A new method for state of charge (SOC) assessment of Li/MnO2 primary batteries is disclosed. The method is based on thermodynamics data measurements and processing. Entropy ‘ΔS’ and enthalpy ‘ΔH’ data are collected during intermittent discharge process and tentatively linked to SOC. Using Li/MnO2 cell sets from two different manufacturers, we found a linear relationship between SOC, ΔS and ΔH; i. e. SOC = α+βΔS+λΔH. In the equation α, β and λ coefficients are manufacturer dependent. This result has been assigned to the physical chemical characteristics MnO2, which vary according the cathode material synthesis conditions specific to each manufacturer. The SOC assessment method was initially developed by us for rechargeable lithium ion battery systems. It surprisingly applies to primary batteries too despite cathode processes are irreversible in nature. It is suggested the method may extend to online SOC assessment owing to an imbedded chip, which collects and processes voltage and temperature data to convert them into thermodynamics data. The method can also be used to characterize MnO2 materials from the thermodynamics properties perspective and to trace Li/MnO2 cell manufacturer.