Aluminum electrolytic capacitors are perhaps the oldest type of capacitor. Even the early Leyden Jar was a kind of electrolytic capacitor that operated like modern-day aluminum electrolytic capacitors. Oftentimes, the first exposure engineering students have to capacitors have is with the electrolytic types. So, it is fair to say that they have been around for a very long time. Aluminum electrolytic capacitors are among the most well-known and understood. One of the things that isn’t often taught is what happens to an electrolytic capacitor when it sits in storage, unused for extended periods of time. It is an interesting revisable electrochemical process that is necessary to understand when designing systems with capacitors that will be sitting in an unused state for a long time.
Aluminum electrolytic capacitors are not all that dissimilar from rolls of paper. An etched sheet of aluminum is put through an electrolysis process that causes it to grow a thin layer of oxide. The original sheet of aluminum serves as the anode and that oxide layer, Al2O3, serves as the dielectric. The purpose of the etching is to increase the surface area, which is a key component to determine the final capacitance. Bonus trivia: an electrolytic capacitor is one where the dielectric layer is grown on the anode using a redox process, hence the name aluminum electrolytic.
That’s the anode side. The cathode side is also an etched roll of aluminum and porous carrier paper. What does this porous carrier paper carry? An electrolyte. Both of these together serve as the current carriers of the cathode layer. The anode structure and cathode structure are rolled together and tabs are welded to opposite ends that ultimately go out to the capacitor terminals.
There is an acidic nature to the electrolyte used in aluminum electrolytic capacitors. Acids are very reactive to oxides. The dielectric used in aluminum electrolytic capacitors is Al2O3 and the electrolyte in that carrier paper also etches away at the dielectric during storage. This sounds undesirable but is expected. The very nature of the materials and chemicals used in these capacitors allow for a healing of this effect. When an aged capacitor with the now etched dielectric has current applied to it the redox reaction that caused the etching of the dielectric reverses. Oxygen is pulled from the dielectric and regrows the dielectric layer, effectively healing the capacitor. The electrolyte is an aqueous solution meaning that it is water-based. The oxygen being pulled out of the electrolyte to re-form the dielectric comes from that water. Water is, of course, H2O and without the oxygen there is an evolved amount of hydrogen gas that must be safely vented from the capacitor. We have another piece <here> that discusses what is to be done with that vented hydrogen. Eventually this “drying out” mechanism of the capacitor is what determine its life.
That leads to the question of how long these capacitors can be stored without any bias. Like many questions in life the answer beings with “It depends”. In general, if a capacitor has been stored for more than 18 months under these conditions (+5 to +35°C and less than 75% in relative humidity) and it shows increased leakage current, then a treatment by voltage application is recommended. This comment is generally noted in our catalog. Storage temperature also plays a role in shelf life. Temperature has an effect on the rate of this effect too. We can get deep into the weeds on reaction rate and the Arrehenius equation but suffice it to say that the higher the temperature, the faster the oxide layer will decay.
If your particular time and temperature takes you in the yellow region in the figure above then re-aging is necessary.
If you happen to be in that yellow “danger zone” then not all is lost. As mentioned earlier, the consumption of the dielectric by the electrolyte acid is actually a reversible chemical reaction and as such it is a relatively simple process to recover from that. All that needs to happen is that the rated voltage of the capacitor needs to be applied for a period of one hour or until the leakage has reached a steady state below the specified limit. The charging current should be limited to twice the specified leakage current or 5mA, whichever is greater. After such time the dielectric will have been regrown and the capacitor is effectively rejuvenated. If only that worked on humans too.
One of the main figures of merit for electrolytic capacitors is their lifetime. KEMET has many long-life aluminum electrolytic capacitors suitable for a plethora of applications.
