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Start-stop, aka engine-stop or idle-stop, technologies are increasingly being applied to automotive vehicles to increase fuel economy. Start-stop vehicles turn off the engine during periods of zero speed and/or during prolonged coast down. During engine-stop, the vehicle electronics are powered solely by the battery. To replenish the battery, the battery needs to be recharged. In typical ICE vehicles, the battery is continuously charged. However, fuel economies can be improved if strategic charging of the battery can be achieved through selective charging through the alternator or through regenerative braking. To optimize fuel economy, an accurate estimation of the battery state of charge (SOC) during vehicle operation is required. Although state of charge estimation has mainly focused on Li-ion batteries, lead-acid batteries may be used successfully in start-stop applications.

The majority of studies in automotive electrification technology focus on the performance of high voltage HEV and EV powertrains. With the introduction of microhybrid systems as a near term technology trend, this work focuses on an analysis of low voltage (<60V) systems across multiple vehicle segments and region-specific regulatory drive cycles. Vehicle simulation results are presented for 12V and 48V vehicle systems equipped with start-stop and regenerative braking, features commonly associated with microhybrid vehicles. Simulation results show that fuel economy benefits from start-stop vary significantly between drive cycles. In contrast, total energy recuperation is similar across all vehicle classes for 12V microhybrid systems. For 48V systems, total recuperated energy increases with vehicle mass while the percent fuel economy benefit is highest for lighter vehicles.