Rechargeable battery systems and methods having a plurality of power modules and a battery management system. The invention relates in particular to a wind turbine blade, a wind turbine and a method of controlling a wind turbine. The methodology includes looking-up a name and address for telephone numbers, which are categorized as unknown telephone numbers, in a current directory service. Accordingly, it is possible to know the exact amount of a liquid substance in the container by using a scale provided on the sheets constituting the container even if a crease is formed in the sheets due to the liquid substance being in the container.