The dynamic behavior of the NiMH battery – Creating a versatile NiMH battery model

Sammanfattning: To facilitate the shift from fossil to renewable energy sources, energy storage is needed to cope with the intermittent nature of technologies such as solar, wind, and wave power. One storage alternative is battery-based stationary energy storage. There are many battery types to choose from, but Nickel Metal Hydride (NiMH) is a type that is especially well suited. These batteries have a high energy density, a large temperature operating window and are a safe alternative for large scale energy storage.In this thesis, the behavior of the NiMH battery is studied with the aim to develop a dynamic battery model, a model that is capable of reproducing the battery voltage and pressure, also for dynamic usage. Such a model can be used to facilitate development of NiMH batteries, improvement of the algorithms used in the Battery Management System (BMS), quality control, and dimensioning of energy storage systems. These improvements can lead to stationary energy storage with a higher efficiency and longer usable lifetime.To increase the understanding of the battery function, deeper study was carried out of two behaviors that are typical for the NiMH battery and are deemed to have a large impact on the battery: Open circuit voltage (OCV) hysteresis and the battery gas phase behavior. The OCV hysteresis complicates modelling because it causes the battery rest voltage at a certain degree of charge to depend on the charge/discharge path taken to get there. OCV hysteresis is not noticeable for all batteries, and it is especially prominent for the NiMH battery. The gas phase in the NiMH battery is active since the electrolyte is water based and the voltage window during operation causes oxygen evolution at the positive electrode. The oxygen is then recombined into water at the negative electrode. The amount of hydrogen in the gas phase varies over a cycle due to the the dependence on temperature and state of charge of the hydrogen equilibrium pressure over the negative metal hydride electrode.Two models were developed separately to study these behaviors. The models showed good qualitative reproduction capabilities. The hysteresis phenomenon was also studied using structural analysis methods. Differences were identified in the material structure between two samples of the positive electrode material at the same state of charge but different hysteresis states. These differences were found in both the bulk and the surface region of the particles. The differences in bulk were related to degree of disorder and the differences in the surface region to inhomogeneity in Li distribution in the cobalt oxyhydroxide layer. The gas composition was studied using mass spectrometry. The gas phase was mostly composed of nitrogen, but hydrogen was responsible for the majority of the pressure changes of the battery during a charge/discharge cycle. Oxygen could be detected at the end of charge, where it is produced due to high voltage on the positive electrode.Finally, the two models were added to a P2D-model. This model type is commonly used to simulate battery behavior, and is based on electrochemical theory with approximations used for the porous electrode behavior. The spacial distribution is modeled in one dimension with an additional dimension added locally to simulate intra particle diffusion. The combined model showed that the behavior seen from a NiMH during dynamic usage could be recreated qualitatively through adding OCV hysteresis and the gas phase behavior to this standard model type.