ebc - aging of batteries

your battery too young to die

Battery degradation

Degradation of battery leads to a loss in capacity and fade in performance/power. This leads to a reduction in functionality, micro-short-cuts and reliability (1).

Various parameters are influencing battery-cell aging:

• Thermodynamics
• Electric-chemical Stress
• Mechanical Stress

Degradation of the Li-Ion battery cell occurs in all components of the cell, but some are dominating the aging process and can be differentiated in calendrical and cycling aging (2). The calendrical aging is most influenced by stress factors like e.g. temperature and state-of-charge (SoC) (3). While cycling aging is most influenced by the charging and discharging process. These dependencies have been shown in various tests arrangements and studies e.g. (4), (5), (6).

Inhomogeneity in aging

More and more sophisticated production methods and increase in experience in the production process has reduced the spread of various parameters especially capacity and internal resistance significantly. Nevertheless S. Rohr could prove that cell manufacturers grade cells and use the high graded cells for automotive purposes only (7). In addition, battery design, cell design and method of production, as well as mechanical stress during operation such as vibration, false battery and car design / battery integration and other sources of high pressure are an initial start for lithium plating and other degradation processes.

Bach et.al. showed the influences of mechanical stress in postmortem analysis (8).

Cell spreading

Cell capacities and internal resistance within the system show great variations. S.Paul et.al. in their studies found a distribution in initial capacity spread of 1,3% deviation in capacity and 5.8% in internal resistance. The aged cells had an average capacity loss of 18% with a spread of 10% between the weakest (25%) and strongest (14%) cell (9). Baumann et.al. found in their studies for prismatic cells in a Daimler Vito-e-cell a deviation of 6,9% of capacity spread between the weakest and strongest cell (10).

Stopping and reversing cell degradation

If specific factors as described above are influencing the SoH of the individual cells and cell aging, specific operation, for example, less stress or more moderate conditions can lead to the opposite effect and can delay or even withdraw lithium plating (11) and other degradation processes.