Matthew Keyser

Thermal characteristics of selected EV and HEV batteries

Battery management is essential for achieving desired performance and life cycle from a particular battery pack in electric and hybrid electric vehicles (EV and HEV). The batteries must be thermally managed in addition to the electrical control. In order to design battery pack management systems, the designers need to know the thermal characteristics of modules and batteries. Thermal characteristics that are needed include heat capacity of modules, temperature distribution and heat generation from modules under various charge/discharge profiles. In the last few years, the authors have been investigating thermal management of batteries and conducting tests to obtain thermal characteristics of various EV and HEV batteries. They used a calorimeter to measure heat capacity and heat generation from batteries and infrared equipment to obtain thermal images of battery modules under load. In this paper, they present their approach for thermal characterization of batteries (heat generation, heat capacity, and thermal images) by providing selected data on valve regulated lead acid, lithium ion, and nickel zinc battery modules/cells. For each battery type, the heat generation rate depends on the initial state of charge, initial temperature, and charge/discharge profile. Thermal imaging indicated that the temperature distribution in modules/cells depends on their design.

Search for an optimized cyclic charging algorithm for valve-regulated lead–acid batteries

Abstract Valve-regulated lead–acid (VRLA) batteries are characterized by relatively poor performance in cyclic applications of the order of two hundred to three hundred 100% depth-of-discharge (DoD) cycles. Failure is due to sulfation of the negative plate and softening of the positive active-material. It is felt that this failure mode arises from abnormally high levels of oxygen recombination that arise due to decreases in separator saturation levels as VRLA batteries age. Charging algorithms have been developed to address this changing condition throughout life. The key step is the finish of charge where, traditionally, low currents and low overcharge limits have been employed with poor results. It has been found that using high finishing currents in an alternating charge–rest algorithm results in proper recharge of the negative plate without creating unacceptable temperature increases. This has resulted in deep-discharge lifetimes of 800 to 1000 cycles, particularly when using a charging algorithm employing only partial recharges (97–100% return) interspersed with full conditioning recharges every 10th cycle. With such minimal average overcharge levels, deep-cycle lifetimes approaching 1000 cycles have been achieved without experiencing failure due to massive grid corrosion.

Reducing cold-start emissions by catalytic converter thermal management

Vacuum insulation and phase-change thermal storage have been used to enhance the heat retention of a prototype catalytic converter. Storing heat in the converter between trips allows exhaust gases to be converted more quickly, significantly reducing cold-start emissions. Using a small metal hydride, the thermal conductance of the vacuum insulation can be varied continuously between 0.49 and 27 W/m{sup 2}K (R-12 to R-0.2 insulation) to prevent overheating of the catalyst. A prototype was installed in a Dodge Neon with a 2.0-liter engine. Following a standard preconditioning and a 23-hour cold soak, an FTP (Federal Test Procedure) emissions test was performed. Although exhaust temperatures during the preconditioning were not hot enough to melt the phase-change material, the vacuum insulation performed well, resulting in a converter temperature of 146{degrees}C after the 23-hour cold soak at 27{degrees}C. Compared to the same converter at ambient conditions, overall emissions of CO and HC were reduced by 52 % and 29 %, to 0.27 and 0.037 g/mile, respectively. The maximum converter temperature during the FTP cycle was 720{degrees}C. This limited testing was performed with a nearly-fresh palladium-only catalyst, but demonstrates the potential of this vacuum insulation approach for emissions reduction and thermal control. Further testing is ongoing. An initial assessment of several production issues is made, including high-volume fabrication challenges, durability, and cost.

Thermal analysis and testing of a vacuum insulated catalytic converter

Based on a recent U.S. Environmental Protection Agency (EPA) study, about 95% of all trips start after a cold-soak period of 16 hours or less. By preserving the heat in the catalyst between trips, exhaust gases could be processed without warm-up delay and without the usual cold-start emissions. Vacuum insulation and phase-change thermal storage have been incorporated into a catalytic converter design to enhance its heat-retention time. Laboratory testing of a bench-scale prototype showed that a `light off` temperature (above 350{degree}C) could be maintained during a 10-hour cold soak. Design improvements currently being tested should increase this heat-retention time to more than 16 hours. The thermal conductance of the vacuum insulation will be made continuously variable to prevent overheating and excessive thermal cycling. This approach to thermal management may be more durable and less costly than quick-heat methods using electric or fuel-fired preheat catalysts. 10 refs., 4 figs.

Identifying the Cause of Rupture of Li‐Ion Batteries during Thermal Runaway

Abstract As the energy density of lithium‐ion cells and batteries increases, controlling the outcomes of thermal runaway becomes more challenging. If the high rate of gas generation during thermal runaway is not adequately vented, commercial cell designs can rupture and explode, presenting serious safety concerns. Here, ultra‐high‐speed synchrotron X‐ray imaging is used at >20 000 frames per second to characterize the venting processes of six different 18650 cell designs undergoing thermal runaway. For the first time, the mechanisms that lead to the most catastrophic type of cell failure, rupture, and explosion are identified and elucidated in detail. The practical application of the technique is highlighted by evaluating a novel 18650 cell design with a second vent at the base, which is shown to avoid the critical stages that lead to rupture. The insights yielded in this study shed new light on battery failure and are expected to guide the development of safer commercial cell designs.

Life prediction model for grid-connected Li-ion battery energy storage system

Lithium-ion (Li-ion) batteries are being deployed on the electrical grid for a variety of purposes, such as to smooth fluctuations in solar renewable power generation. The lifetime of these batteries will vary depending on their thermal environment and how they are charged and discharged. To optimal utilization of a battery over its lifetime requires characterization of its performance degradation under different storage and cycling conditions. Aging tests were conducted on commercial graphite/nickel-manganese-cobalt (NMC) Li-ion cells. A general lifetime prognostic model framework is applied to model changes in capacity and resistance as the battery degrades. Across 9 aging test conditions from 0°C to 55°C, the model predicts capacity fade with 1.4% RMS error and resistance growth with 15% RMS error. The model, recast in state variable form with 8 states representing separate fade mechanisms, is used to extrapolate lifetime for example applications of the energy storage system integrated with renewable photovoltaic (PV) power generation. Uncertainty quantification and further validation are needed.