The Advanced Light-Duty Powertrain and Hybrid Analysis (ALPHA) tool was created by EPA to evaluate the Greenhouse Gas (GHG) emissions of Light-Duty (LD) vehicles. It is a physics-based, forward-looking, full vehicle computer simulator capable of analyzing various vehicle types combined with different powertrain technologies. The software tool is a freely-distributed, MATLAB/Simulink-based desktop application. Version 1.0 of the ALPHA tool was applicable only to conventional, non-hybrid vehicles and was used to evaluate off-cycle technology such as air-conditioning, electrical load reduction technology and road load reduction technologies for the 2017-2025 LD GHG and Fuel Economy rule. The next version of the ALPHA tool extends its modeling capabilities to include power-split and P2 parallel hybrid electric vehicles and their battery pack energy storage systems. Future versions of ALPHA will incorporate plug-in hybrid electric vehicle (PHEV) and electric vehicle (EV) architectures. This paper describes the battery pack model for ALPHA.Battery packs are key components in the various HEV, PHEV and EV architectures because they are among the costliest of drivetrain components and because Hybrid-Electric Vehicle (HEV), PHEV and EV efficiencies and driving capabilities are heavily dependent on battery pack system performance and life. This paper presents lithium-polymer battery pack model development and validation integrated into the ALPHA tool. The battery pack model consists of a battery cell, a thermal and battery management system (BMS) models. The battery pack model was validated with 2010 Toyota Prius battery pack voltage and 2011 Hyundai Sonata Hybrid vehicle data obtained from chassis dynamometer tests. The simulated battery voltages, currents, and State of Charge (SOC) are in excellent agreement with the vehicle test data on a number of drive schedules.