The performance and life of lithium-ion battery packs are affected significantly by battery operating temperatures. Thermal management is one of the main factors to be considered in pack design, especially for those with indirect air or indirect liquid cooling since the cooling medium is not in contact with the battery cells.
Plug-in hybrid electric vehicles (PHEVs) have two operating modes: charge depleting (CD) and charge sustaining (CS). The battery pack capacity is determined by the range of the CD mode. In CD mode, the state of charge (SOC) of the battery cells typically drops from 90 to 25% (i.e., the usable capacity SOC change = 65%). If the electrical energy consumption at the CD is assumed to be about 250-300 W·h/mi (400-500 W·h/km) for passenger cars, then the pack capacity must be in a range of 15 to 20 kW·h to travel 40 mi (64 km) in CD mode.
Considerable heat can be generated in the battery cells at the end of the CD mode as a result of high discharge/regen pulse currents and high cell internal resistances that increase with decreasing SOC. Because a battery pack for PHEV applications operates at its full capacity only at the CD mode, the maximum battery heat generation is encountered at the end of the CD mode. The more heat accumulated in the cells, the higher their temperatures.
The maximum temperature and the maximum temperature difference in a cell are crucial factors in cell durability and safety. To achieve optimal performance for a given battery pack, working temperatures of the cells in the pack should be controlled to within a proper range. Temperature distributions in the cells should be as uniform as possible to ensure the durability and safety of the battery pack under specified duties and ambient conditions.
Good thermal management of a battery pack depends on good understanding of the thermal behavior of the cells in the pack under various pack operation and cooling conditions. Battery thermal modeling can play an important role in the pack cooling system design. Researchers from AVL Powertrain Engineering Inc. recently undertook a theoretical investigation on thermal behavior of a Li-ion battery pack with indirect liquid cooling under operation conditions simulating PHEV applications.
The pack consisted of 96 high-power cells stacked in eight modules of equal cell count. The cells were connected in series for a pack capacity of 20 kW·h. Each of the modules was cooled in a parallel architecture (thermally symmetric) via cold plate. Each cell was cooled with an aluminum fin 1.5 mm (0.06 in) thick in close contact with a cold plate.
The researchers used a simplified FEA model. Both single cold plate and dual cold plate cooling were analyzed. Influences of the cold plate surface temperature gradients on cell temperature distributions were also studied.
The researchers came to the following conclusions:
• Cell temperature distribution can be influenced significantly by the location of the cold plate.
• The direction of the coolant flow in the cold plate has insignificant influence on single cold plate cooling, but it can have considerable impact on the maximum differential cell temperature for dual cold plate cooling.
• The cooling capacity of dual cold plates is about twice that of a single cold plate; as a result, both the maximum cell temperature and the maximum differential cell temperature for dual cold plate cooling are much lower than those for single cold plate cooling under the same cell load.
• The maximum cell temperatures for single cold plate and dual cold plates occur at different locations: near the positive terminal for single cold plate cooling and in the area between the positive and negative terminals for dual cold plate cooling.
• The cell terminal temperatures for dual cold plate cooling are much lower than those with single cold plate cooling because the ohmic heat generated in the cell terminals and busbars can be short-circuited to the cold plates in the dual cold plate configuration. For single cold plate cooling, the cold plate is generally arranged at the opposite end of the cell terminals. The ohmic heat from the cell terminals and busbars can only be dissipated via the heat transfer path across the cell height. The increase in thermal resistance results in more terminal/busbar ohmic heat to accumulate in the cell, resulting in higher cell temperatures near the terminal tabs.