Stiffening Capacitors FAQs

Stiffening capacitors can help your automotive amplifier deliver more output on peak bass transients. Low frequency causes the most power draw in an amplifier, so automotive subwoofer amplifiers are the main application for stiffening capacitors. Like any other electrical power supply system, automotive charging systems have a finite capability in the amount of power they can supply both continuously and at peak. Adding a stiffening capacitor to your system can improve transient response in an audio system while run at high output levels, and can reduce flickering dash lights, to a point. It should stand to reason that an amplifier cannot draw power from the charging system beyond what the charging system is designed to produce.

What is a stiffening capacitor?

A stiffening capacitor is an extremely large value electrolytic capacitor. Capacitors have the ability (capacity) to store a charge on their "plates". The larger the capacitance, measured in units of Farad, the greater the charge a capacitor can store for a given voltage. Most automotive capacitors can store a charge at 16 to 20 volts, and are rated from about .5 Farad up to 2 Farad or even more. Capacitors have an extremely low internal resistance to current flow, much lower than that of a battery. Capacitors can therefore charge and discharge power much faster than a battery can.

How does it work?

All capacitors consist of two "plates", separated by an insulator called the "dielectric". Each plate has an external connector. Connected to a source of voltage, electrons are forced onto one plate, and off of the other plate. When the capacitor is fully charged, no more electrons will flow in or out. Disconnected from the source of voltage, the charge will remain between the plates, and the voltage can be measured between the capacitor’s terminals with a volt meter. A capacitor can store a charge for a very long time.

The capacitor is connected to the positive power line to the amplifier, and then grounded to the vehicle chassis. When initially connected, the capacitor will very quickly charge up to the voltage of the vehicle's battery, usually a little over 12V. So quickly, in fact, that a large spark will occur unless a small resistance is used to slow the rate of charge. More on this in a moment.

Low frequency causes the output transistors in any audio amplifier to remain on and flowing current to the speakers for a longer period than midrange or high frequencies. Low frequency therefore creates the greatest current draw in any audio system. As the source of low frequency in music is usually a drum or bass guitar, the low frequency is mostly in short bursts, or "transients". So, subwoofer amplifiers generally draw the most power from an automotive charging system.

When a motor vehicle is running, the power for the electrical system comes from the alternator. Some very high-power subwoofer amplifiers are capable of drawing more power from a vehicle's charging system than the alternator can produce. At very high output levels, this can cause the bass "hits" to begin to lose volume, punch, and overall clarity, and can cause lights to dim or flicker. When the amplifier tries to draw more power than the alternator can produce, the voltage in the power line to the amplifier (and everywhere else in the electrical system for that matter) begins to drop. As the voltage in the capacitor is still at 12V, the drop-in voltage in the line will cause the capacitor to push charge onto the power line. This bolsters or "stiffens" the voltage in the electrical system until the transient bass "hit" passes. At that time the demand by the amplifier drops, the alternator brings the voltage back up, and the capacitor recovers its charge. It is then ready for the next bass transient.

This power "reservoir" in the capacitor only works to a point. If the power draw from the amplifier or multiple amplifiers demands so much power that the alternator can't bring the charging system and capacitor back up between bass transients, then reduced bass response and flickering lights will again occur. Adding more capacitance may help, but an alternator with a higher current rating would be a better solution.

Where should the capacitor be mounted?

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

Power cable does display resistance to current, and dissipates a bit of power itself. The stiffening capacitor should be connected into the circuit as close as possible to the amplifier for the greatest benefit to the sound system.

What size capacitor do I need?

As a general rule, 100,000 Microfarad (0.1 Farad) per 100 watts of amplifier power is recommended. So a 1 Farad capacitor should work up to about 1000 watts. Remember that the subwoofer amplifier is not the only drain upon the charging system, just the one that causes its limitations to show up first.

Can I use multiple caps?

Yes, capacitors add in value when wired in parallel. Be sure to use high surface area connectors such as spade connectors and distribution blocks to hold down losses.

What voltage capacitor do I need?

Automotive battery charging systems typically vary from as low as 12VDC up to about 14.5VDC, with the average alternator output at 13.8VDC. Most stiffening capacitors are rated at 16V/20 surge, or 20V/24 surge. Do not use capacitors rated below 16VDC, though a greater voltage rating is fine. Over-voltage can quickly destroy a capacitor, and must be avoided. Over-voltage transients can occur in the charging system, which is why a 20V or greater stiffening capacitor is generally recommended.

Do stiffening capacitors require special handling?

Handle stiffening capacitors with the same care as you would a car battery. Also of concern is the initial charging of the capacitor, or if you want to discharge the capacitor for storage. Having an extremely low internal resistance and great capacity for holding charge, they can create arcing and sparks during charge and discharge. As a result, you should not connect 12VDC directly to the terminals of an uncharged capacitor. At best, the arcing will deface the connectors. Most capacitors are supplied with a charge/discharge resistor, through which the capacitor is to be charged or discharged. It can take up to about a minute for the cap to reach full charge or complete discharge when using the resistor. The resistor may become hot during the process, so be careful not to get burned.

The arc or sparking during charge or discharge can also be a source of ignition for combustibles. Never discharge a capacitor by touching metal between its terminals for these same reasons. Always use a charging/discharging resistor to stop arcing. A standard 12V turn signal lamp can be used to charge or discharge a stiffening capacitor if you don't have a resistor. Wired in series, the lamp will glow while the capacitor is charging or discharging. It will go out when the capacitor is near full charge, or discharge. Never store a charged capacitor as it does represent a fire hazard.