Side reactions leading to gas evolution are undesirable in batteries and result in reduced coulombic efficiency and shortened lifetime. Quantitative analysis of the gases that evolve is therefore important to improve understanding of the reactions occurring in the battery during cycling and could be used to optimize battery operation. However, the measurements are challenging because batteries are by their nature closed with limited gas space. Nickel metal hydride (NiMH) batteries are widely used due to their good rate capability, reliability, and environmental friendliness. The battery type has been extensively studied in terms of degradation and performance. However, very few studies have been conducted on the gas composition created during a work cycle. In this study, two methods for investigating the internal NiMH battery gas phase composition during different charge/discharge cycles using a 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 most abundant component in the measured gas phase was nitrogen, present in the cell from the assembly stage, followed by hydrogen. A clear rising trend of hydrogen pressure as depth of charge (DOC) increased was recorded, while oxygen levels were low except around the end of charge. The methods were found to be a reliable means of investigating NiMH gas composition without negatively affecting the battery and may be adapted to other battery chemistries. In this study, two methods for investigating the internal NiMH battery gas phase composition during different charge/discharge cycles using a mass spectrometer (MS) were developed.

Side reactions that lead to gas evolution are undesirable in batteries, and result in reduced Coulombic efficiency and shortened lifetime. Quantitative analysis of the gases that develop istherefore important to increase the understanding of the reactions occurring in the batteryduring cycling and could be used to optimize battery operation. However, the measurementsare associated with challenges because batteries are by their nature closed with limited gasspace. Nickel metal hydride (NiMH) batteries are widespread due to good rate capability,reliability, and environmental friendliness. Although the battery type is thoroughlyinvestigated, studies of battery gas composition formed during a work cycle are few. In thisstudy, two methods for investigating the internal NiMH battery gas phase composition duringdifferent charge/discharge cycles using a mass spectrometer (MS) were developed. In the firstmethod, the battery module was connected by a sampler system. In the second method, thebattery was connected directly using a microcapillary, and the gas composition wascontinuously measured. In addition to the gas composition the voltage, pressure, andtemperature of the battery were recorded. The biggest contributor in the measured gas phasewas nitrogen, present in the cell already from the assembling, followed by hydrogen. A clearrising trend of hydrogen pressure as depth of charge (DOC) increased was recorded, whileoxygen levels were low except around end of charge. The methods were found to be a reliableway of investigating NiMH gas composition without negatively affecting the battery and maybe adapted to other battery chemistries.

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.