does a capacitor really harm more than do good?

[helotaxi]

Several factors involved in building an adequate power supply system.

1) The alternator is the source for everything. Period. If the alternator can't keep up, you will have long term problems. The Alt needs to be able to run the cars electrics (spark, fuel pump, lights, AC clutch, etc...) and the stereo without reaching it's max capacity. Ideally you want a healthy reserve of at least 25% of the alt capacity at the average current draw at a normal (for you) listening level. This current draw depends on the size and effiicency of the amps as well as how loud you play the system.

2) The "big 3" are important as well. The chassis ground for the battery and engine block/alternator and the + wire between the alt and the battery must be large enough to carry the current from the alt with minimal voltage drop. It is not possible for this wire to be too big.

3) Your alt only supplys the current demanded of it at any given time. For the alt to up its current output requires a finite amount of time. When the current demand exceeds the amount being produced (during music transients for example) the voltage output of the alt starts to dip. The battery does nothing to help the situation until the voltage of the alt drops to the discharge voltage of the battery (12-13.2V depending on the battery) at which point the voltage will stabilize and the battery will contribute current to the system. Understand that even if the alt is more than up to the task of providing the amount of current demanded by the system, this voltage dip will still occur in the time period that it takes for the alt to up its output. The duration of this transition depends on the particular alt and its voltage regulator.

4) A cap can helo in the above situation to alleviate the dimming of headlights. Since the cap charges to the voltage of the alt and begins to discharge from that voltage, a large cap can reduce the transient voltage drop and since it is a gradual drop from the working voltage rather than a quick one, the dimming is not as severe and not as obvious, though it does still occur.

5) If on the other hand your alt is straining to supply the normal operating current of the system, adding a cap or another battery or anything else is not going to help at all and may well hurt. Once the capacity of the alt is exceeded and the battery(ies) are partially discharged or the cap is discharged, it becomes another load on the alt until it is fully recharged. If the alt was on the edge before the transient with the bat/cap fully charged and the bat/cap are partially discharged and now demanding current from the alt to recharge, the alt is in over its head with no chance of catching up.

[knukonceptz]

While this has been gone round and round on several topics/posts, the main concenus is they can/do help musically. If SPL is your goal a battery or other upgrades would be the better choice.