AbstractEfficiency losses due to side reactions are one of the main challenges in battery development. Despite providing valuable insights, the results of standard analysis on the individual components cannot be simply extrapolated to the full operating system. Therefore, non‐destructive, and high resolution approaches that allow the investigation of the full system are desired. Herein, we combined neutron radiography and tomography with electrical monitoring of the state of charge of commercial Ni‐mischmetal hydride batteries, to track the exchange and transport of hydrogen under operating conditions. This non‐destructive approach allowed both the quantification of the hydrogen distribution in the electrodes in 4D, and the distinction between the electrochemically exchanged hydrogen and the hydrogen gas pressure generated by side reactions, as a function of the applied potential and current. One of the most counter‐intuitive observation is that the generation of hydrogen gas during discharge depends on the charging state of the battery. The results presented provide critical new insights in the mechanisms governing the electrochemical processes during Nimischmetal hydride battery operation, and also pave the way for the extrapolation of this approach to the investigation of state‐of‐the‐art Li‐ions batteries.