even weld the ring terminal to the cap.
Similarly, it's capable of discharging it's entire charge in a brief moment.

Even if you upgrade your alternator, large systems can easily exceed 150a, 175a, even 200a... and possibly more current draw at transient peaks in the music.

When this happens...
...what's covering the gap in time in YOUR system, as the alternator voltage is falling, before the battery is able to respond to augment the supply?

Yes, this period in time is only milliseconds in duration... thankfully this is why larger amounts of capacitance aren't necessary in most systems.

But don't discount its value.

Here's a graphic to show the comparison of voltage level, comparing a system reaching a large transient peak, one with a capacitor, one without:


Too many people that misinterpret Richard Clark's capacitor testing that proved that caps don't help amps make more power than they are rated for...