Lead-acid batteries are a type of rechargeable battery that uses a chemical reaction to store and release electrical energy. They consist of lead dioxide (PbO2) as the positive electrode, spongy lead (Pb) as the negative electrode, and a diluted sulfuric acid (H2SO4) electrolyte. The chemical reactions during discharge and charge are as follows:
During discharge:
PbO2 + Pb + 2H2SO4 → 2PbSO4 + 2H2O
During charge (recharge):
PbSO4 + PbSO4 + 2H2O → PbO2 + Pb + 2H2SO4
Characteristics of a lead-acid cell include its voltage, capacity, and characteristic curves. Let's focus on the characteristic curves.
Voltage vs. State of Charge (SoC) Curve:
This curve illustrates the relationship between the voltage of the battery and its state of charge. As the battery discharges, the voltage gradually decreases. Conversely, as the battery charges, the voltage increases. The curve is not linear; the voltage drop tends to be steeper as the battery discharges further.
Voltage vs. Current (Load) Curve:
This curve shows how the voltage of the battery changes in response to different levels of current draw (load). At low current draw, the battery voltage remains relatively stable. However, as the current draw increases, the internal resistance of the battery causes the voltage to drop more significantly. This is often referred to as the "Peukert effect."
Charge and Discharge Efficiency Curve:
Lead-acid batteries are not 100% efficient in terms of energy conversion during charge and discharge cycles. The efficiency decreases at high discharge rates and can also be affected by the depth of discharge. This curve illustrates the relationship between the energy input during charging and the energy output during discharging.
Temperature vs. Capacity Curve:
Temperature can significantly affect the performance of lead-acid batteries. At lower temperatures, the capacity of the battery decreases, and at higher temperatures, the self-discharge rate increases. The curve depicts how battery capacity changes across different temperature ranges.
Cycle Life Curve:
The cycle life curve shows the number of charge and discharge cycles a lead-acid battery can undergo before its capacity drops below a certain threshold (e.g., 80% of its initial capacity). Cycle life is influenced by factors such as depth of discharge, charging methods, and operating temperature.
These characteristic curves are important for understanding the behavior and performance of lead-acid batteries under different conditions. It's worth noting that while lead-acid batteries have been widely used in applications such as automotive starting, lighting, and ignition (SLI) systems, they are being gradually replaced by other battery technologies like lithium-ion due to their higher energy density and longer cycle life.