The NiMH battery is resilient against mistreatment and can be charged in a large temperature window. It also has a high power-capability, which has made it popular in for instance power tools. Today, the battery is often found in large scale stationary energy storage, as well as applications which require high power capability.

In large scale energy storage, small differences in run cycles can have large total effects on power efficiency and safety. It is therefore desirable to develop better methods of battery management. In recent years, development in system hardware has made it possible to implement on-line models in battery management systems. Such an implementation requires a model capable of working in dynamic conditions simulating voltage, temperature, and pressure. In this study, a model for this purpose is developed and verified.

There are two major features that present a challenge when modelling a NiMH-system dynamically: Open Circuit Voltage (OCV) hysteresis and pressure build-up. Handling the hysteresis effect is important to accurately predict voltage and OCV.[1] OCV is especially important, as OCV is used to estimate the battery state of charge (SOC). Pressure build-up can be an important tool for charge termination, as it is the build-up of oxygen pressure and subsequent temperature increase that signal end of charge, Figure 1. In addition, the hydrogen pressure behavior changes with battery age and can be used to estimate end-of-life.

OCV hysteresis is when the open circuit voltage at a certain SOC depends on the path taken to reach that charge state. This mean that a battery that has been charged to 50% will have a drastically different OCV than when the same battery has been discharged to 50%. This effect is believed to arise primarily from structural changes in the positive electrode, where the potential of the electrode surface is dependent on the material structure of the surface. An example of open circuit voltage hysteresis in a NiMH cell is found in Figure 1.

The gas phase in the battery consists of four gases: nitrogen, hydrogen, oxygen and water. Nitrogen is an inert remnant from the battery manufacture.[2] Water is in equilibrium with the aqueous electrolyte. Hydrogen is in equilibrium with intercalated hydrogen in the negative metal hydride (MH) electrode. Finally, oxygen is the product of a side reaction that occurs at high potentials on the positive electrode. The production of oxygen at the positive electrode and ensuing oxygen recombination on the negative electrode produces a great deal of heat, which heats the battery and produces the temperature response that can be used to set a temperature derivative determined end of charge criteria. However, the presence of oxygen and high temperatures also drives the major aging mechanism: oxidation of the negative electrode. As the negative electrode is consumed, the capacity decreases. This leads to a difference in the hydrogen equilibrium pressure, as reaching a higher intercalation fraction increases the equilibrium pressure. [3]

An implementation of a model for system battery management could help improve the following functions for a NiMH system: SOC estimation, SOH estimation, charge termination, fault detection and aging prevention. Therefore, development of a fully dynamic NiMH model has great value in improving over all system function for large scale energy storage applications. This study presents a fully dynamic and experimentally verified model for the NiMH battery that takes both pressure and OCV hysteresis into consideration.

References

1. Axén, J. B.; Ekström, H.; Zetterström, E. W.; Lindbergh, G. Evaluation of Hysteresis Expressions in a Lumped Voltage Prediction Model of a NiMH Battery System in Stationary Storage Applications. J. Energy Storage 2022, 48 (January). https://doi.org/10.1016/j.est.2022.103985.
2. Mank, A. J. G.; Belfadhel-Ayeb, A.; Krüsemann, P. V. E.; Notten, P. H. L. In Situ Raman Analysis of Gas Formation in NiMH Batteries. Appl. Spectrosc. 2005, 59 (1), 109–114. https://doi.org/10.1366/0003702052940503.
3. Tserolas, V.; Katagiri, M.; Onodera, H.; Ogawa, H. Thermodynamical Modeling of P-C Isotherms for Metal Hydride Materials. Trans. Mater. Res. Soc. Japan 2010, 35 (2), 221–226. https://doi.org/10.14723/tmrsj.35.221.

Figure 1 Left: Open Circuit Voltage of a NiMH cell with exhibited hysteresis behavior. Right: Pressure and Temperature behavior of a NiMH module during a charge discharge cycle.