Influence of the vehicle-to-grid strategy on the aging behavior of lithium battery electric vehicles

Abstract

The main goal of this paper is to study the effect of a vehicle-to-grid (V2G) strategy on the lifetime of two different lithium-ion batteries. The work investigates how the aging effect on the electric vehicles’ (EV) battery packs due to the additional V2G use can be reduced: it is assumed that the grid is able to identify the cars within the fleet for which the ulterior aging effects caused by V2G usage are restrained in respect of the others. The chosen EVs have to contain enough energy to satisfy the grid requests in terms of power regulation. In order to analyze the possible effects on the EVs due to the mentioned strategy, a V2G simulation environment has been implemented. The system consists of 100 EVs and a grid management strategy subsystem. Each EV is represented by a battery electrical model based on electrical impedance spectroscopy (EIS) data and an aging prediction model parameterized through accelerated aging tests. In order to reproduce real scenario conditions, both the electrical battery model and the aging prediction model have been parameterized for two different cells, a LiFePO4-cathode based and an NMC-cathode based lithium-ion cell. In particular, the accelerated aging tests have been carried out for more than one year, both for calendar and cycling operation, involving around 45 cells for each of the two technologies. The grid subsystem is represented by an algorithm which is able to consider information in terms of aging and type of battery installed in the EV. This subsystem helps to make decisions related to the optimal additional use of each car for a V2G operation. In order to show the applicability and feasibility in terms of battery pack lifetime of the considered V2G management strategy, different scenarios for a period of one year have been simulated. These scenarios consider two different locations with two significantly distinct ambient temperatures, in which the starting conditions of each car in terms of aging state have been selected randomly. The implemented system can be used as a perfect tool to test different grid strategies, taking the aging of the EVs as well as the request in terms of grid power regulation at the same time into account. Furthermore, the entire strategy has been tested including in the system two assembled battery packs, with two li-ion battery chemistries as mentioned earlier. The individual battery management system (BMS) for each technology has been developed in terms of hardware and software requirements. Moreover, the information exchange in terms of aging data between grid and BMS for the V2G strategy has been implemented and tested on a real-time simulation unit.