The invention relates to an object's state or properties being monitored, such as a lithium-iron-phosphate (LFP) or other type of electrical battery containing one or more LFP or other type of electrical battery cell within the battery, and one or more sensors for measuring the state or property of the object, such as the state of charge of the battery cells, by measuring the magnetic susceptibility or other properties, such as the magnetic susceptibility of electrolyte or the electrodes within the battery cells. The state of charge of the battery cells within the battery are monitored by the one or more sensors, which are coupled to electrical circuits.