Cycle life of lithium-ion power battery
During the use of electric vehicles and other new energy vehicles, the lithium-ion power battery as the power source is in different charging and discharging states, and at the same time is affected by external conditions such as vibration and temperature. As the number of charging and discharging increases, the capacity of lithium-ion power batteries will inevitably have varying degrees of attenuation. The capacity loss of lithium-ion batteries is divided into reversible loss and irreversible loss: reversible loss can be restored by recharging, generally refers to self-discharge; irreversible loss can not be restored by recharging. The cycle life of lithium-ion power batteries is an important parameter for battery production and use, and the attenuation of battery capacity is a long-term and complicated process of change. Accurately detecting or predicting the life state of power batteries is a common concern for automobile manufacturers and users. This section mainly introduces the relevant regulations of lithium-ion power battery life.
The life of a lithium-ion power battery usually includes storage life, service life and cycle life. The storage life refers to the time that the battery has been stored in a static state until it expires. Service life refers to the total discharge time accumulated before the battery fails. Cycle life refers to the total number of charging and discharging of the battery before failure. The cycle life of lithium-ion power batteries is directly related to the cost-effectiveness of the vehicle and the performance of the vehicle. Quick and accurate evaluation of the cycle life of the battery is also one of the key issues that power batteries must solve urgently. At present, most studies often use cycle life to study the life state of lithium-ion power batteries. This article focuses on the cycle life of lithium-ion power batteries.
The cycle life defined in the “USAB Battery Test Manual” is the number of cycles that the battery can perform before it reaches the end-of-life conditions when the battery is subjected to a cyclic charge and discharge test according to the standard charge and discharge system. “GB/Z18333.1–2001 Lithium-ion Battery for Electric Road Vehicles” stipulates that lithium-ion power batteries follow the prescribed charging and discharging system (charged until the voltage reaches 4.2V, and then discharged with 1I3 current to 80% of the rated capacity) Carry out the charge and discharge cycle until the battery capacity decays to 80% of the rated capacity, then the total number of charge and discharge cycles in the whole process is its cycle life. According to IEE1188-1996, when the remaining capacity of the battery is less than 80% of the rated capacity of the new battery, the performance of the battery cannot meet the needs of the vehicle and should be replaced. Therefore, the decay of the battery capacity to 80% of the rated capacity is often used as the end of battery life. SOH is generally used to represent the health of the battery. It is defined as the percentage of the remaining dischargeable capacity of the battery to the rated capacity under standard operating conditions, as shown in the formula.
In the formula, Qres represents the remaining dischargeable capacity of the battery, and Qrat represents the rated capacity of the battery.
The life of lithium-ion power battery is affected by the design, production, use conditions and other aspects. For lithium-ion power battery packs for vehicles, its life is also affected by the design and consistency of the battery pack. From the perspective of use, vehicle manufacturers and users are most concerned about the life status of the power battery system, not just the life of the battery cells or individual modules. The main factors affecting the life of lithium-ion power batteries include: battery design plan, production level, battery pack structure design, group structure, monomer consistency, vehicle usage conditions, charge and discharge current intensity, depth of discharge, SOC working window, and charging system Wait.