Li-ion batteries are continually developed for a wide range of use. New variants appear on the market, including Li-ion technologies that allow supplying very high electrical power. By consequence, the battery can be used as an energy buffer in vehicles. The autonomy is limited and it is necessary to determine the State of Charge of each element that makes up a battery, although these elements (Li-ion cells) are never fully discharged or charged. In this unusual context, a solution may be to use a Kalman filter at the cost of significant computing power. More computation-wise economical observers can beings used, in particular Luenberger observers. The results obtained for this type of specific use, give similar final results, with a significant gain on the computing power required.