The discharge curve is drawn using the working voltage of the battery as the vertical axis, discharge time, capacity, state of charge (SOC), or discharge depth (DOD) as the horizontal axis. The information contained in the charge discharge curve is very rich, including capacity, energy, working voltage and voltage platform, the relationship between electrode potential and state of charge, etc. The main data recorded during discharge testing is the time evolution of current and voltage, and many parameters can be obtained from these basic data. Measuring the discharge curve of a battery is one of the basic methods for studying battery performance. Based on the discharge curve, it is possible to determine whether the battery's performance is stable and the maximum current allowed for stable operation.
Lithium battery discharge curve stage
The discharge curve basically reflects the state of the electrode, which is the superposition of the changes in the state of the positive and negative electrodes. The voltage curve of lithium batteries during the entire discharge process can be divided into the following three stages:
1. In the initial stage, the terminal voltage of the battery drops rapidly, and the larger the discharge rate, the faster the voltage drops;
2. The battery voltage enters a stage of slow change, which is called the platform area of the battery. The smaller the discharge rate, the longer the platform area lasts, the higher the platform voltage, and the slower the voltage drop.
3. As the battery level approaches full discharge, the load voltage of the battery begins to drop sharply until it reaches the discharge cut-off voltage.
Common discharge curves of lithium batteries
When a lithium battery is discharged, its working voltage always changes continuously over time. The most basic form of the discharge curve is the voltage time and current time curves. Common discharge curves include voltage capacity (specific capacity) curve, voltage energy (specific energy) curve, voltage SOC curve, etc.
1. Voltage
In the discharge test of lithium-ion batteries, voltage parameters mainly include voltage platform, median voltage, average voltage, cut-off voltage, etc.
Platform voltage refers to the voltage value corresponding to the minimum voltage change and the significant capacity change.
The median voltage is the voltage value corresponding to half of the battery capacity. For materials with obvious platforms, such as lithium iron phosphate and lithium titanate, the median voltage is the platform voltage.
The cut-off voltage refers to the minimum voltage allowed for battery discharge. If the voltage is lower than the discharge cut-off voltage and discharge continues, the voltage at both ends of the battery will rapidly decrease, forming excessive discharge. Overdischarge may cause damage to the electrode active material, loss of reaction ability, and shorten the battery life.
2. Capacity
The electrical energy that a battery can output for external work under certain conditions is called the energy of the battery, usually expressed in units of Wh.
Battery capacity refers to the amount of electricity released by a battery under a certain discharge system (under certain discharge current I, discharge temperature T, and discharge cut-off voltage V conditions), which characterizes the battery's ability to store energy, expressed in Ah or C. Capacity is influenced by many factors, such as discharge current, discharge temperature, etc. The capacity is determined by the amount of active substances in the positive and negative electrodes. The larger the discharge current of the battery, the smaller the output capacity; The discharge temperature of the battery decreases, resulting in a decrease in output capacity.
3. Energy
Theoretical energy: The discharge process of the battery is in a balanced state, the discharge voltage maintains the value of electromotive force (E), and the utilization rate of the active substance is 100%. Under these conditions, the output energy of the battery is the theoretical energy, which can be used to reverse the maximum work done by the battery under constant temperature and pressure.
Actual energy: The actual output energy of a battery during discharge is called actual energy. At room temperature, the energy (Wh) released when the battery discharges at a current of 1I1 (A) and reaches the termination voltage is called rated energy.
Specific energy: The energy provided by a battery per unit mass and per unit volume, known as mass specific energy or volume specific energy, also known as energy density. The unit is wh/kg or wh/L.
4. SOC, DOC
SOC (State of Charge) is the state of charge, which represents the ratio of the remaining capacity of a battery after being used for a period of time or left unused for a long time to its fully charged capacity at a certain discharge rate.
DOD (Depth of Discharge) is a measure of discharge depth, representing the degree of discharge, as a percentage of discharge capacity to total discharge capacity. The depth of discharge is closely related to the lifespan of the battery: the deeper the discharge depth, the shorter its lifespan. The relationship between the two is SOC=100% - DOD.