Battery energy refers to the energy that a battery can release under a certain discharge regime, usually expressed in W·h or kW·h. Battery energy is mainly divided into the following types:
(1) Theoretical Energy
Assuming that the battery is in equilibrium during discharge, its discharge voltage maintains the value of the electromotive force (E), and the utilization rate of the active material is 100%, i.e., the discharge capacity is the theoretical capacity, then the energy output under these conditions is the theoretical energy W₀, i.e.
(2) Actual Energy
Actual energy refers to the energy actually output by the battery during discharge. Numerically, it is equal to the integral of the battery's actual discharge voltage, discharge current, and discharge time, i.e.
In practical engineering applications, the actual energy of a battery is often estimated using both the rated capacity of the battery pack and the average discharge voltage of the battery.
Because the active material cannot be fully utilized, the battery's operating voltage is always less than its electromotive force, therefore the battery's actual energy is always less than its theoretical energy.
(3) Total Energy
Total energy refers to the total electrical energy output of a battery during its lifespan, measured in W·h.
(4) Charging Energy
Charging energy refers to the electrical energy fed into the battery through charging, measured in W·h.
(5) Discharging Energy
Discharging energy refers to the electrical energy output by the battery during discharge, measured in W·h.The energy of a battery, or the energy that a unit mass or unit volume of battery can output, is correspondingly called mass energy.Battery density (W·h/kg) and volumetric energy density (W·h/L), also known as specific energy or volumetric energy, are important indicators for evaluating the quality of power batteries. In electric vehicle applications, the specific energy of a battery affects the overall vehicle weight and driving range, while the volumetric energy affects the battery's placement space. Specific energy is also a crucial indicator for comparing the performance of different types of batteries. Specific energy is divided into theoretical specific energy (W₀) and actual specific energy (W').
Theoretical specific energy corresponds to theoretical energy, referring to the energy that can theoretically be output when a unit mass or unit volume of battery reactants is fully discharged. Actual specific energy corresponds to actual energy, representing the actual energy released when a unit mass or unit volume of battery reactants is fully discharged. It is characterized by the ratio of the battery's actual output energy to its mass (or volume).
or
In the formula, C--represents the mass of the battery; V--represents the volume of the battery. Due to various factors, the actual specific energy of a battery is much lower than its theoretical specific energy. The relationship between actual and theoretical specific energy can be expressed as follows:
In the formula, K_E represents voltage efficiency; K_B represents reaction efficiency; and K_m represents mass efficiency.
In the application of power batteries in electric vehicles, the actual specific energy of the battery pack is lower than the specific energy of the individual battery cells because battery pack installation requires corresponding battery boxes, connecting wires, current and voltage protection devices, and other components. The specific energy of the battery pack is calculated by multiplying the specific energy of the individual battery cells by the packaging factor. The packaging factor for a typical battery is 0.6 to 0.8. As the design level of the battery pack improves, the integration of the battery pack increases. Therefore, the mass specific energy of the battery pack is often an important indicator of battery pack performance. Generally speaking, the mass specific energy of the battery pack is more than 20% lower than the specific energy of the individual battery cells.
