In this paper we present an innovative and precise way to calculate the available capacity in a battery. This quantity is essential to assess the ageing process during real use or ageing tests. The most common way to measure the battery capacity consists on a constant current discharge. It is quite simple to implement but very dependent of impedance and relaxation state of the battery. Consequently, this method is not suitable to quantify reversible and irreversible capacity losses occurring on batteries. We propose an indirect measure of available capacity that reduces the distortion caused by battery relaxation and impedance changes.