Abstract
Ambient conditions can have a significant impact on the average and maximum temperature of the battery of electric and plug-in hybrid electric vehicles. Given the sensitivity of the ageing mechanisms of typical battery cells to temperature, a significant variability in battery lifetime has been reported with geographical location. In addition, high battery temperature and the associated cooling requirements can cause poor passenger thermal comfort, while extreme battery temperatures can negatively impact the power output of the battery, limiting the available electric traction torque. Avoiding such issues requires enabling battery cooling even when the vehicle is parked and not plugged in (key-off), but the associated extra energy requirements make applying key-off cooling a non-trivial decision. In this paper, a representative plug-in parallel hybrid electric vehicle model is used to simulate a typical 24-h duty cycle to quantify the impact of hot ambient conditions on three performance attributes of the vehicle: the battery lifetime, passenger thermal comfort and fuel economy. Key-off cooling is defined as an optimal control problem in view of the duty cycle of the vehicle. The problem is then solved using the dynamic programming method. Controlling key-off cooling through this method leads to significant improvements in the battery lifetime, while benefiting the fuel economy and thermal comfort attributes. To further improve the battery lifetime, partial charging of the battery is considered. An algorithm is developed that determines the optimum combination of key-off cooling and the level of battery charge. Simulation results confirm the benefits of the proposed method.
Highlights
The most significant obstacle in the way of the transition from conventional vehicles to electric vehicles (EVs) is the under-developed traction battery technology [1,2]
The high weights assigned to fuel economy and thermal comfort make it imperative that the battery is sufficiently charged to deliver maximum CD operation in both trips, as well as an intense key-off precooling. This effectively limits the window of state of charge (SoC) that is available for partial charging, so the choice is between higher charge that enables more key-off cooling, or lower charge that only supports the CD operation and precooling
Three performance attributes of plug-in parallel hybrid electric vehicles (PPHEVs), namely their fuel economy, battery lifetime, and passengers’ thermal comfort, can be negatively affected by the high battery temperature that results from hot climate conditions
Summary
The most significant obstacle in the way of the transition from conventional vehicles to electric vehicles (EVs) is the under-developed traction battery technology [1,2]. As this transition takes place, most technology road maps and policy trends indicate that plug-in hybrid electric vehicles (PHEVs) will dominate the passenger vehicles markets in the decade [3,4,5]. Ageing of battery cells, defined as the irreversible reduction of their energy storage capacity and power [8,9], is one such challenge [10]. Lithium-ion cells, which are believed to be the most promising choice for automotive applications [11,12] have ageing mechanisms that are sensitive to real world usage conditions [13]. Cycling ageing (ageing due to the conditions of charge-discharge cycles) accelerates at high temperature, as well as with a high degree of discharge, and high charge-discharge rates [14,15]
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