Model-based Validation of Lithium-ion Batteries for Vehicle Use
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C OVER STORY Energy Systems
LOAD AND CAPACIT Y OF BATTERIES IN VEHICLE OPERATION
AUTHORS
Dr. Franz Langmayr is Managing Director of Uptime Engineering in Graz (Austria).
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Ulrich Penitz is Student Research Assistant at Uptime Engineering in Graz (Austria).
The actual vehicle duty cycles of cus tomers represent the references against which validation is evaluated. The level and dynamics of the electrical and ther mal load for BEV operation result from the vehicle operating load. These data can be simulated for future operational profiles. The quasi-stationary charging process is defined by the voltage level. In hybrid operation, the operating strat egy dominates the load process. The determination of such references there fore requires extensive load simulation and measurement. Relevant data for life cycle assess ments of batteries result from the current level and the load dynamic. Thus, extreme users can be identified, like the city driver, who is AC charging and the sporty highway driver, with DC charging. In commercial vehicles the differences in use are pronounced.
Model-based Validation of Lithium-ion Batteries for Vehicle Use The reliability and lifetime of lithium-ion batteries is essential for the long-term acceptance and economic efficiency of electric and hybrid vehicles. Damage mechanisms of lithium-ion systems have been documented in the literature, but they are insufficiently applied for the design of validation programs. Using two damage mechanisms, Uptime Engineering explains how this understanding can be used for test optimization.
Borderline cases would be represented by the city bus and the long-distance truck. For machines the extremes lie for example in crane operation and the combine harvester. The resulting dif ferences in duty cycles can be distinct. They must be considered for target iden tification when a battery system is vali dated for several applications or climates. The permissible electrical power is the relevant property of batteries. Char ging and discharging requires the cyclic intercalation and removal of lithium ions in the electrodes. This represents the limiting factor for the current den sity. Because batteries are ionic conduc tors, conductivity decreases sharply with temperature. This effect is intensified by resistance jumps at internal boundary surfaces. The sensitivity makes external control of the battery load in vehicle use necessary. Protective operation is achieved by temperature control and by limiting the current density. The tradeoff between charging time and service life is evident. It is addressed by reducing the charging power with rising State of Charge (SoC). Narrow temperature win dows must be guaranteed despite only small differences, which is a challenge for the cooling system, especially during charging. ATZ electronics worldwide 12|2020
VALIDATION
The requirements for reliability demon stration are derived from the lifetime risks. A failure potential analysis, like a fault tree, identifies damaging operating states and
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