The discharge characteristics of lead-acid batteries are influenced by various factors including temperature, discharge rate, and battery age.
Voltage Profile During Discharge
- Initial Voltage Drop: When a lead-acid battery starts discharging, there is an initial voltage drop from its fully charged state (typically around 12.6 to 12.8 volts for a 12V battery).
- Stable Discharge Phase: The voltage remains relatively stable during the bulk of the discharge cycle. This is the phase where the battery delivers most of its capacity.
- End-of-Discharge: As the battery nears the end of its capacity, the voltage drops more rapidly. For a 12V battery, this is usually around 10.5 volts. Discharging below this voltage can cause damage to the battery.
Discharge Rate (C-Rate)
- C-Rate Definition: The C-rate indicates the rate at which the battery is discharged relative to its maximum capacity. For example, a 1C rate means the battery will be fully discharged in one hour.
- Effect of High C-Rate: Higher discharge rates (e.g., 2C or more) lead to higher internal resistance and heat generation, causing a steeper voltage drop and reduced capacity.
- Effect of Low C-Rate: Lower discharge rates (e.g., 0.1C) result in a more stable voltage profile and higher usable capacity.
Depth of Discharge (DoD)
- Shallow Discharge: Regularly discharging only a small portion of the battery’s capacity (e.g., 20-30%) can extend the battery’s overall lifespan.
- Deep Discharge: Frequently discharging the battery to a high depth (e.g., 80-100%) reduces the number of cycles the battery can handle before its capacity significantly degrades.
Temperature Effects
- High Temperatures: Increased temperatures can improve the discharge efficiency temporarily but accelerate the aging process and reduce overall battery lifespan.
- Low Temperatures: Decreased temperatures can reduce the battery’s discharge capacity and efficiency due to increased internal resistance.
Peukert’s Law
- Peukert’s Coefficient: This law describes how the available capacity of a lead-acid battery decreases with increasing discharge rates. A higher Peukert exponent indicates greater loss of capacity at higher discharge rates.
- Practical Implication: At higher discharge rates, the battery’s capacity appears to be lower than its rated capacity because it cannot maintain high efficiency.
Discharge Curves
- Typical Curves: Discharge curves typically show the battery voltage against the state of charge (SoC) or the time. These curves help in understanding the behavior of the battery under different loads.
- Interpretation: A steeper curve indicates a higher discharge rate, while a flatter curve suggests a more moderate rate.
Capacity and Cycle Life
- Cycle Life: The number of complete charge-discharge cycles a battery can perform before its capacity drops to a certain percentage of its original capacity (usually 80%).
- Factors Affecting Cycle Life: Depth of discharge, discharge rate, temperature, and maintenance practices.
Self-Discharge
- Rate of Self-Discharge: Lead-acid batteries naturally lose charge over time when not in use. The rate can be around 3-5% per month at 25°C (77°F).
- Impact of Temperature: Higher temperatures increase the self-discharge rate, while lower temperatures decrease it.
Understanding these discharge characteristics is crucial for optimizing the performance and lifespan of lead-acid batteries in various applications, from automotive to renewable energy storage systems.
