As a part of battery system operation, battery models are often used to determine battery characteristics such as the state of charge (SOC) and the state of health (SOH). A phenomenon that has a large impact on battery model accuracy for NiMH batteries is open circuit voltage (OCV) hysteresis, which causes the OCV to differ not only with the SOC of the battery but also with the charge-discharge history. This characteristic is especially influential when running the system in applications with dynamic current patterns. A model including a way to emulate battery hysteresis behavior would improve the battery management system function. In this study a lumped battery model for cell voltage prediction was expanded to include an OCV hysteresis model. Different expressions to describe the hysteresis behavior were explored. The different models were then evaluated using both synthetic and real-life application experimental data. In all cases the error was reduced by adding a hysteresis component to the model. Using this type of model in the battery management system of stationary energy storage systems based on NiMH batteries could help make the state prediction more accurate. This, in turn, would allow for better optimization of the system operation, something that could help increase system efficiency and lifetime.

Powder X-ray diffraction (XRPD), X-ray absorption spectroscopy (EXAFS and XANES) and Raman spectroscopy were used to study chemical changes in the polycrystalline nickel hydroxide positive electrode material of a NiMH battery at four states of charge: 0%, 50% and 100% charged, and 50% discharged. The two 50% samples were at the same state of charge but in different hysteresis states, manifested by differences in the open circuit potential. The nickel hydroxide electrodes consist of particles in the µm size range, and all measurements were performed ex situ. The material studied was taken from commercial batteries and as such contained both metallic nickel particles, cobalt, and zinc dopants in the active Ni(OH)2 material as well as a cobalt oxide surface layer. Combining the results from all the characterization methods was necessary to better understand the chemistry behind the physio-chemical hysteresis behavior in this complex system. Our results show that there are structural differences between the two 50% samples. Comparison of the XRPD results and the EXAFS results on the nickel edge indicate a presence of the kinetically favored TP2-NiOOH phase in the transition between β-Ni(OH)2 and β-NiOOH and that the amount differs between the two hysteresis states. The measurements on the zinc edge using EXAFS and XANES suggest short range differences in the active material bulk that stems from disorder. Raman spectroscopy measurements show a difference in degree of lithium intercalation in the LiCoO2 surface layer between the hysteresis states. As electrochemistry takes place on the surface of the particles, it is likely that differences in the surface structure are responsible for the open circuit voltage hysteresis. However, due to the coherence of the structure differences detected, it is probable that they are all connected and have a part in the observed behavior.

Batteries in energy storage systems are exposed to electrical noise, such as alternating current (AC) harmonics. While there have been many studies investigating whether Lithium-ion batteries are affected by AC harmonics, such studies on Nickel Metal Hydride (NiMH) batteries are scarce. In this study a 10 Ah, 12 V NiMH battery was tested with three different harmonic current frequency overlays during a single charge/discharge cycle: 50 Hz, 100 Hz, and 1000 Hz. No effect on battery internal temperature or gas pressure was found, indicating that NiMH battery aging is not affected by the tested harmonic AC frequencies. This can reduce the cost of energy storage systems, as no extra filters are needed to safeguard the batteries. Instead, the capacitive properties of the batteries give the possibility to use the battery bank itself as a high pass filter, further reducing system complexity and cost.

NiMH batteries are popular in the electromotive industry due to good rate capability, reliability, and environmental friendliness. Although the battery type is thoroughly investigated, studies of battery gas composition formed during a work cycle are few. The gas composition would be useful to understand the reactions occurring in the battery during cycling and could be used to optimize battery operation. 

In this study, two methods for investigating the internal NiMH battery gas phase composition during different charge/discharge cycles using mass spectrometer (MS) were developed. In the first method, the battery module was connected by a sampler system. In the second method, the battery was connected directly using a microcapillary, and the gas composition was continuously measured. In addition to the gas composition the voltage, pressure, and temperature of the battery were recorded. 

The biggest contributor in the measured gas phase was N2, followed by H2. A clear rising trend of H2 pressure as SOC increased was recorded, while O2 levels were low except around end of charge. Thus, the methods were found to be a reliable way of investigating NiMH gas composition without negatively affecting the battery.

The Nickel Metal Hydride (NiMH) battery has an active gas phase during operation. This is due in part to the aqueous electrolyte causing oxygen evolution on the positive nickel electrode, and in part due to the hydrogen stored in the negative metal hydride (MH) electrode being in equilibrium with gaseous hydrogen. The gas phase reactions are closely connected to the battery function and must therefore be accounted for when creating a successful battery management system (BMS).

This study explores a pressure model for management of the NiMH battery. By using measured current, voltage, and temperature as input, the total pressure and gas composition can be modeled. Model parameters are fitted by comparing the modeled total pressure to the measured pressure. By using the system voltage signal, the difficulty of simultaneously modeling the voltage based on the current is circumvented. A model like this opens the way to new ways of battery system management through use of calculated partial pressures and deviations from the modeled total pressure. This can help increasing safety and longevity of battery systems.

In this study, a predictive voltage and pressure nickel metal hydride (NiMH) battery model is presented. The model was validated under conditions that would be seen in applications, with mixed charge and discharge usage patterns. The model is based on an extended P2D model using concentrated electrolyte and porous electrode theory. On top of the charging and discharging processes, the NiMH battery has additional side reactions that affects the battery behavior. These processes are important to include for a model to accurately reproduce the voltage and pressure behavior under usage like conditions. Two processes were identified as necessary for the model to be predictive: Open circuit voltage hysteresis and the gas phase reactions involving oxygen and hydrogen. To take these into account, results from two previous studies that modeled these phenomena separately was introduced into the model. Hysteresis was described using empirical mathematical expressions and the gas phase reactions were described using electrochemical rate equations.

The results show that the resulting model is capable of qualitatively reproducing NiMH battery voltage and pressure behavior, both for a continuous charge/discharge cycle and a varied usage pattern with mixed charge and discharge pulses using different currents. The model was used to study the effect of changes in electrode thickness on the energy and power density during discharge. The mechanism behind the drop in cell voltage at the end of charge was also investigated and found to be connected to the temperature dependence of the oxygen evolution equilibrium potential. Although the model can be fine-tuned further to improve quantitative reproducibility, this study shows that taking the OCV hysteresis and gas phase reactions into account creates a basis for a NiMH battery model that can function for different usage patterns. Such a model has potential to improve the development and use of NiMH batteries, providing a tool to improve battery design and battery management algorithms.