• electrochemical impedance spectroscopy
• intercept frequency
• internal battery temperature
• li-ion batteries
• sensorless temperature measurement
• thermal issues

Li-ion batteries represent the most efficient way to supply portable and mobile electronic devices with electrical energy. However, these batteries imply safety and stability problems. Exceeding of temperature and voltage safety ranges may cause battery degradation, permanent damages and explosions. Electrochemical Impedance Spectroscopy (EIS) is a widely used experimental method to gain deeper information on electrochemical systems. It allows analyzing some battery characteristics, such as state of charge (SOC), state of health (SOH) and internal temperature, via impedance by applying a sinusoidal signal and by measuring the response. Experimental analyses are carried out on two different tablet batteries, a LiFePO4 ANR26650 cylindrical cell and a Panasonic 18650 cylindrical cell. Changes of the battery impedance’s spectrum with respect to temperature and to state of charge are widely investigated. This work confirms the existence of an intrinsic relationship between battery internal temperature and three different battery parameters: module and phase of the impedance and the frequency at which the imaginary part is zero. The three methods are exhaustively analyzed and compared in terms of accuracy and sensitivity. It is demonstrated that it is possible to monitor battery internal temperature by means of impedance measurements without the need of external sensors or inserted thermocouples.