(1) The Charging Process.
The process of imparting a charge to a capacitor (thereby storing electric charge and electrical energy) is known as charging. When one plate of a capacitor is connected to the positive terminal of a power source and the other plate to the negative terminal, the two plates acquire equal amounts of opposite charges. Once charged, an electric field is established between the capacitor's two plates; the charging process effectively stores the electrical energy obtained from the power source within the capacitor.
(2) The Discharging Process.
The process by which a charged capacitor loses its charge (releasing both charge and electrical energy) is known as discharging. For instance, if the two terminals of a capacitor are connected via a conductive wire, the charges on the terminals neutralize each other, causing the capacitor to release its stored charge and electrical energy. Following discharge, the electric field between the capacitor's plates dissipates, and the electrical energy is converted into other forms of energy.
Battery self-discharge refers to a battery's ability to retain its stored charge while in an open-circuit state. The mechanisms of self-discharge in lithium-ion batteries can be broadly categorized into physical self-discharge and chemical self-discharge. Individual battery cells are assembled into modules through series and parallel connections; if the self-discharge rates among the individual cells within a module lack consistency, it can lead to voltage imbalances across the internal cells after a period of storage. Consequently, during subsequent charging and discharging cycles, some cells may reach their target voltage while others remain at significantly higher or lower voltages. This discrepancy can result in the overcharging or over-discharging of individual cells-potentially even giving rise to safety hazards-and poses a significant challenge to the module's ability to maintain voltage balance. Self-discharge is, therefore, a critical performance metric for lithium-ion capacitors.