Blog by Professor James Marco and Dr Truong Bui
A number of studies have identified the strategic importance of charging infrastructure to support the introduction of electrified vehicles (EVs) and to underpin consumer acceptance of the technology. For this reason, considerable research has been undertaken to evaluate the different facets of the technology, including the ability to charge at higher rates of electrical power, the introduction of smart charging (V1G) that allows dynamic management of the charging process in terms of both power and charge time and vehicle-to-grid (V2G) that enables bidirectional energy flow between the vehicle and the supply. In recent years, the term V2G has become more generalised to V2X, to acknowledge the variability in which the EV may be integrated, either to the grid or behind-the-meter, for example within a local electrical network, such as a building (V2B).
Irrespective of the exact nature of the integration method, understanding the impact of different strategies on battery degradation is a key requirement that may ultimately underpin consumer acceptance. Only a few studies have examined the potential impact of V2G operation on battery degradation. Often their assertion is that the increased charge-throughput will negatively impact battery life. These works often neglect that the battery will equally degrade through a process of calendar-ageing in which the retained capacity of the battery reduces as the battery is stored at no-load.
As highlighted in these results from the EV-elocity project, the nature of the degradation is highly complex with dependencies that crosscut: the state of charge (SOC) of the battery with respect to the optimal SOC storage condition and the duration of the parking interval. To further compound the challenge, experimental results shows that the optimal SOC point varies with battery life.
In this study, WMG researchers have evaluated the impact on battery degradation of different charging strategies relative to a baseline standard (STD) charging process. These include:
- Time-shifted (TS) charging in which the charge is delayed to an optimal time-point to commence charging.
- V1G charging in which the battery is pre-conditioned to an optimal SOC for vehicle parking. The battery is stored at an optimal SOC before performing charging.
- V2G charging in which the ability to exchange energy with the supply is exploited to optimise preconditioning of the battery while the vehicle is parked.
- VxG charging in which the ability to switch between V1G and V2G to optimise the battery degradation during parking.
Table 1 Characteristics of different charging strategies
|Strategies||Delayed charge time||Full charge start time||Resting SoC||Prior knowledge of parking period||Required V2G charger|
|Standard||No||When connected to the charger||100%||No||No|
|TS||Yes||At an appropriate time before next departure||Arrival SoC||Yes||No|
|V1G||Yes||Preconditioning to a local optimal SoC, then full charge just before next departure||Arrival SoC or higher SoC with smaller calendar ageing rate||Yes||No|
|V2G||Yes||Preconditioning to a global optimal SoC for resting, then full charge just before next departure||Any SoC with least calendar ageing rate||Yes||Yes|
|VxG||Yes||Preconditioning to a global optimal SoC for resting, then full charge just before next departure||Any SoC with either least calendar ageing rate or having smaller cycling ageing rate||Yes||Yes|
A validated semi-empirical ageing model is used that captures the impact of calendar ageing (as a function of temperature, resting period and SOC) and cyclic ageing (as a function of temperature, C-rate, depth-of-discharge and charge throughput). The model leverages over 12 months of experimental data from the WMG battery ageing laboratory for parameterisation and validation.
To support our evaluation, we introduce two driving profiles, which are defined as “gentle” and “intensive” profiles, representing two different drivers to evaluate the battery degradation behaviour through different charging strategies. Simulation results on the validated model show that state of health (SOH) improvements of up to 30-34% may be possible for a new EV battery, reducing to 8-12% as the battery progressively degrades through normal operation as shown in Figure 1.
The TS and V1G strategies perform consistently well in reducing battery ageing under the intensive driving profile while they create negative impact on the battery degradation under the gentle driving profile. The V2G and VxG strategies mitigate battery ageing in most cases, especially when the battery is new. However, their capability reduces as battery SOH progresses.
It is beyond the scope of this initial study to report on the relative impact of different V2G applications (e.g. energy arbitrage or peak shaving). However, the results presented here would scale into a future holistic management strategy for grid integrated EV batteries.
The description of the full research undertaken including a detailed description of the experiment methods, model creation and validation can be found in the related article that is available open source:
- M. N. Bui, M. Sheikh, T. Q. Dinh, A. Gupta, D. W. Widanalage and J. Marco, “A Study of Reduced Battery Degradation through State-of-Charge Pre-Conditioning for Vehicle-to-Grid Operations,” in IEEE Access, doi: 10.1109/ACCESS.2021.3128774.