An infrared thermal video imaging system (Model TVS2000) has been used to follow the evolution of heat in VRLA/AGM cells and monoblocs. The system has the capability of resolving 0.05 degrees C temperature differences within such cells. Minute thermal inhomogeneities are easily detectable and allow the dynamics of the heat evolution to be followed during discharge and charge processes. In large 1000 Ah VRLA/AGM cells, electrolyte movements are traceable with this method through the associated shifting of the site of the oxygen recombination reaction and the subsequent local evolution of heat. The areas of heat evolution match those with the most significant degree of local grid-growth.