Sensorless Battery Internal Temperature Estimation using a Kalman Filter with Impedance Measurement

This study presents a method of estimating battery cell core and surface temperature using a thermal model coupled with electrical impedance measurement, rather than using direct surface temperature measurements. This is advantageous over previous methods of estimating temperature from impedance, which only estimate the average internal temperature. The performance of the method is demonstrated experimentally on a 2.3 Ah lithium-ion iron phosphate cell fitted with surface and core thermocouples for validation. An extended Kalman filter, consisting of a reduced order thermal model coupled with current, voltage and impedance measurements, is shown to accurately predict core and surface temperatures for a current excitation profile based on a vehicle drive cycle. A dual extended Kalman filter (DEKF) based on the same thermal model and impedance measurement input is capable of estimating the convection coefficient at the cell surface when the latter is unknown. The performance of the DEKF using impedance as the measurement input is comparable to an equivalent dual Kalman filter using a conventional surface temperature sensor as measurement input.Comment: 10 pages, 9 figures, accepted for publication in IEEE Transactions on Sustainable Energy, 201

Recent U.S. Trade Policy and its Global Implications

The purpose of this paper is to describe United States trade policy since World War II, and to assess the possibility for ongoing U.S.trade-policy leadership. U.S. trade policy has shown remarkable consistency since World War II. It has never been as purely free-trade-focussed as some commentators suggest, but it has not recently shifted toward isolationism as dramatically as alarmists fear. It has almost always been best described as "open, but fair," with injury to import competitors being the measure of "fairness." The general consistency of U.S. trade policy over time is quite remarkable given the frequent change of political party in power, especially in the executive branch, but also in the Congress. U.S. trade-policy leadership seems still potentially strong despite a decline in U.S. hegemony. It is clearly strong in a protectionist direction.Any shift toward aggressive insularity justifies parallel trade-policy aggression in the eyes of trading partners. It is arguably strong ina liberalizing direction as well. The U.S. seems ideally poised for aggressive trade-policy peacemaking; perhaps multilaterally, but perhaps also bilaterally; perhaps with its traditional industrial trading partners, but perhaps also with Japan and newly industrializing Asian countries that play so importanta role in U.S. trade, and that, on many matters,may be closer in spirit to U.S. economic philosophy than Europe, Canada, or Latin America.

The Information in Long-Maturity Forward Rates: Implications for Exchange Rates and the Forward Premium Anomaly

The forward premium anomaly is one of the most robust puzzles in financial economics. We recast the underlying parity relation in terms of cross-country differences between forward interest rates rather than spot interest rates with dramatic results. These forward interest rate differentials have statistically and economically significant forecast power for annual exchange rate movements, both in- and out-of-sample, and the signs and magnitudes of the corresponding coefficients are consistent with economic theory. Forward interest rates also forecast future spot interest rates and future inflation. Thus, we attribute much of the forward premium anomaly to the anomalous behavior of short-term interest rates, not to a breakdown of the link between fundamentals and exchange rates.

Risk, Research, and Returns: Valuation of the Potential of Improved Citrus Cultivars

Replaced with revised version of paper 02/04/08.Crop Production/Industries, Research Methods/ Statistical Methods, Risk and Uncertainty,

The Myth of Long-Horizon Predictability

The prevailing view in finance is that the evidence for long-horizon stock return predictability is significantly stronger than that for short horizons. We show that for persistent regressors, a characteristic of most of the predictive variables used in the literature, the estimators are almost perfectly correlated across horizons under the null hypothesis of no predictability. For example, for the persistence levels of dividend yields, the analytical correlation is 99% between the 1- and 2-year horizon estimators and 94% between the 1- and 5-year horizons, due to the combined effects of overlapping returns and the persistence of the predictive variable. Common sampling error across equations leads to ordinary least squares coefficient estimates and R2s that are roughly proportional to the horizon under the null hypothesis. This is the precise pattern found in the data. The asymptotic theory is corroborated, and the analysis extended by extensive simulation evidence. We perform joint tests across horizons for a variety of explanatory variables, and provide an alternative view of the existing evidence.

On-board monitoring of 2-D spatially-resolved temperatures in cylindrical lithium-ion batteries: Part II. State estimation via impedance-based temperature sensing

Impedance-based temperature detection (ITD) is a promising approach for rapid estimation of internal cell temperature based on the correlation between temperature and electrochemical impedance. Previously, ITD was used as part of an Extended Kalman Filter (EKF) state-estimator in conjunction with a thermal model to enable estimation of the 1-D temperature distribution of a cylindrical lithium-ion battery. Here, we extend this method to enable estimation of the 2-D temperature field of a battery with temperature gradients in both the radial and axial directions. An EKF using a parameterised 2-D spectral-Galerkin model with ITD measurement input (the imaginary part of the impedance at 215 Hz) is shown to accurately predict the core temperature and multiple surface temperatures of a 32113 LiFePO$_4$ cell, using current excitation profiles based on an Artemis HEV drive cycle. The method is validated experimentally on a cell fitted with a heat sink and asymmetrically cooled via forced air convection. A novel approach to impedance-temperature calibration is also presented, which uses data from a single drive cycle, rather than measurements at multiple uniform cell temperatures as in previous studies. This greatly reduces the time required for calibration, since it overcomes the need for repeated cell thermal equalization.Comment: 11 pages, 8 figures, submitted to the Journal of Power Source