energies Article Water Condensation in Traction Battery Systems Woong-Ki Kim 1, Fabian Steger 1,2 , Bhavya Kotak 1, Peter V. R. Knudsen 3, Uwe Girgsdies 4 and Hans-Georg Schweiger 1,* 1 Technische Hochschule Ingolstadt, Esplanade 10, 85049 Ingolstadt, Germany; [email protected] (W.-K.K.); [email protected] (F.S.); [email protected] (B.K.) 2 Royal Melbourne Institute of Technology, School of Engineering, Melbourne, VIC 3000, Australia 3 Faculty of Engineering, University of Southern Denmark, Campusvej 55, 5230 Odense, Denmark; [email protected] 4 Audi AG, Auto-Union-Straße 1, 85045 Ingolstadt, Germany; [email protected] * Correspondence: [email protected] Received: 1 March 2019; Accepted: 22 March 2019; Published: 26 March 2019 Abstract: Lithium-ion traction battery systems of hybrid and electric vehicles must have a high level of durability and reliability like all other components and systems of a vehicle. Battery systems get heated while in the application. To ensure the desired life span and performance, most systems are equipped with a cooling system. The changing environmental condition in daily use may cause water condensation in the housing of the battery system. In this study, three system designs were investigated, to compare different solutions to deal with pressure differences and condensation: (1) a sealed battery system, (2) an open system and (3) a battery system equipped with a pressure compensation element (PCE). These three designs were tested under two conditions: (a) in normal operation and (b) in a maximum humidity scenario. The amount of the condensation in the housing was determined through a change in relative humidity of air inside the housing. Through PCE and available spacing of the housing, moisture entered into the housing during the cooling process. While applying the test scenarios, the gradient-based drift of the moisture into the housing contributed maximum towards the condensation. Condensation occurred on the internal surface for all the three design variants. Keywords: traction battery system; condensation; durability test; pressure compensation element; behavior of an enclosed system 1. Introduction One of the biggest challenges while designing automotive traction batteries is the durability of the battery system and its components [1,2]. The lifespan of the lithium-ion cell is expressed in terms of a charge cycle. According to William et al. [3], the end life of a battery in electric vehicle (EV) applications can be defined as when the battery capacity reduces to 80% of its initial capacity. Previous studies have shown that the lifespan of batteries is between 5–15 years [4–10]. The life of the battery depends on the cell material, operating conditions, and environmental conditions. The battery system is exposed to fluctuating environmental conditions [11] in vehicle applications. To ensure the safe operation of lithium-ion cells, and to prolong their lifespan, hybrid and electric vehicle battery systems are equipped with cooling systems. Using a liquid or an evaporative cooling system can result in the condensation of water inside the battery system. Condensation occurs if the temperature of the cooling plate is below the dew point. It can damage the electrical components, cause corrosion inside the system, reduce the insulation resistance, as well as it may accelerate the aging of the cells [12]. These effects will shorten the life of the battery system, especially of electrical devices and metallic parts [12]. Energies 2019, 12, 1171; doi:10.3390/en12061171 www.mdpi.com/journal/energies Energies 2019, 12, 1171 2 of 17 If the surface temperature of a component is below the dew point, water will condense [13,14]. Pressure compensation elements (PCE) are usually integrated into the battery housing to compensate for the pressure difference between the interior and exterior of the battery system. The pressure differences are caused by fluctuation of temperature and pressure. During the operation in severe environmental conditions such as in Asia, there exists a risk that the ambient humidity enters through the PCE and increases the amount of vapor in the housing, leading to the higher possibility of system failure. Moreover, the risk of condensation is higher if liquid cooling is implemented in the battery system [13]. To optimize the life span as well for the robust design of battery systems, knowledge of the amount of water condensation is crucial. The knowledge of the external factors (e.g., operational conditions, amount and design of PCEs, causes and influences of water condensation) can lead to the better design of longer lasting battery systems. 2. Materials and Methods To simulate the environmental conditions and to identify if vapor in the housing of the battery condenses during the usage of the battery system, experiments were carried out using a physical simulated battery system (PSBS). This simulator covers the attributes of the battery system which affect the condensation phenomenon inside the housing. This PSBS mimics three battery system designs: (1) a sealed battery system, (2) an open system and (3) a battery system equipped with a pressure compensation element (PCE). The most important environmental quantities that affect the condensation inside the battery housing and performance of the battery system are the air temperature, humidity, and ambient pressure. Therefore, the equipment installed within the simulator was set up to measure the internal air pressure, relative humidity, temperature, and ambient pressure. The battery housing was additionally monitored with the help of a dew sensor and a camera to determine when condensation appears. Using the recorded data, the absolute humidity of air within the housing was calculated (refer to Equation (1)). It was assumed that the amount of condensation was equivalent to the reduction of absolute humidity of air in the housing. The experiments were carried out with two test profiles: (a) normal operation and (b) abnormal operation. Normal operation means simulating the actual operation of the battery and cooling system. The abnormal operation is considered as the worst case scenario of the faulty battery system. 2.1. Design of the Physical Simulator The PSBS was designed as a scaled model to mimic the thermal behavior of a real traction battery system of hybrid or electric vehicles. Instead of real lithium-ion cells, simulated cells were used to ensure safe operation. The PSBS mimics the following factors and parameters of battery systems: • Thermal simulation of lithium-ion pouch cells with a capacity of 20 Ah, similar to the cells of an EV; • Equivalent heat capacity and heat conductivity as real lithium-ion cells; • Active cooling with liquid coolant by means of a cold plate in the bottom of the housing; • Cells in direct contact with the cold plate; • Housing classified as waterproof with leak rate below 1 Pa·dm3/s (leak rate according to DIN EN1330-8 [15]); • A free volume ratio of about 20%. Housing: The housing of dimension of 236.0 mm × 216.0 mm × 166.0 mm (length × width × height) having an accuracy of ±0.5 mm was constructed out of four 8.0 mm aluminum 7020 (AlZn4.5Mg1) plates. The thickness of the aluminum plate was selected according to the insulation requirement of the housing and the availability of the material. To avoid bending caused by pressure changes, a thick stainless-steel plate (1.4301 type, 20.0 mm ± 0.5 mm) was used as a Energies 2019, 12, 1171 3 of 17 top plate. The 18.0 mm ± 0.5 mm thick base plate made up of stainless steel, manufactured by AusterlitzEnergies 2019,electronic X, x FOR PEER GmbH REVIEW (Nuremberg, Germany) had served as a cold plate. 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