S. Chacko and S. Charmer,     Tata Motors European Technical Centre plc, Vehicle Integration, UK

1 Introduction

The thermal management of traction battery that incorporates Li-ion cell is an important design attribute that automotive manufacturers need to consider. A battery thermal management (BTM) strategy is needed to ensure that battery cells remain within a narrow temperature bandwidth regardless of vehicle environment and operating condition, this is required to ensure that attributes such as performance, range, battery life, reliability and safety are met. Tata motors have recently developed an advanced BTM system for Vista EV. This system has been developed to ensure that the temperature of a cell within a battery pack is maintained within an ideal zone. Previous work and our in-house EV vehicle tests has clearly identified that temperature influences a cells ability to accept charge and to discharge; it also influences safety, reliability and the life characteristics of a cell. It is observed that the ideal temperature gradient within a Li-Ion cell, and from cell to cell within a pack, should be held within a 5 to 10 °C window [1]. With the above in mind active indirect liquid cooling/heating would be one of the promising means to achieve BTM [2,3]. The developed system would then be able to remove heat from the battery pack and exchange it to the environment via a pumped coolant network. The BTM system will also have the flexibility to thermally pre-condition the battery, thus ensuring the customer can access optimum performance and range of the vehicle regardless of environmental conditions [4].

2 Li-Ion Battery Pack Configuration

The battery configuration described in this study incorporates conditioned coolant that is pumped through a coolant plate attached to a battery module. Active heating and cooling is achieved by pumping either warmed or cooled/sub cooled coolant through the coolant plate. Coolant temperature can be precisely controlled based on the battery temperature and its operating condition. The design concept for any given operating condition, precisely control coolant temperature, maintaining battery temperature within the ideal range (self-regulating zone in figure 1).

Figure 2(a) presents the layout of a Vista EV battery module with liquid cooling plate, the plate is housed between modules and is designed to transfer heat via coolant which is then dissipated into the environment (via a coolant to air network). The cooling plate as in figure 2(b) is designed to transfer heat at the optimum rate. It is important to achieve optimum heat transfer efficiency with minimum loss, this minimizes parasitic losses within the system and facilitates efficient coolant pump/radiator selection. By optimizing the design the BTM system draws minimum power from the EV system thus maximizing vehicle range [5–9].

Figure 3(a) shows the Vista EV complete assembly with two sets of battery. Each battery pack has within it a number of individual battery modules. A battery module is a combination of stacked Li-ion cells which are arranged with coolant jacket plates placed on two sides as shown in figure 2(a). Each cell within the module is housed in a plastic cassette frame. A heat transfer plate made of a thin aluminum sheet is also wrapped around the cell, this provides an efficient heat transfer path from the cell core to the cooling plate. The heat transfer plate is a planar member covering one longitudinal side of the cell and extending along the transverse side of the cell. The side face of heat transfer plate along with the thickness of the cell connects to the coolant jacket plate, this facilitates a conduction path between cell and coolant plate. The coolant channels are optimized to minimize the pumping pressure required to flow coolant through the channel while maximizing heat transfer efficiency. The inlet portion of the coolant channel is routed to the top of the cells first to allow maximum heat transfer efficiency as it is envisaged that heat generation is at its maximum in the upper portion of a cell. The channel is routed from top to the bottom of the plate, covering a substantial width of the plate. Through these channels conditioned coolant is flowed to thermally manage the cells. Coolant plates are arranged in rows between battery modules. The total arrangement is placed in a tray and coolant inlet and outlet pipes figure 3(b) are routed within this tray.

3 Battery Thermal Management Development Tools

TMETC apply a wide variety of tools and techniques in developing vehicle thermal management systems. The company is well rehearsed in applying advanced testing and CAE methods on ICE powered passenger, commercial and military vehicles. By adopting Thermo fluid CAE tools such as Kuli, RadTherm and Fluent (CFD) the team, were able to develop a robust thermal management system for Vista EV in a timely manner. However one area the team were not initially converse in was the electro thermo characteristics of Li-Ion cells, here the team had to develop an additional tool and technique that would slot well within our tried and tested methods.