Voltage transformation system

A voltage transformation system (2) comprising - a battery system comprising a first battery (4) and a second battery (6) connected to one another in series, - an equaliser (8) connected to at least one of said first and second batteries and adapted to transforming a first voltage to a second voltage, - a battery sensor (10) situated close to said first battery (4) and comprising a first current measuring circuit adapted to determining a value I 1 which represents the current through said first battery. The system further comprises a second current measuring circuit (12) adapted to determining a value I EQ which represents the equaliser's current to or from the battery system, and to conveying said value to the battery sensor, and the battery sensor comprises a calculation unit adapted to receiving the values I 1 and I EQ and to calculating on the basis of them a value I 2 which represents the current through said second battery. The invention comprises also a method in a voltage transformation system.