For an amplifier to be able to produce power, it must consume power. In the case of the car audio system, we are limited to about 12V (up to 14.4V in some vehicles). Given even the most efficient Class-D amplifiers, operating at over 80% efficiency, we need about one-Amp of current for every 10W being produced. So, a 1,000W amp requires about 100-Amps of current. Remember, this is an approximation and depends greatly on the efficiency of your amplifier.
Ohms law states that for every Amp of current flowing through an Ohm of resistance, one Volt will be dropped across the load. If we double the current, we get more voltage drop. Conventional lead-acid batteries have somewhat high internal resistances, which is why the output voltage drops as the current draw increases. If you put an analog voltmeter on your car’s battery as you start it, you will see the voltage drop from 13.6V down to the mid 11s while the starter motor is running. It’s also the same reason why our headlights and dash lights dim when a stereo is drawing large amounts of current from the electrical system.
A capacitor is similar to a battery in that it can store an electrical charge. However, its internal resistance is much lower than that of a battery, so the voltage drop when large amounts of current are drawn from it are much smaller, almost negligible.
The other problem with batteries is that there is a limit to how much current can be drawn instantaneously. Batteries are slow-motion devices; meant to store a charge for a long time. A capacitor is the opposite. It can completely discharge itself almost instantaneously, given the correct load conditions.
So what does this do for us? A capacitor is installed in parallel with the battery, preferably as close as possible to your amplifier. When the amplifier requires a great deal of instantaneous current (more than the battery can supply), the capacitor supplies it. This helps improve the transient (dynamic) response of the amp, but doesn’t help in long-term (more than a second) power delivery.
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since it only drops for a second when the base hits and the base is only drawing 12.5 amps this is all you need. an example of this is if you turn your lights on the battery gauge will drop for a second then come back up, this is the voltage regulator telling the alternator to put out more juice