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Aluminum electrolytic capacitors are made by layering the electrolytic paper between the anode and cathode foils, and then coiling the result. The process of preparing an electrode facing the etched anode foil surface is extremely difficult. Therefore, the opposing electrode is created by filling the structure with an electrolyte. Due to this process, the electrolyte essentially functions as the cathode. The basic functional requirements for the electrolyte are as follows:

(1) Chemically stable when it comes in contact with materials used in the anode, cathode, and electrolytic paper.
(2) Easily wets the surfaces of the electrode.
(3) Electrically conductive.
(4) Has the chemical ability to protect the anode oxide thin film and compensate for any weaknesses therein. 
(5) Low volatility even at high temperatures
(6) Long-term stability and characteristics that take into consideration such things as toxicity.

Aluminum electrolytic capacitors are constructed from two conducting aluminum foils, one of which is coated with an insulating oxide layer, and a paper spacer soaked in electrolyte. The foil insulated by the oxide layer is the anode while the liquid electrolyte and the second foil acts as the cathode. This stack is then rolled up, fitted with pin connectors and placed in a cylindrical aluminum casing. The two most popular geometries are axial leads coming from the center of each circular face of the cylinder, or two radial leads or lugs on one of the circular faces.

In aluminum electrolytic capacitors, the layer of insulating aluminum oxide on the surface of the aluminum plate acts as the dielectric, and it is the thinness of this layer that allows for a relatively high capacitance in a small volume. This oxide has a dielectric constant of 10, which is several times higher than most common polymer insulators. This layer can withstand an electric field strength of the order of 25 megavolts per meter which is significant fraction of that of common polymers. This combination of high capacitance and reasonably high voltage result in high energy density.

The above are the most common schematic symbols for electrolytic capacitors. Some schematic diagrams do not print the "+" adjacent to the symbol. Older circuit diagrams show electrolytic capacitors as a small positive plate surrounded below and on the sides by a larger dish-shaped negative electrode, usually without "+" marking.

Today electrolytic capacitors or as they are more correctly termed, Aluminium Electrolytic Capacitors are used in huge quantities. They are very cost effective and able to provide a larger capacitance per volume than other types of capacitor. This gives them very many uses in circuits where high currents or low frequencies are involved. Aluminium Electrolytic Capacitors are typically used most in applications such as audio amplifiers of all types (hi-fi to mobile phones) and in power supply circuits.

The plates of an electrolytic capacitor are constructed from conducting Aluminium foil. As a result they can be made very thin and they are also flexible so that they can be packaged easily at the end of the production process. The two plates, or foils are slightly different. One is coated with an insulating oxide layer, and a paper spacer soaked in electrolyte is placed between them. The foil insulated by the oxide layer is the anode while the liquid electrolyte and the second foil act as cathode.

In order to package them the two Aluminium foils with the electrolyte soaked paper are rolled together to form a cylinder, and they are placed into an Aluminium can. In this way the electrolytic capacitor is compact while being robust as a result of the protection afforded by the can.

There are two geometries that are used for the connection leads or tags. One is to use axial leads, one coming from each circular face of the cylinder. The other alternative is to use two radial leads or tags, both of which come from the same face of the cylinder.

The lead styles give rise to the descriptions used for the overall capacitors. Descriptions of axial and radial will be seen in the component references.

Aluminum Electrolytic Capacitors include axial, snap-in and screw-terminal types. These capacitors offer very long life in applications requiring high ripple currents and temperatures up to 150°C. Other models have rated voltages as high as 550 VDC.

In addition to those products shown in the datasheets, KEMET can optimize the construction to offer application-specific balance between required life, temperature, ripple current, physical size, and cost.

Motor start capacitors are also offered.

A high performance electrolytic capacitor designed for automotive applications with high vibrations and high ambient temperatures.

An electrolytic capacitor with outstanding electrical performance. It is polarized with the negative pole connected to the case. The outer case has a plastic cover.

The low ESR is a result of a low resistive electrolyte/paper system. Together with the TDC thermal concept, this range has very high ripple current capability. The capacitor is suitable for both mobile and aircraft applications, with operation up to 105°C.

1. Aluminum electrolytic capacitors have a bi-polar structure. This is marked on the body of the capacitor. A capacitor must not be mounted with reversed polarity. The application of an AC or reverse voltage may cause a short circuit or damage the capacitor. Bi-polar capacitors must not be used in AC applications, where the polarity may be reversed in the circuits or is unknown.

2. The DC voltage applied to the capacitor terminal must not exceed its rated operating voltage, as this will result in a rapid increase of the leakage current and may damage the capacitor. It is recommended to operate the capacitor at 70 – 80% of its rated voltage to optimize its service life.

3. The ripple current applied to the capacitor must be within the permitted range. An excessive ripple current leads to impaired electrical properties and may damage the capacitor. Note that the sum of the peak values of the ripple voltage and the DC operating voltage must not exceed the rated DC voltage.

4. Capacitors must be used within their permitted range of operating temperature. Operation at room temperature optimizes their service life.

5. Capacitors with case diameter 8 mm are equipped with a safety vent. In capacitors fitted with a lead or soldering lug, the safety vent is usually located at the base of the case. It needs sufficient space around it to operate optimally. The following dimensions are recommended: for case diameter d = 8 to 16 mm, more than 2 mm; for d = 18 to 35 mm, more than 3 mm; and for d = 42 mm or more, more than 5 mm.

6. Capacitors should not be mounted with the safety vent face down on the board. Do not locate any wire or copper trace near the safety vent. Do not reverse the voltage, as this may result in excess pressure and the leakage of electrolyte.

7. Gas is released through the safety vent when the pressure inside the capacitor is too high. A gaseous liquid around the safety vent does not indicate a leakage of electrolyte.

8. The capacitor should be stored under conditions of normal temperature and in a non-acid, non-alkali environment of normal humidity. Exposure to high temperatures, for example under direct sunlight, will reduce its operating life. If the capacitor is stored in an environment containing acids or alkalis, the solder ability of the leads may be affected.

9. Containing acids or alkalis, the solder ability of the leads may be affected. The leakage current of an aluminum electrolytic capacitor may increase after a long period of storage. After such storage, the capacitor must be aged by applying the rated operating voltage for 6 – 8 hours before use.

An electrolytic capacitor is a type of capacitor that uses an electrolyte, an ionic conducting liquid, as one of its plates, to achieve a larger capacitance per unit volume than other types. They are often referred to in electronics usage simply as "electrolytics". They are used in relatively high-current and low-frequency electrical circuits, particularly in power supply filters, where they store charge needed to moderate output voltage and current fluctuations in rectifier output. They are also widely used as coupling capacitors in circuits where AC should be conducted but DC should not. There are two types of electrolytics; aluminum and tantalum.

A common modeling circuit for an electrolytic capacitor has the following schematic:

Where Rleakage is the leakage resistance, RESR is the equivalent series resistance (ESR), LESL the equivalent series inductance (L being the conventional symbol for inductance).

RESR must be as small as possible since it determines the loss power when the capacitor is used to smooth voltage. Loss power scales quadratically with the ripple current flowing through and linearly with RESR. Low ESR capacitors are imperative for high efficiencies in power supplies. Low ESR capacitance can sometimes lead to destructive LC voltage spikes when exposed to voltage transients.

This is only a simple model and does not include dielectric absorption (soakage) and other non-ideal effects associated with real electrolytic capacitors.

The service life of an aluminum electrolytic capacitor for mid- to high-voltage filters is affected by the applied voltage. If the applied voltage is between 60% and 100% of the rated voltage, the estimated service can be extended by lowering the applied voltage below the rated voltage. However, if the applied voltage is less than 60% of the rated voltage or the capacitor is used in low-pressure (100 WV or less) applications, the impact of the applied voltage on the service life is negligible.

Therefore, service life is estimated assuming no impact from voltage. Continuous application of a voltage over the rated voltage rapidly increases leakage current in a capacitor. This may increase internal pressure due to generation of gases, resulting in activation of the safety vent in a short time and/or formation of an internal short circuit. For this reason, the applied voltage must be maintained below the rated voltage during use.

Besides, it should be noted that the circuit design is such that the applied voltage will remain 80% or less of the rated voltage during use.

Where more than one capacitor connected in series is used, the applied voltages across the individual capacitors may become out of balance, resulting in the application of excessive voltage to them. To avoid this, either choose a rated voltage allowing for voltage imbalances, or connect a voltage divider(resistors) to the capacitors. Please be careful about charge/discharge.

Aluminum electrolytic capacitors ('capacitors') may cause explosion, fire, or other serious hazard if used outside the specified operating conditions. Please familiarize yourself with the instructions below before using these capacitors.

1.Check the operating and installation environment and use the capacitor within the range of the rated performance specified in the catalog or specifications.

2.Maintain operating temperature and ripple current within the specified ranges. Base your choice of capacitors on the maximum load conditions. A capacitor will overheat under excessive current, potentially resulting in short circuit, fire, or other major failure.

3.A capacitor also generates the self heating. Please bear in mind that the capacitor heats up the interior of the equipment, and take appropriate precautions. Operate the unit under normal conditions and check the temperature of the area surrounding the capacitor.

4.The permissible ripple current declines with the rise in ambient temperature (the temperature of the capacitor's surroundings).Consider the permissible ripple current at the maximum predictable ambient temperature.

5.Electric characteristics change as frequencies change. Check frequency changes in order to choose the right capacitor. Special attention needs to be given to the self heating and short life time both low and high frequency, when equivalent series resistance and inductance change.

Unlike capacitors that use a bulk dielectric made from an intrinsically insulating material, the dielectric in electrolytic capacitors depends on the formation and maintenance of a microscopic metal oxide layer. Compared to bulk dielectric capacitors, this very thin dielectric allows for much more capacitance in the same unit volume, but maintaining the integrity of the dielectric usually requires the steady application of the correct polarity of direct current else the oxide layer will break down and rupture, causing the capacitor to lose its ability to withstand applied voltage (although it can often be "reformed"). In addition, electrolytic capacitors generally use an internal wet chemistry and they will eventually fail if the water within the capacitor evaporates.

Electrolytic capacitance values are not as tightly-specified as with bulk dielectric capacitors. Especially with aluminum electrolytic, it is quite common to see an electrolytic capacitor specified as having a "guaranteed minimum value" and no upper bound on its value. For most purposes (such as power supply filtering and signal coupling), this type of specification is acceptable.