Since the question keeps coming up in threads, and Steve and myself, and numerous others with whom I am regretably less familiar keep replying to them as the questions arise, I am starting this thread as a reference for people who somehow haven't read the FAQ's or found the answers they seek in a search. Feel free to reply, but this wasn't meant to start a discussion, only provide information.
Any guru's please feel free to back me up so that others know this is accurate and credible, or to expound on any part of it - if, in your expertise, you find any part of it lacking.
Not to be nasty, but if there be any individual who should feel compelled to disagree with this info based on what their "friend who knows a lot about this stuff" told them, I would ask said individuals to refrain from posting here without doing some heavy duty homework. This post is meant to clear things up for people who have heard different things from different sources and need help, not add to their confusion on the issue. That said -
The most frequently recomended step to help with a low voltage situation is to add a capacitor of one sort or another. This advice is given even by seasoned professionals in the industry. Unfortunately, it is more often than not the wrong advice.
Capacitors (caps) are a good thing - if they're used correctly. When added to an electrical system that can keep up, they provide the extra on-demand power a big amplifier craves. However, if the electrical system is already lagging, a cap helps until it's discharged, then it becomes one more thing for the already overworked charging system to keep up with. Once drained, a capacitor becomes a point of resistance in the system untill it is able to regain a voltage equal to or greater than that demanded "down stream", as it were. Since caps drain very quickly, and the demand that drained them in the first place will draw the current it wants with or without the cap, they do not recharge until the demand drops. So -
Make sure the alternator is in good shape. Then check the existing battery. If either of the above is wanting, replace it. If both are good, then add a second battery to reduce the load on the front end of the charging system. Upgrade the alternator if you are hard on your system to the point where even the 2nd battery is running behind. Once all this is in place, and your voltage no longer drops at the front, add capacitors as you see fit to bolster the amplifiers supply voltage. This is proper use of a cap.
Note that not all situations will call for the additional battery and/or upgraded alternator. If the power shortage is slight, as with a moderately sized amplifier added to a stock charging system in good condition and not overtaxed, a cap MAY be helpful. For most imports, that would be a sub amp fused at less than 40 amps, with few or no other additional components drawing significant power. There are no "exceptions" per se, but there are borderline situations where a cap may be benificial, and the preceeding was a VERY GENERALIZED example of such a situation.
As to what to add if you do in fact need to beef up the electrical system's foundation, here are some Cliff's Notes on what to do when.
When adding a second battery, try to mount it as close to amplifiers as possible. Avoid standard lead-acid batteries - for both safety and practicality. Gel Cell batteries (most popularly the Optima brand Red or Yellow Top units) are good, and will do the job. Redtops work best as a PRIMARY battery REPLACEMENT, Yelows are better as a second, dedicated battery. Drycell batteries (I prefer the Stinger brand, there are other very good ones out there) are better. They have a lower internal resistance, recharge faster, can discharge more current on demand, and are more durable.
Any time a second battery is added, the power wire connecting the two MUST be fused within 18 inches of BOTH batteries positive posts. A dual battery isolator or relay is not NECESSARY, but is a very good idea. It is OK to ground the 2nd battery to the frame if you'd rather not run two wires through the car, but all system components should be grounded to the system batteries negative post (this is usually more convenient install-wise anyway). It is not in fact necessary to have both batteries be of the same type - they will be wired in paralell, so the resistance will therefor average out as far as the alternator is concerned, although the battery with the lower internal resistance will charge faster.
Upgrading the alternator can be done two ways, by having your unit rewound to a higher amperage, or by replacing it altogether with a high output type unit. The former is less expensive, but typicaly less reliable, the latter usually more reliable - but MUCH more expensive. I am not going into adding additional alternators here, it is VERY seldom necessary, and if you need it, you need more help than would be useful here.
I REALLY hope there won't be many questions remaining after this, I tried to make it easy to grasp. Sorry it was so long, I hope it will be worth it!! Coming soon - Subwoofer enclosure types and their best applications. Stay tuned!!
