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Stiffening capacitors can help your amplifiers deliver up to 50% more output on those demanding peak bass notes. Most automotive alternator/battery systems simply lack the ability to produce large amounts of instantaneous power. This is exactly the type of power that car audio amplifiers crave. Adding a stiffening capacitor to your system can add tremendous bass punch and improve transient response.





Stiffening capacitors FAQ's



What is a stiffening capacitor?

Computer grade power caps (also known as stiffening capacitors) store energy and then very quickly release it on demand to your power amplifier(s). The term stiffening capacitor as a generic term refers to any large capacitor (100,000 Microfarad and larger) placed in parallel with an amplifier and battery.



How does it work?

A stiffening capacitor charges like a battery. But unlike a battery the stiffening capacitor is designed to quickly release power on demand. This quick burst of power aids your amplifier in producing deep bass notes when it needs it. In between deep bass notes your battery and alternator re-charge the cap allowing it to be ready for the next deep bass note. This battery voltage assistance reduces voltage induced amplifier clipping, increases transient response, increases bass punch, and increases amplifier life.



Where should my capacitor be mounted?

Stiffening capacitors should be mounted as close to the amplifier as possible, as is shown in the diagram above.



One of the other benefits of stiffening capacitors is the ability to reduce line loss. The power cable itself creates line loss. Long runs of power cable associated with mounting amplifiers in automotive trunks create line loss, especially during deep bass notes. Stiffening capacitors help maintain appropriate system voltages at the amplifier.



What size capacitor do I need?

As a general rule we recommend 100,000 Microfarad (.1 Farad) per 100 watts of amplifier power, or 1 Farad per 1000 Watts. Use the simple chart below to determine what size capacitor you need.



Total Amplifier Power: Capacitor needed:

Up To 500 Watts .50 Farad

500-1000 Watts 1.0 Farad

1000-1500 Watts 1.5 Farad

2000 Watts 2 Farad



Can I use multiple caps?

Yes, Capacitors add in value when wired in parallel. Example: Two 1 Farad capacitors wired in parallel sum to 2 Farad. This is the perfect size for a competition system with up to 2000 Watts of trunk-mounted amplifiers.



What voltage capacitor do I need?

Automotive battery charging systems typically vary between as low as 12VDC up to as high as 14.5VDC with the average alternator output at 13.8VDC. Generally speaking stiffening capacitors these days are rated at 20VDC average and 24VDC surge. We do not recommend using capacitors rated below 16VDC but higher is always better.



Do stiffening capacitors require special handling?

Handle stiffening capacitors as you would a charged battery. The main thing you need to worry about is initial installation charging and discharging. Most capacitors are supplied with a charging/discharging resistor or lamp. Capacitors draw a very large amount of current during installation. DO NOT attempt to connect 12VDC directly to the terminals or arching and permanent terminal damage will occur. Utilize a charging resistor or lamp in series with the cap and follow the instructions supplied with your capacitor to slowly energize the capacitor and eliminate arching. NEVER short the leads of a stiffening capacitor. When un-installing a capacitor remove the source voltage then use the same resistor or lamp to slowly de-energize it.



Anything else I should know?

Yes, ALWAYS install an appropriate fuse or circuit breaker between your amplifier/capacitor in the trunk and your battery power source.



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Capacitance (C) is the measure of a capacitor’s ability (capacity) to store an electric charge

on its plates. A capacitor is an electrical component that consists of two conductors (plates) separated by an insulator called the dielectric.



The voltage rating of a capacitor, often labeled as WVDC for Working Voltage Direct Current, is the maximum voltage that can safely be applied across the capacitor without arcing, or voltage breakdown, occurring in the dielectric. The difference of potential between the capacitor’s plates creates a stress on the atomic structure of the dielectric. If the voltage is too high, electrons will be torn from their orbits in the dielectric material, producing an electric arc. A small carbon path known as a puncture is created. A punctured capacitor is not useable as the puncture creates a leakage path for current between the plates.



When a capacitor is connected to a DC voltage source, electrons accumulate on the negative plate. The other plate becomes positively charged by electrostatic conduction. The presence of the negative charge (surplus electrons) on one plate repels negative charges (electrons) off the neighboring plate. Thus, the opposite plate receives an equal, though opposite, positive charge of “holes”. A hole is a vacant spot in the valence shell of an atom where an electron may be captured.



The amount of capacitor charge is determined by the amount of applied voltage and the capacitance, in Farads, of the capacitor. When a DC voltage is first applied, the capacitor acts like a short and demands a great amount of current. At this first instant of time when voltage is applied, the circuit resistance is the main limiting factor to current flow. Current flow will be at its greatest. As capacitor charge increases, the rate of charge decreases as the voltage across the capacitor approaches the source voltage. When the source voltage is reached, current flow stops. Remember that there is actually no current flowing from one plate of the capacitor to the other. There are only free electrons moving onto and leaving the plates by the leads. As like charges repel (electrons have a negative charge), the plate with an excess of electrons will repel electrons from the other plate giving it a positive charge.



AC Capacitor Current A capacitor is able to pass alternating current even though there is an insulator between the plates (no actual current path). In an RC (resistive/capacitive) transient circuit, the capacitor charge current will flow until the capacitor is fully charged, and discharge current will flow until the capacitor is fully discharged. When an alternating voltage is applied to the plates of a capacitor, the capacitor’s plates are forced to follow repeated cycles of charge and discharge.



The RC Time Constant It takes time for a capacitor to charge through a circuit resistance. Capacitors do not charge at a linear rate, they charge at an exponential rate. The RC Time Constant (t) formula is





t = R·C





One time constant is equal to the product of the circuit resistance times the capacitance. A capacitor will reach a full charge and show the source voltage after a period of 5 time constants. This applies to every capacitor in every circuit, and applies to inductors as well! For example, a 5 uF capacitor, charging through a 1 KW resistor will have a time constant equal to 5 E-6 X 1K = .005 seconds, or 5 milliseconds. Therefore, with this capacitor/resistance combination, the capacitor will reach full charge and show the full source voltage in 5 X 5 milliseconds or 25 milliseconds.



In the first 5 ms, the capacitor will reach 66.6% of the total charge; it will reach 86% after 10 ms, 95% after 15 ms, 98% after 20 ms, and 99% after 25 ms. As you can see, the rate of charge slows as the capacitor takes on charge and the voltage across the cap approaches the source voltage. After 5 time constants, current flow stops, and capacitor voltage equals the source voltage.