The usable capacity of a battery is limited by several factors, primarily related to operational conditions and usage patterns. While the fundamental theoretical capacity is determined by the amount and type of active materials, the actual capacity available during use is significantly influenced by various dynamic elements.
According to ScienceDirect, the battery capacity also depends on the operational conditions such as the load, discharge rate, depth of discharge, cut-off voltage, temperature, and cycle history of the battery. These factors play a crucial role in determining how much energy can be extracted from a battery in a given scenario.
Factors Influencing Usable Battery Capacity
Several key operational conditions and historical usage patterns act as practical limits on the amount of energy a battery can deliver. Understanding these factors is essential for predicting battery performance and lifespan.
Operational Conditions
These are conditions under which the battery is currently operating.
- Load: The demand placed on the battery (how much current is being drawn). Higher loads can reduce available capacity due to internal resistance losses.
- Discharge Rate (C Rating): This specifies how quickly the battery is discharged relative to its rated capacity. The reference notes that "Usually, the battery capacity will be specified for a given discharge/charge rating or C rating." Discharging at higher C rates (faster) typically results in less usable capacity compared to slower discharge rates.
- Example: A battery rated at 2000 mAh at a 1C rate might only provide 1800 mAh when discharged at a 5C rate.
- Cut-off Voltage: This is the minimum voltage allowed before the battery is considered fully discharged. Discharging below the recommended cut-off voltage can damage the battery, while setting a higher cut-off voltage limits the usable capacity.
- Temperature: Extreme temperatures (both hot and cold) can significantly impact battery performance and capacity.
- Cold temperatures reduce chemical reaction rates, decreasing available capacity and voltage.
- Hot temperatures can increase chemical degradation, reducing lifespan and potentially affecting immediate capacity and safety.
Usage History
How a battery has been used over its lifetime also limits its current capacity.
- Depth of Discharge (DoD): How deeply the battery is discharged in each cycle affects its lifespan. Repeated deep discharges (high DoD) can accelerate capacity fade compared to shallower discharges (low DoD).
- Cycle History: The total number of charge and discharge cycles a battery has undergone contributes to its aging. Each cycle causes some wear and tear, leading to a gradual reduction in maximum capacity over time – a phenomenon known as capacity fade.
Summary Table of Limiting Factors
| Factor | Description | Impact on Capacity |
|---|---|---|
| Load/Discharge Rate | How quickly energy is drawn | Higher rates often reduce usable capacity |
| Cut-off Voltage | Minimum allowed voltage during discharge | Higher cut-off limits usable capacity |
| Temperature | Operating temperature of the battery | Extremes reduce performance & potentially capacity |
| Depth of Discharge | How deeply discharged in each cycle | Deeper cycles can accelerate capacity fade |
| Cycle History | Total number of charge/discharge cycles | Leads to gradual capacity fade over time |
These operational and historical factors mean that a battery's stated capacity is often a theoretical maximum or a rating specified under specific conditions (like a certain C rate), and the actual capacity experienced by a user can vary significantly.
