Battery Degradation and How It Impacts Vehicle Performance
Since the release of the first commercially successful low-cost fully electric vehicle (EV) this century as the Nissan Leaf by Nissan in 2010, as well as the commercial success of the higher end Tesla Model S which was first released in 2012, whose introductions proved that electric cars could perform as well as their petrol and diesel equivalents and have a decent battery range, there has been a huge increase in the number of vehicle manufacturers (OEMs) who have started to offer their own fully electric vehicle models. The initial surge in sales of EVs was partly due to emissions related government incentives in various countries since the high cost of Lithium Ion (Li-Ion) battery packs made the price of these vehicles uncompetitive in comparison to traditional gasoline and diesel vehicles.
In recent years, however, the price of Li-Ion batteries and battery packs has been steadily decreasing, bringing down the vehicle costs to be more competitive with that of a traditional vehicle. When calculating the EV value proposition, arguably the most critical factor and point of debate are EV battery lifetimes which affects the usable lifetime of the vehicle, and the objective of this paper is to explore current battery technologies, battery degradation and how it affects the battery lifetime, ways in which this can be mitigated and finally conclude as to the current and future EV value proposition against their traditional diesel and gasoline counterparts.
Effect and Measurement of Degradation
There are various parameters of battery performance which are affected by degradation. Typical parameters shown below can be used to determine the state of degradation or conversely the state of health (SOH) and can also be used by battery monitoring systems for this purpose:
Internal Resistance. When batteries internal resistance is increased due to degradation the output current is restricted, the battery heats up more due to power loss and the output voltage is lowered.
Capacity. A batteries capacity or its ability to store charge is reduced due to degradation.
Self-Discharge Rate. A batteries self-discharge rate increases due to degradation and loses its ability to hold charge.
While the various battery components undergo different aging mechanisms, the anode undergoes a multitude of aging mechanisms that degrade the electrochemical performance of the lithium-ion battery. A few of these degradation mechanisms for Li-Ion batteries are described below:
Anode Impedance. The SEI layer is a key element of traditional Li-ion batteries and acts as a safety feature by maintaining a protective barrier between the negative electrode and the electrolyte. The growth of a passive surface layer on top of the SEI layer is caused by by-products which plate the SEI layer during unwanted conditions such as overcharging, short-circuiting and temperature. This passive surface layer creates resistance to lithium-ion flow, which results in a rise in the charge transfer resistance and the impedance of the anode.
Anode Degradation Due to Structural Changes. Cycling the lithium-ion batteries at a high discharge rate and high state of charge (SOC) induces mechanical strain on the graphite lattice of the anode electrode. The nature and orientation of the graphite particles influences the reversible capacity of the anode.
Prevention of Battery Degradation
By mitigating the degradation mechanisms as much as possible, the battery performance and life cycle can be extended. This can be done by proper management of the battery during storage and use during charging and discharging. These are functions such as:
Charge Control. Over-current and over-voltage prevention during charging. This could be done by adhering to a recommended charge profile to ensure that the maximum recommended charging current is not exceeded.
Discharge Control. Over-current and under-voltage prevention during discharging. This could be done by implementing a cut-off threshold for voltage and maximum current.
Temperature Control. Maintaining the environment at operating temperature if possible, or shut down battery use outside of its operating environment range.
Battery Balance. Battery Balancing ensures that a pack is able to be properly utilized since we will have to stop charging when any of the pack cells reach their maximum voltage or stop discharging when any of the cells reach their minimum voltage. The goal of balancing is to try and maintain the cells at more or less the same voltage to ensure that this cut-off point is not reached prematurely due to an imbalanced pack. Battery Balancing could also be seen as a form of degradation prevention since it could, for example, prevent an over-current situation in a cell at a different voltage to the pack cell average, when individual cell currents are not being monitored.
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