One of the most important performance characteristics of energy storage batteries is their discharge performance. To characterize the battery's discharge behavior under different conditions, it is necessary to measure the battery's discharge curve, which is usually a curve showing the change of discharge voltage over time. Different discharge conditions are characterized by discharge strategies, and different discharge strategies will result in different discharge curves. Discharge strategies typically include discharge method, discharge current, termination voltage, and ambient temperature.
Discharge method
There are three ways a battery can discharge: constant current discharge, constant resistance discharge, and constant power discharge. Typical discharge curves are shown in Figure 1-5, which illustrates the changes in discharge current, voltage, and power over discharge time under these three discharge modes.
During constant-resistance discharge, the battery's operating voltage and discharge current gradually decrease over time. Similarly, under constant-current discharge, the operating voltage also decreases as the discharge process continues. This decrease in operating voltage with prolonged discharge time is due to the increase in the battery's internal resistance. Furthermore, with the increasing use of battery power in power tools, electric vehicles, and other applications, constant-power discharge is becoming more prevalent. During constant-power discharge, the battery voltage continuously decreases while the discharge current continuously increases as the discharge progresses.
Discharge current
During battery operation, the current it outputs is called the discharge current. The discharge current is also commonly referred to as the discharge rate, and is often expressed using the hourly rate (also known as the hourly rate) and the multiplier.
The discharge rate refers to the rate at which a battery discharges, measured in discharge time. Specifically, it's the time required to fully release the battery's capacity using a specific discharge current, usually expressed in hours (h). For example, for a battery with a rated capacity of 10 amp-hours (A·h), if it's discharged with a current of 2A, the corresponding discharge rate is 5 hours (10A·h/2A=5h), meaning the battery is discharging at a 5-hour rate.
Discharge rate refers to the current value, expressed as a multiple of the battery's rated capacity, when the battery's full capacity is fully released within a specific time. For example, 2C discharge means the discharge current is twice the battery's rated capacity, usually represented by 2C (where C represents the battery's rated capacity). For a battery with a rated capacity of 10A·h, 2C discharge (there is a dimensional issue here, i.e., the units of capacity and current are not the same, but this is a common usage, so it will not be changed) means the discharge current is 2 x 10 = 20 (A), corresponding to a discharge rate of 0.5h. Different types and designs of batteries have different adaptability to discharge conditions: some are more suitable for low-current discharge, while others perform better at high currents. Generally, discharge rates less than or equal to 0.5C are called low rates; those between 0.5C and 3.5C are called medium rates; those between 3.5C and 7C are called high rates; and those exceeding 7C are called ultra-high rates.
Termination voltage
During battery discharge, the initial voltage value is defined as the starting operating voltage; when the voltage drops to a threshold where further discharge is no longer suitable, this voltage point is called the termination voltage. The specific value of this termination voltage is usually set by the tester based on actual test requirements and past experience.
The set termination voltage varies depending on the different discharge conditions and their impact on battery capacity and lifespan. Lower termination voltages are typically used in low-temperature environments or under high-current discharge conditions, while higher termination voltages are usually set under low-current discharge conditions. This is because polarization between the battery electrodes increases significantly during low-temperature or high-current discharge, resulting in incomplete utilization of active materials and a faster voltage drop. Therefore, appropriately lowering the termination voltage helps release more energy. Conversely, when using low-current discharge, the active components in the battery are utilized more fully. In this case, increasing the termination voltage to limit deep discharge can effectively extend the overall battery lifespan.
Ambient temperature
As shown in Figure 1-6, ambient temperature has a significant impact on the discharge curve. At higher temperatures, the discharge curve exhibits a relatively gentle trend; however, as the temperature decreases, this change becomes increasingly drastic. The fundamental reason is that at low temperatures, the migration rate of ions decreases, leading to an increase in ohmic internal resistance. In extreme cases, if the temperature is too low, the electrolyte may freeze, thus hindering the normal discharge process of the battery. Furthermore, at lower temperatures, electrochemical polarization and concentration polarization are correspondingly enhanced, further accelerating the decay rate of the discharge curve.
Figure 1-6 Discharge curves of lead-acid batteries at different ambient temperatures
Capacity and specific capacity
Battery capacity refers to the amount of electricity that can be obtained from a battery under certain discharge conditions. The unit is usually expressed as ampere-hour (Ah). Depending on the actual situation, battery capacity can be further divided into theoretical capacity, actual capacity, and rated capacity.
Theoretical capacity (Co) refers to the amount of electricity that can be provided under ideal conditions when the active material fully participates in the electrochemical reaction of the battery. This value is calculated based on the mass of the active material, following Faraday's law. Faraday's law states that there is a direct proportional relationship between the mass of the material participating in the reaction at the electrode and the amount of charge it transfers; when 1 mol of active material participates in the electrochemical process of the battery, it can release a charge equivalent to 26.8 A·h or 1 farad (F). Therefore, the following calculation formula exists:
In the formula, m is the mass of the active substance when it reacts completely; n is the number of electrons gained or lost during the flow reaction; and M is the molar mass of the active substance.
In the formula, K is called the electrochemical equivalent of the active substance.
As shown in equation (1.5), the theoretical capacity of an electrode is related to the mass of the active material and the electrochemical equivalent. With the same mass of active material, the smaller the electrochemical equivalent, the larger the theoretical capacity. The electrochemical equivalents of some electrode materials are shown in Table 1-3.
Table 1-3 Electrochemical Equivalents of Some Electrode Materials
| Negative Electrode Material | Density (g/cm³) | Specific Capacity (mA·h/g) | Positive Electrode Material | Density (g/cm³) | Specific Capacity (mA·h/g) |
|---|---|---|---|---|---|
| H₂ | - | 0.037 | O₂ | - | 0.30 |
| Li | 0.534 | 0.259 | SOCl₂ | 1.63 | 2.22 |
| Mg | 0.74 | 0.454 | AgO | 7.4 | 2.31 |
| Al | 2.699 | 0.335 | SO₂ | 1.37 | 2.38 |
| Fe | 7.85 | 1.04 | MnO₂ | 5.0 | 3.24 |
| Zn | 7.1 | 1.22 | NiOOH | 7.4 | 3.42 |
| Cd | 8.65 | 2.10 | Ag₂O | 7.1 | 4.33 |
| (Li)Cl₂ | 2.25 | 2.68 | PbO₂ | 9.3 | 4.45 |
| Pb | 11.34 | 3.87 | I₂ | 4.94 | 4.73 |
In addition, the concepts of actual capacity and rated capacity are often used. Actual capacity refers to the total amount of electricity a battery can provide under specific discharge conditions. Actual capacity is limited not only by the theoretical maximum value but also by the specific discharge conditions.
Rated capacity, on the other hand, is a standard set for the battery during the design and manufacturing process; that is, the minimum output capacity that the battery should achieve under specified discharge conditions, also known as nominal capacity.
When comparing different types of batteries within the same series, specific capacity is typically used for evaluation. Specifically, specific capacity refers to the amount of electricity a battery can provide per unit mass or volume, i.e., mass specific capacity (Ah/kg) and volumetric specific capacity (Ah/L). It is important to note that when calculating the mass and volume of a battery, in addition to considering the electrode materials and electrolyte, other components of the battery must also be taken into account, such as the casing, separator, and related conductive components. Especially for storage batteries and fuel cells, the total mass and volume also include all necessary auxiliary equipment, such as tanks for storing liquids, activation devices (for storage batteries), or active material storage and supply systems, control systems, heating units, etc. (for fuel cells).
By introducing the concept of specific capacity, we can compare the performance of batteries of different types and sizes. Battery capacity is divided into theoretical capacity and actual capacity; correspondingly, specific capacity also has theoretical and actual aspects.
Energy and specific energy
Battery energy refers to the total electrical energy output by the battery when performing work under specific discharge conditions, generally expressed in watt-hours (W·h). Battery energy also has a theoretical energy and an actual energy.
Assuming the battery remains in equilibrium during discharge and its discharge voltage is constant equal to its electromotive force, and also assuming all active materials participate in the chemical reaction, then the energy provided by the battery should be equal to its theoretical maximum energy Wo.
The theoretical energy of a battery is the maximum non-volume work done by the battery under constant temperature, constant pressure, and reversible discharge conditions.
Actual energy (W) refers to the energy actually provided by a battery under certain discharge conditions. It is numerically derived by multiplying the actual capacity by the average operating voltage. Because the active materials inside the battery cannot be fully utilized, and its operating voltage is usually lower than the theoretical electromotive force, actual energy is always less than theoretical energy.
Specific energy refers to the energy released by a battery per unit mass or unit volume. The energy output per unit mass of battery is defined as mass specific energy, typically measured in watt-hours per kilogram (Wh/kg). The energy output per unit volume of battery is defined as volumetric specific energy, typically expressed in watt-hours per liter (Wh/L). Furthermore, the concept of specific energy can be further subdivided into theoretical (W) and actual (W), where theoretical mass specific energy can be calculated using equation (1.9):
In the formula, K+ is the electrochemical equivalent of the positive electrode material; K- is the electrochemical equivalent of the negative electrode material; and E is the battery electromotive force.
Power and specific power
Battery power refers to the energy output of a battery per unit time under specific discharge conditions, and its unit of measurement is watt (W) or kilowatt (kW). When this output power is considered in relation to the battery's mass or volume, the concept of specific power is obtained. Specifically, mass specific power measures how many watts of power a unit mass of battery can provide, and its unit is W/kg; while volumetric specific power reflects the power generated by a unit volume of battery, and its corresponding unit is W/L.
Power and specific power indicate the discharge rate of a battery. A higher battery power means the battery can discharge at high current or high rates. For example, a zinc-silver battery can achieve a specific power of over 100 W/kg when discharging at a medium current density, indicating low internal resistance and good high-rate discharge performance. In contrast, a zinc-manganese dry cell battery can only achieve a specific power of 10 W/kg when operating at a low current density, indicating high internal resistance and poor high-rate discharge performance. Similar to battery energy, power also has theoretical power and actual power.
The theoretical power of a battery can be expressed as:
In the formula, t is time; Co is the theoretical capacity of the battery; and I is the current.
The actual power of the battery should be：
In the formula, I2R represents the power consumed by the battery's internal resistance. This power is useless to the applied load; it is essentially converted into heat energy and released as heat.
Cycle life
For batteries, cycle life, or usage cycle, is one of the key indicators for evaluating battery performance. Each complete charge-discharge cycle is considered a period of time for a battery.
Under specific charge-discharge conditions, the number of cycles a battery can withstand before its capacity drops to a certain specified value is defined as its cycle life or usage cycle. The longer the cycle life, the better the battery's cycle performance.Different types of batteries exhibit different cycle lives; for example, nickel-cadmium batteries can achieve thousands of cycles, while zinc-silver batteries have relatively fewer cycles, some even less than a hundred. It is worth noting that even batteries of the same type can have different cycle lives due to differences in their internal structure.
The cycle life of a battery is affected by a variety of factors. In addition to proper use and maintenance, the following key aspects also apply: ① During charge-discharge cycles, the surface area of the active material gradually decreases, leading to an increase in operating current density and intensified polarization; ② Active components on the electrodes may detach or transfer; ③ During battery operation, some electrode materials may be corroded; ④ Dendrites formed on the electrodes during cycling may cause short circuits inside the battery; ⑤ The separator may be damaged; ⑥ The crystal morphology of the active material changes during repeated charge-discharge cycles, thereby reducing its activity.
Storage performance
Battery storage performance refers to the degree of natural energy loss within the battery when it is in an open-circuit state under specific environmental conditions (such as temperature and humidity). This phenomenon is also known as self-discharge. If the proportion of energy loss during storage is small, it indicates that the battery has excellent storage performance.
When a battery is in an open-circuit state, although it is not supplying electrical energy to the outside, it still undergoes a self-discharge process. This phenomenon is mainly due to the thermodynamic instability of the electrodes in the electrolyte environment, leading to spontaneous redox reactions between the electrodes. Even under dry conditions, if the seal is not tight enough, the infiltration of external factors such as air or moisture can still trigger a self-discharge effect inside the battery.
The self-discharge rate can also be expressed as the number of days it takes for the battery's capacity to decrease to a specified value when stored, known as shelf life.There are dry shelf life and wet shelf life. For example, a storage battery, without adding electrolyte before use, can be stored for a long time; such a battery can have a long dry shelf life. Storage with electrolyte is called wet storage; wet storage results in a stronger self-discharge effect and a relatively shorter wet shelf life. For example, a zinc-silver battery can have a dry shelf life of 5–8 years, while its wet shelf life is typically only a few months.