Method to fill oxygen in a nimh battery

A method for filling oxygen into a nickel metal hydride, NiMH, battery module with at least one battery cell, the battery module having a casing encompassing the at least one battery cell and a gas space, wherein each battery cell in the battery module has a first electrode, a second electrode, a porous separator, and an aqueous alkaline electrolyte arranged between the first electrode and the second electrode, wherein the porous separator, the first electrode and the second electrode are configured to allow exchange of hydrogen and oxygen by allowing gas to migrate between the electrodes, and wherein the casing has an inlet for adding oxygen to the gas space of the at least one battery cell, the method comprising:

• regularly determining present internal resistance of the at least one battery cell of the battery module to calculate a resistance ratio, R, in relation to an initial internal resistance of the at least one battery cell of the battery module;
• filling the battery module with oxygen when the resistance ratio, R, is higher than a maximum value, Rmax;
• monitoring battery module temperature and initiating filling of oxygen at an initial flow rate;
• and
• regulating the oxygen filling flow rate based on the monitored battery module temperature to avoid exceeding an upper temperature limit.