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A new cell balancing technology was developed under a Department of Energy contract which merges the DC/DC converter function into cell balancing. Instead of conventional passive cell balancing technology which bypasses current through a resistor, or active cell balancing which moves current from one cell to another, with significant cost and additional inefficiencies, this concept takes variable amount of current from each cell or small group of cells and converts it to current for the low voltage system.

In a laboratory environment, it is cost prohibitive to run automotive battery aging experiments across a wide range of possible ambient environment, drive cycle, and charging scenarios. Because worst-case scenarios drive the conservative sizing of electric-drive vehicle batteries, it is useful to understand how and why those scenarios arise and what design or control actions might be taken to mitigate them. In an effort to explore this problem, this paper applies a semi-empirical life model of the graphite/nickel-cobalt-aluminum lithium-ion chemistry to investigate calendar degradation for various geographic environments and simplified cycling scenarios. The life model is then applied to analyze complex cycling conditions using battery charge/discharge profiles generated from simulations of plug-in electric hybrid vehicles (PHEV10 and PHEV40) vehicles across 782 single-day driving cycles taken from a Texas travel survey.

A previously validated 1-D electrochemical model of a 72 cell, 6 Ah, 276 V nominal lithium-ion hybrid vehicle battery pack is used to predict maximum discharge current for discharge pulses ranging from zero to twenty seconds in duration from various state of charge (SOC) initial conditions. Ohmic drop from open-circuit potential limits instantaneous pulse power capability (83 kW from 50% SOC) while Li+ diffusion inside negative electrode active material solid particles limits long-time capability (28 kW for a 20 second pulse from 50% SOC). A simple lumped negative electrode solid diffusion model is derived to dynamically describe electrode-averaged solid phase concentration distribution inside of active material particles as a function of measured current time history.