Nassim A. Samad

Optimal power management for a series hybrid electric vehicle cognizant of battery mechanical effects

This paper presents an optimal power management strategy for a series hybrid electric vehicle (SHEV) with the consideration of battery bulk mechanical stress. The relation between mechanical stress and state-of-charge (SOC) is characterized first. Then, this relation is used to penalize the battery usage leading to capacity fade due to particle fracture in the negative electrode. The optimal power management strategy is found using Dynamic Programming (DP) not only for maximizing fuel economy but also for minimizing the battery cumulative bulk mechanical stress. DP results suggest that battery SOC needs to be regulated around a lower value for prolonged battery life. Moreover, it is found that the cumulative bulk mechanical stress can be significantly reduced at a small expense of fuel economy.

Parameterization and Validation of a Distributed Coupled Electro-Thermal Model for Prismatic Cells

The temperature distribution in a prismatic Li-ion battery cell can be described using a spatially distributed equivalent circuit electrical model coupled to a 3D thermal model. The model represents a middle ground between simple one or two state models (generally used for cylindrical cells) and complex finite element models. A lumped parameter approach for the thermal properties of the lithium-ion jelly roll is used. The battery is divided into (m × n) nodes in 2-dimensions, and each node is represented by an equivalent circuit and 3 temperatures in the through plane direction to capture the electrical and thermal dynamics respectively. The thermal model is coupled to the electrical through heat generation. The parameters of the equivalent circuit electrical model are temperature and state of charge dependent. Parameterization of the distributed resistances in the equivalent circuit model is demonstrated using lumped parameter measurements, and are a function of local temperature. The model is parameterized and validated with data collected from a 3-cell fixture which replicates pack cooling conditions. Pulsing current experiments are used for validation over a wide range of operating conditions (ambient temperature, state of charge, current amplitude and pulse width). The model is shown to match experimental results with good accuracy.© 2014 ASME

Parameterization of Battery Electrothermal Models Coupled with Finite Element Flow Models for Cooling

Developing and parameterizing models that accurately predict the battery voltage and temperature in a vehicle battery pack are challenging due to the complex geometries of the airflow that influence the convective heat transfer. This paper addresses the difficulty in parameterizing low-order models which rely on coupling with finite element simulations. First, we propose a methodology to couple the parameterization of an equivalent circuit model (ECM) for both the electrical and thermal battery behavior with a finite element model (FEM) for the parameterization of the convective cooling of the airflow. In air-cooled battery packs with complex geometries and cooling channels, an FEM can provide the physics basis for the parameterization of the ECM that might have different convective coefficients between the cells depending on the airflow patterns. The second major contribution of this work includes validation of the ECM against the data collected from a three-cell fixture that emulates a segment of the pack with relevant cooling conditions for a hybrid vehicle. The validation is performed using an array of thin film temperature sensors covering the surface of the cell. Experiments with pulsing currents and drive cycles are used for validation over a wide range of operating conditions (ambient temperature, state of charge, current amplitude, and pulse width). [DOI: 10.1115/1.4035742]

Estimating state-of-charge imbalance of batteries using force measurements

This paper addresses the problem of estimating SOC-imbalance between two battery cells connected in series. Particularly, the effectiveness of using force measurements for the SOC-imbalance detection against pack/total voltage measurements is studied. SOC imbalance estimation during charging using pack voltage measurement was previously demonstrated for the LiFePO4/graphite battery chemistry. However, the Li-ion battery with LiNiMnCoO2/graphite, which is of great interest in a hybrid electric vehicle application, exhibits an almost linear relation between SOC and voltage when battery SOC is greater than 15%. This characteristic makes SOC imbalance estimation using pack voltage challenging. The use of other novel measurands, related to volumetric change of the electrode materials during battery operations, make the problem feasible. To estimate SOCs of two batteries connected in series, a Moving Horizon Estimation (MHE) technique is applied and three different measurement sets are considered: (1) total voltage, (2) force, and (3) both voltage and force. Simulations results show that, for the batteries of interest, the inclusion of force measurements significantly improves the estimation of SOC-imbalance.

Observability analysis for surface sensor location in encased battery cells

Compact battery pack design and cooling requirements present significant challenges for the placement of temperature sensors. Typically, temperature sensors are located near the edge of the battery, and away from the hottest regions, which leads to slower response times and increased errors in the prediction of interior cell temperature. New sensor technology allows for sensor placement between cells to improve sensor performance. With the ability to place sensors anywhere on the exterior of the cell, an observability analysis is necessary to determine the optimal locations for these sensors. The analysis is performed using a spatial discretization of a validated electrothermal model. This model describes a 5 Ah Li-ion cell harvested from a Ford C-max 2013 pack. Given that the spatial discretization of the heat partial differential equation (PDE) governing the system results in singular values that are very small (numerically zero), two methods are presented in this paper to quantify observability and address the issue of optimal sensor placement. The optimal sensor placement between cells yields a 240% improvement (for 3 sensors on the surface) and 15% improvement (for one sensor on the surface) in observability over existing sensor placement near the top edge of the cell. The pack geometry and airflow conditions impact sensor placement. It is preferable to place the sensors towards the outlet side of the airflow as opposed to the inlet side, resulting in a 13% improvement in observability.

Influence of Battery Downsizing and SOC Operating Window on Battery Pack Performance in a Hybrid Electric Vehicle

This paper provides a parametric study for (1) downsizing a battery pack (reducing the number of battery cells), potentially reducing cost and weight; and (2) lowering the nominal operating SOC to reduce degradation. The downsized pack design with shifted SOC window is evaluated in a light- duty hybrid electric vehicle (HEV) where the power demanded by the battery pack is specified prior to downsizing. A calibrated electro-thermal model and a semi-empirical capacity fade model are used to capture voltage, state-of-charge, temperature and capacity loss of the downsized battery. The capacity fade model is developed based on a novel set of experiments designed to clarify the influence of nominal operating SOC on battery degradation. The parametric study shows that the pack size could be reduced from 76 to 64 cells while shifting nominal operating SOC from 50% to 35% without experiencing battery power denials. This would result in a 20% increase in energy utilization per cell, with only a 0.4% increase in capacity fade.

Novel thin temperature and expansion sensors for li-ion battery monitoring

This paper introduces a combined temperature and displacement sensor for new measurements of physical parameters which can inform multi-physics based models of Li-ion batteries. These flexible sensors can be placed directly on the cell to measure intercalation effects which can improve battery state estimation. The sensors were characterized on individual Panasonic cells and subsequently, packaged into a 76 cell Ford Focus hybrid electric vehicle (HEV) pack, which was cycled for 5000 equivalent miles. Results showed improvement in temperature estimation capability with the combination of sensor and model. However, the low expansion of the hard cased cells limited utility of the eddy current sensors.