A Concept for Assessing the Impact of Enhanced Fast Charging Availability on Battery Operation and Design

摘要

In recent years, increasing efforts have been made to establish fast charging infrastructure as a way to solve the problem of range anxiety associated with battery electric vehicles. Global ventures and societies increase almost annually the amount of fast charging stations and possible charging power. Currently, a European-wide network of 350 kW fast charging stations is in ramp up phase enabling charging times equivalent to refueling times of combustion vehicles. Alongside with increased fast charging possibility, also usage will increase, if vehicles will be adequately equipped. There has been much discussion about fast charging and its impact on battery performance. Fast charging can be defined as reducing charging time by operating the battery beyond the cell manufacturer specifications close to the physical and safety limits of a battery cell. Many parameters affect battery fast charging capabilities, changing and interacting over vehicle lifetime. Recent publications investigated the limiting effects, such as lithium plating and thermal development, and provided different charging regimes to solve the contradictions charging time, accelerated aging and energy density to optimize fast charging procedures. Despite existing literature in this field of research, the choice for the right fast charging strategy is still not trivial to make and depends on an in-depth analysis of all occurring effects over vehicle lifetime. We present an approach to analyze and model the interacting effects during fast charging on battery performance, incorporating electrical and thermal effects up to battery pack level on a life-cycle timescale. Focus is set on verifying and transferring results from a cell level perspective to an automotive life-cycle scale. The modelling approach will reveal the answers to the questions: (1) How does fast charging affect battery pack lifetime performance and what will be the driving mechanisms? (2) How can battery packs be optimally designed in advance and managed by the battery management system in operation, i.e. which charging strategy is optimal in terms of operating current, voltage range and temperature to avoid negative effects on battery lifetime performance and customer acceptance? To do so, adequate cell models will be utilized to imitate cell electrical and thermal behavior and extended with an aging model covering battery degradation under fast charging procedures. The cell model will be scaled to battery pack system level incorporating balancing currents and cell-to cell thermal interdependencies. Realistic automotive use-case scenarios will be applied over a full vehicle life cycle to reveal which phenomenona will occur and finally lead to battery pack aging and failure, e.g. which can be either the weakest cell in the battery pack because of inhomogeneities or the general battery pack average degradation. With the model and a broad understanding of the interdependencies during fast charging at hand, measures can be derived to optimize design and operation of battery packs in electric vehicles with fast charging capability, e.g. battery management strategies can be optimized based on various effects over vehicle lifetime. Also, a strategy can be developed ensuring that charging speed is not limited during long-term operation of the vehicle, since range anxiety will not completely vanish if charging time reliability is not maintained over vehicle lifetime.