We investigate the evolution of battery pack capacity loss by analyzing cell aging mechanisms using the “Electric quantity – Capacity Scatter Diagram (ECSD)” from a system point of view. The results show that cell capacity loss is not the sole contributor to pack capacity loss.
The cells are connected in series at the beginning of the second stage, and the environment is kept unchanged. The battery pack is cycled 200 time at a 1C charge and discharge rate, during which it is also rested for 10 days after the 60th cycle so as to simulate a real pack aging process which should also consider calendar aging.
The battery’s aging is generally affected by three factors: the active present in the cells, the storage and the length of time it remains idle. During storage, batteries self-discharge and their contents are prone to decomposition.
Impedance growth of an aged battery pack with cells connected in series is simply the sum of the impedance growth of each cell, while capacity loss of an aged pack is more complex. Hence, we will only focus on capacity loss of battery packs and impedance growth of single cells will not be addressed in this paper when we refer the term “cell aging”.
Note that aging doesn’t necessarily mean the battery will be dead after a certain amount of time. Battery aging is expressed in % of remaining capacity, which is measured at a 1C discharge rate, much higher than normal use rate of a typical battery.
The surrounding overall system - pack or vehicle - is relevant in that it defines the boundary conditions to which the battery cell is exposed. Therefore, to prevent premature aging, the influences of the critical factors must be uncovered and specifically translated into hardware design and operational strategy requirements.