The temperature-dependent behavior of the resistance and overpotential of a lithium-iron-phosphate (LiFeP04) battery cell is explored in this paper. Offline experimental results from hybrid pulse power characterization (HPPC) tests and electrochemical impedance spectroscopy (EIS) methods for resistance and overpotential are explained using Arrhenius equations. Using a nonlinear regression technique, simulated drive cycle data are used to confirm the experimental findings and construct a generic cell model that explicitly takes temperature effects on the resistance and overpotential into account. The significance of the work lies in its confirmation of the inadequacy of the baseline linear-circuit model for lithium batteries at low temperatures and its presentation of a modeling approach that provides much better agreement with measured battery characteristics. Index Terms -State-of-Charge (SOC), battery management system (BMS), electric vehicle (EV), lithium iron phosphate battery, Kalman filter, Hybrid Power Pulse Characterization I. I NTRODUCTION Battery monitors for electric vehicles (EVs) are attracting the attention of many researchers [1 -4]. The goals are to provide accurate and robust state-of-charge (SOC), power capability prediction, and state-of-health (SOH) monitoring for the battery pack. To monitor the battery pack online, recursive estimation techniques based on a battery equivalent circuit are typically implemented [2][3][4]. Unlike the offline modeling approach in [1], the recursive approach can adjust the battery model parameters to adapt to changes in the battery characteristics caused by various factors such as aging or temperature changes. That is, the configuration of the battery equivalent circuit model remains fixed, while the recursive estimation scheme adjusts the model parameters when the battery temperature changes without specifically including temperature as a model input.This paper investigates the validity of the baseline linear model that is often adopted for lithium-iron-phosphate (LiFeP04) battery cells. After exposing its limitations, the paper proposes a modeling approach that includes temperature as a model input so that the battery resistance and overpotential values are defined functions of the temperature.In [2], a Butler-Volmer (BV) equation-based nonlinear model was adopted for the recursive estimation of lead-acid batteries in an EV. Statistical analysis confirmed the benefits of the BV model over the baseline linear-circuit model. For 978-1-4799-2262-8/14/$31.00 ©2014 IEEE lithium batteries, the literature has typically adopted a linear circuit model [3,4], and, for the most part, this approach has not been questioned. In this paper, a LiFeP04 cell ' s overpotential is demonstrated to exhibit significant nonlinear BV behavior at low temperatures, even though such behavior is not observed at room temperature. Additionally, the cell ' s ohmic resistance is shown to increase rapidly as the temperature drops.Increased cell resistance and overpotential at low temperatures h...