The aluminium electrolytic capacitor has a limited shelf life, which means it can degrade over time even without being used. This occurs because the electrolyte in the element eventually dissipates, evaporates, or leaks out. This can cause the capacitance to decrease, the equivalent series resistance (ESR) to increase, and the leakage current to increase. These changes can affect the performance and reliability of the capacitor and the circuit it is used in.
The shelf life of an aluminium electrolytic capacitor depends on several factors, such as:
- The quality and design of the component
- The storage temperature and humidity
- The applied voltage and current
- The reforming process
The quality and design of the component refers to the materials, construction, and specifications of the capacitor. Different manufacturers may use different types of electrolytes, foils, seals, terminals, and cases, which can affect the shelf life of the capacitor. The specifications of the capacitor, such as the rated voltage, capacitance, ESR, and ripple current, can also influence the shelf life.
The storage temperature and humidity refers to the environmental conditions where the capacitor is stored before use. Higher temperatures and humidity can accelerate the degradation of the electrolyte and the oxide layer, reducing the shelf life of the capacitor. Lower temperatures and humidity can preserve the electrolyte and the oxide layer, extending the shelf life of the capacitor.
The applied voltage and current refers to the electrical stress that is applied to the capacitor during use or testing. Higher voltage and current can cause more heat generation and electrolyte evaporation, reducing the shelf life of the capacitor. Lower voltage and current can cause less heat generation and electrolyte evaporation, extending the shelf life of the capacitor.
The reforming process refers to the procedure of applying a voltage to a capacitor that has been stored for a long time or has degraded performance. The purpose of reforming is to restore or regenerate the oxide layer on the aluminium foil, which can improve the capacitance, ESR, and leakage current of the capacitor. Reforming should be done gradually and carefully, using a series resistor or a current-limited power supply, to avoid damaging or overheating the capacitor.
The manufacturer’s stated shelf life for an aluminium electrolytic capacitor can range from 2 to 10 years, depending on the quality of the component. However, this is only an estimate based on typical storage conditions and usage scenarios. The actual shelf life of a capacitor may vary depending on how it is stored and used.
Aluminium electrolytic capacitors consist of very thin aluminium sheets that have been etched to enlarge their surface area and then wound with an electrolyte in between. A common misconception is that the electrolyte acts as the dielectric. However, the actual function of the electrolyte is to assist in the formation of aluminium oxide, which is the real dielectric material. The thin layer of aluminium oxide is formed on the surface of the aluminium anode foil through a process called “forming” or “anodization,” in which the foil is subjected to a high voltage in an electrolytic solution. The aluminium oxide layer acts as an insulator, separating the anode and cathode and allowing the capacitor to store charge.
When aluminium electrolytic capacitors are not used for a long time, the aluminium oxide layer becomes thinner. This affects the capacitance of the capacitor, which may increase the ripple in the circuit. It also affects the leakage current, which is the current that flows through the dielectric layer. A thinner aluminium oxide layer means more leakage current or sometimes, much more. But this is not entirely negative, because it is this leakage current that enables the electrolytic process that restores the dielectric layer. As the capacitor is used again, the electrolyte repairs the aluminium oxide layer and the capacitor resumes its normal performance.
The study of aluminium electrolytic capacitors, their ageing process, and their reformation is a complex and fascinating topic. A major implication for consideration is that leakage currents rise with disuse and that leakage current variations between capacitors can become more pronounced when this happens, which is a crucial factor to consider when selecting capacitors' maximum rated voltages for connecting several capacitors in series across a high DC voltage.