Transient Finite Element Simulation of a Lithium-Ion Battery Pack Thermal Management System Based on Latent Heat System Materials

Abstract

Battery packs in high-intensity use run the risk of overheating, potentially inducing catastrophic thermal runaway. Thermal runaway occurs when an individual cell in a connected system fails and subsequently applies increased stress on the remaining cells, leading to cascading failures and destruction of the battery pack as a whole. One method for controlling battery temperature during usage is utilizing the battery pack casing as a latent heat system (LHS), specifically incorporating phase-change materials (PCMs). Using this method, the transient thermal response of a 84-cell, 270 V lithium-ion battery pack for an aircraft application was simulated using ANSYS FLUENT. Heat generation rates and specific heat capacity of a single cell were experimentally measured and used as input to the thermal and mechanical model. A heat generation load was applied to the entire battery pack, and natural convection film boundary conditions were applied to the exterior of the enclosure. The maximum temperature of the battery pack without any casing reached 69.8 ᵒC after 800 s of usage at a simulated peak power draw of 1013 W or 16 A. This exceeds the manufacturer’s maximum recommended operating temperature of 60 ᵒC. For other materials, we found the maximum temperature of the batteries reached 57.0 ᵒC with plastic casing and 45.0 ᵒC with aluminum casing. We present a design of the pack that incorporates a passive thermal management system using a composite material consisting of phase-changing material, Puretemp60®, mixed with expanded graphite (EG). Simulations of the battery pack showed a decrease in the maximum temperature with the Puretemp60® as the sole material reaching a maximum of 54.2 ᵒC and the Puretemp EG composite matrix having a maximum temperature of 51.1 ᵒC, both after 800 s at the same power draw.The optimal material combination was then determined through evaluating several PCM and EG composites in several ratios using ANSYS FLUENT, followed by experimental testing. The mixture was cost effective, easy to mix, and demonstrated good mechanical properties. Results illustrated that a mixture of 25% expanded graphite with 75% Puretemp60® (PCM) was capable of maintaining battery temperatures within the acceptable operating range under moderate load conditions and therefore preventing thermal runaway. A resulting weight reduction of about 23.5% was also achieved using the Puretemp EG composite matrix in comparison to aluminum casing, and about a 7.2% reduction compared to plastic casing.Differential scanning calorimetry (DSC) was utilized to determine the phase transformation of the composites. The addition of expanded graphite minimally affected the phase transformation temperature, while decreasing the latent heat of transformation by about 40%. Scanning electron microscope (SEM) pictures visualized the morphology of the graphite during each stage of the composite fabrication process. The Puretemp60® and expanded graphite were easily mixed and the backscattering electron images indicated the composition was homogenous.The heat generation rate of the battery was calculated using the q̇gen= mCp(dT/dt) + UA∆T. The change in temperature with respect to time was experimentally determined from a series of discharges. Heat generation in the batteries was modeled as uniformly distributed. The average of the experimentally measured heat generation rates for a 1P constant power discharge were divided by the volume of the battery to yield a curve for the average body heat flux load in W/m3 over the discharge of the battery. A polynomial curve fit to the average heat generation rates was applied as a heat load to each battery in the simulation. The heat generation due to discharge was tested with a setup where the battery was insulated in polyurethane foam in order to reduce convection heat transfer during the experiments. A polyurethane foam block was cut such that the battery could be placed in the center, along with three T-type thermocouples to measure the surface temperature and external air temperatures. Two metal cylinders, one consisting of copper and the other of 6061 aluminum, were machined to the same cylindrical dimensions as the battery. These cylinders were then used to determine the heat loss, or UA value, of the foam insulation block setup. A National instruments NI 9147, Chroma Programmable DC Power Supply, and LabView software were used for the experiments. Figure 1