Clearly there is nowadays an increasing use of portable electric devices (mobile phones, laptops, electric cars, sensor networks ...). These devices use batteries as a power source. Therefore, it is increasingly important to have a good knowledge of the remaining battery power, to exactly know at what point should be recharged. Moreover its wear out must also be known in order to replace it when necessary. Li-ion batteries have provided many advantages over their predecessors. They provide a higher voltage, they are lighter and the energy density with respect to their weight and volume is much higher. This project aims to estimate the actual remaining battery charge through temperature variations that occur inside the battery both during charging and discharging. The aim is to find a simple and cheap solution, which does not require external elements. For this reason, temperature measurements are carried out using a thermistor that is already present in lithium batteries. At present, its purpose is to avoid overheating. A Li-ion battery with 740mAh capacity provided by VARTA Microbattery was used. From measurements of current, voltage and temperature we calculated the internal resistance, the heat flow inside the battery and the thermal impedance. It has been shown that it is possible to relate temperature variations, current and voltage to the heat generated in the battery. Thus, it is possible to estimate the state of charge of the battery (SoC) from temperature fluctuations. In addition, consecutive charge and discharge cycles were carried out. Injected and extracted charge was calculated for each of them. Then, it was related to SoH. Finally, new definitions of SoC and SoH which consider their relationships are proposed.