A battery sensor for measuring the state of charge, temperature and electrolyte level of a battery. The state of charge is determined by a concentration cell. The concentration cell comprises an ionic fluid of known concentration separated from the electrolyte by an ionically porous plug. When a first electrode is placed in the electrolyte and a second electrode is placed in the fluid, a voltage, which is a function of the difference in the concentration of each, is generated so that a state of charge of the electrolyte may be determined. Temperature determination is made by positioning thermistors above the electrolyte thereby measuring the gas temperature over the aqueous electrolyte. By positioning one of the thermistors in contact with a catalyst for recombining the gaseous hydrogen, produced by a fully charged battery, with oxygen, sensing means are established for indicating an overcharge situation. The porous plug is positioned at the minimum safety level for the electrolyte such that when the electrolyte falls below the minimum level, the concentration cell voltage between the two electrodes becomes erratic and can be used to activate a warning signal. The sensor is constructed such that the sensing apparatus and voltage generating device can be contained within a battery cap.