In theory, a capacitor never fully discharges to zero charge or voltage. This is due to the exponential nature of the discharge process, described by the equations V = V₀e^(-t/τ) and Q = Q₀e^(-t/τ). However, in practical terms, a capacitor is considered 'fully discharged' when its voltage or charge drops to a negligible level. Typically, this is around 5 time constants (5τ), at which point the voltage or charge is less than 1% of its initial value. This implies that while the discharge process continues indefinitely, it reaches a point of practical completion within a finite, predictable time frame.

The current direction changes when a capacitor starts discharging due to the reversal of the flow of charge. During charging, current flows into the capacitor, accumulating charge on its plates. This results in a build-up of voltage across the capacitor. When discharging begins, the stored charge flows out of the capacitor, reversing the current direction. The current during discharge flows in the opposite direction to that during charging. This change is a fundamental characteristic of capacitor behaviour, reflecting the shift from energy storage to energy release within the circuit.

Temperature can significantly affect the discharge of a capacitor, primarily through its impact on the resistance (R) of the circuit and the dielectric properties of the capacitor. As temperature increases, the resistance of conductive materials generally increases, leading to a higher time constant (τ = RC), and hence, a slower discharge rate. Additionally, the dielectric strength of the capacitor may decrease at higher temperatures, affecting its ability to store charge efficiently. However, the specific effects depend on the materials used in the capacitor and the circuit. In some cases, capacitors may have temperature coefficients designed to compensate for these changes.

The dielectric material in a capacitor influences its capacitance, which in turn affects the time constant, τ = RC. A dielectric material increases the capacitance of a capacitor by reducing the electric field between its plates, allowing it to store more charge for a given voltage. This increased capacitance leads to a higher time constant, resulting in a slower discharge rate. The nature of the dielectric – its dielectric constant and breakdown voltage – plays a crucial role in determining these characteristics. Different materials have varying dielectric constants, impacting the overall effectiveness of the capacitor in storing charge and influencing the time constant in discharging circuits.

Predicting the life span of a capacitor based on its discharge characteristics is complex and depends on various factors beyond just the discharge rate. While the time constant (τ = RC) provides insight into the discharge behaviour, the life span is also influenced by factors like the quality of the dielectric material, the operating voltage, temperature, and environmental conditions. Capacitors often degrade over time due to dielectric breakdown, electrolyte drying (in electrolytic capacitors), or leakage currents. Therefore, while discharge characteristics can give some indication of performance, they are not definitive predictors of lifespan. Regular testing and monitoring of key parameters like ESR (Equivalent Series Resistance) and capacitance loss are more reliable methods for assessing a capacitor's health and estimating its remaining life span.