Tech Tip: Does a “Stiffening Capacitor” behave like you think it does?

Does a “Stiffening Capacitor” behave like you think it does? It’s been a popular approach to supplement an audio system consuming a lot of power by adding a power storage capacitor (often called “stiffening capacitors”). The question is, do they really behave like you think?

Any capacitor takes a certain degree of time to charge as well as discharge. The time required for charging and discharging depends on the internal equivalent series resistance (ESR) of the capacitor as well as its amount of capacitance (or conductive plate surface area). As resistance increases, it takes more time for the capacitor to charge and discharge.

Lengthy cables and connectors only add to the series resistance, which is why many companies recommend that power storage capacitors be installed as close as possible to the amplifier(s) in a mobile audio system. The common (tall beer can sized) electrolytic capacitors available follow that recommendation.

Some newer capacitor designs specifically intended for use in power supplies employ a technology called electrochemical double layering. These are known as EDL power supply capacitors. The main difference is that the surface area of the conductive plates has been greatly increased by using porous carbon as the plate material. Carbon is an inherently low-voltage material, which means that many banks of the plates are connected together in a series configuration internally to gain the capacity of a voltage level appropriate to work with a vehicle’s electrical system. These capacitors can take on very different physical packaging because the conductive plates are not one continuous roll. EDL capacitors can have as much as 300 times the surface area as a comparable electrolytic capacitor of the same physical size. EDL capacitors also cost more than electrolytic power supply capacitors.

All capacitors use a formula called a time constant to establish the rate of charge and discharge. It takes five time constants to become completely full or empty (see graph).

Capacitor time constant chart.

Image courtesy of Jethro Donaldson

A capacitor is considered “fully” charged (99.2 percent of supply voltage level) or “fully” discharged (0.8 percent of beginning “full” charge) after five time constants. The way that charging and discharging affect important applications (such as “stiffening” power supply capacitors) is important to consider when the time constant percentage of charge or discharge is considered. Useful applications need the five time constants to charge a capacitor and make effective as an opposition to changes in voltage. Yet just one time constant of discharge makes a capacitor useless as a tool to stabilize DC voltage in a demanding mobile audio system and then becomes another load to supply until charged again.

A capacitor is charged (or discharged) by the following percentages:

• After Time Constant 1 = 63.2% (either charged or discharged)
• After Time Constant 2 = 86.5% (either charged or discharged)
• After Time Constant 3 = 95.0% (either charged or discharged)
• After Time Constant 4 = 98.1% (either charged or discharged)
• After Time Constant 5 = 99.2% (either charged or discharged)

The formula for 1 time constant is T = R x C

• T = time in seconds.
• R = resistance in ohms (and don’t forget to move the decimal if given as milliohms).
• C = capacitance in farads (and don’t forget to move the decimal if given as microfarads).

That means if the battery is 12.6 volts and the system voltage (with the car running) is around 14 volts, it takes between the second and third time constant to even come up to battery voltage, then it slowly rises through the 13 volt range until reaching the fifth time constant at fully charged (99.2%). The real shocker is that on the first time constant of discharge, it’s down from the supply voltage by 63.2%, and it’s no longer any help at all to stabilizing the voltage of the audio system. What matters here is how long and how often the capacitor is called upon to stabilize. Does it really behave like you thought?

The end result is that for use as a real power supply reserve, the stiffening capacitor takes effectively five times longer to charge (for stabilizing the system’s supply voltage) than it does to dump its effective energy and become a load on the electrical system. For the occasional times when a bass note or dynamic music is in demand of an otherwise lacking audio system, this can be a good strategy given the recharge time. For systems that are constantly starving for stabilized voltage, a capacitor might not be as good an answer as an alternator upgrade (if it drives) or a battery (if it’s parked a lot at shows).

Finally, if you have a customer with dimming headlamps, the capacitor is probably better installed on the lighting circuit near the headlamps than the audio system (it will require less capacitance, too). The headlamps present a more constant load that can visually be improved by installing a capacitor, even if it’s installed for the audio system. Since an audio system is more dynamic in nature, it’s harder to “see” the benefit, let alone hear it. If you have installed one and it makes the system sound better, then you have your proof. If it hasn’t made improvements in the sagging voltage problem, sounds like an alternator upgrade or cutting back on the power consumption of amplifiers is necessary.

Technical data provided by Ramsey Consulting Group, Inc. www.ramseyconsultinggroup.com.