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differing degrees of all electric range and battery electric vehicles (EV) that rely entirely on electric drive and battery electric energy storage. Different HEV, PHEV and EV concepts utilize these mechanisms differently, so they are treated separately for the purposes of this analysis. Below is a discussion of battery energy storage and the major hybrid concepts that were determined to be available in the near term. i. Batteries for HEV, PHEV and EV Applications The design of battery secondary cells can vary considerably between HEV, PHEV and EV applications. MHEV systems will likely continue to use lead-acid batteries due to their lower voltage (12-42 VDC) and relatively low power and energy requirements. However, technology used is expected to be upgraded over conventional (non-MHEV) lead acid batteries to meet the charge cycling demands of MHEV applications, and is likely to include extended-cycle-life flooded (ELF) lead-acid batteries or absorptive glass matt, valve-regulated lead-acid (AGM/VRLA) batteries. HEV applications operate in a narrow, short-cycling, charge-sustaining state of charge (SOC). Energy capacity in HEV applications is somewhat limited by the ability of the battery and power electronics to accept charge and by space and weight constraints within the vehicle design. HEV battery designs tend to be optimized for high power density rather than high energy density, with thinner cathode and anode layers and more numerous current collectors and separators (Figure V-12). EV batteries tend to be optimized for high energy density and are considerably larger than HEV batteries. PHEV battery designs are intermediate between power-optimized HEV and energy-optimized EV battery cell designs. PHEV batteries also must provide both charge depleting operation similar to an EV and charge sustaining operation similar to an HEV. Unlike HEV applications, charge sustaining operation with PHEVs
206occurs at a relatively low battery state of charge (SOC) which can pose a significant challenge with respect to attaining acceptable battery cycle life. In the case of the GM Volt, this limits charge depleting operation to a minimum SOC of approximately 30%.146Figure V-12 Schematic representation of power and energy optimized prismatic-layered battery cells Collector (-)Cathode (-)SeparatorAnode (+)Collector (+)HEV Power-optimized Battery CellEV Energy-optimized Battery CellPower-split hybrid vehicles from Toyota, Ford and Nissan, integrated motor assist hybrid vehicles from Honda and the GM 2-mode hybrid vehicles currently use nickel-metal hydride (NiMH) batteries. Lithium-ion (Li-ion) batteries offer the potential to approximately double both the energy and power density relative to current NiMH batteries, enabling much more electrical-energy-intensive automotive applications such as PHEVs and EVs. Li-ion batteries for high-volume automotive applications differ substantially from those used in consumer electronics applications with respect to cathode chemistry, construction and cell size.
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