As part of the Midterm Evaluation of the 2017-2025 Light-duty Vehicle Greenhouse Gas Standards, the U.S. Environmental Protection Agency (EPA) developed simulation models for studying the effectiveness of stop-start technology for reducing CO2 emissions from light-duty vehicles.Stop-start technology is widespread in Europe due to high fuel prices and due to stringent EU CO2 emissions standards beginning in 2012. Stop-start has recently appeared as a standard equipment option on high-volume vehicles like the Chevrolet Malibu, Ford Fusion, Chrysler 200, Jeep Cherokee, and Ram 1500 truck. EPA has included stop-start technology in its assessment of CO2-reducing technologies available for compliance with the standards. Simulation and modeling of this technology requires a suitable model of the battery.The introduction of stop-start has stimulated development of 12-volt battery systems capable of providing the enhanced performance and cycle life durability that it requires. Much of this activity has involved advanced lithium-ion chemistries, variations of lead-acid chemistries, such as absorbed-glass-mat (AGM) designs, and lead-carbon formulations.EPA tested several AGM batteries that are used in OEM start-stop systems. The purpose of this testing was to develop an equivalent circuit model for integration into EPA's ALPHA vehicle simulation model. Testing was performed at the Battery Test Facility (BTF) at the U.S. Environmental Protection Agency (EPA) National Vehicle and Fuel Emissions Laboratory (NVFEL) in Ann Arbor, Michigan. The Duracell batteries referenced are model number SLI49AGM with a rating of 92 Ah and the X2 Power batteries referenced are model number SLI34-78AGMDP with a rating of 68 Ah. Both batteries are 6 cell 12 volt using AGM technology. For compatibility with the voltage specifications of the BTF equipment, tests were performed on two batteries connected in series (nominal 24 volts).This paper presents the development and validation of the lead-acid battery model. The battery model is a standard equivalent circuit model with two Resistance-Capacitance (RC) blocks. Resistances and capacitances were calculated using test data from a Duracell 92Ah lead-acid battery which is aftermarket equipment for the Chevrolet Malibu. The lead-acid battery library in the ALPHA model was validated with data obtained from Argonne National Laboratory (ANL) from their chassis dynamometer testing of the 2010 Mazda 3 Hatchback i-Stop  and 2010 VW Golf TDI Diesel Bluemotion . 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.