Battery system

A battery system comprising:

• a first battery cell including a first positive electrode terminal and a first negative electrode terminal;
• a second battery cell including a second positive electrode terminal and a second negative electrode terminal;
• a third battery cell including a third positive electrode terminal and a third negative electrode terminal;
• a first connection which electrically connects the first negative electrode terminal and the second positive electrode terminal;
• a second connection which electrically connects the second negative electrode terminal and the third positive electrode terminal;
• a power load which is disposed between the first positive electrode terminal and the third negative electrode terminal and is supplied with current from the first, second and third battery cells;
• a first voltage sensor for measuring a cell voltage of the first battery cell via the first connection;
• a second voltage sensor for measuring a cell voltage of the second battery cell via the first connection and the second connection;
• a third voltage sensor for measuring a cell voltage of the third battery cell via the second connection;
• a cell balancing circuit which enables electromotive voltages of the first to third battery cells to be substantially equal;
• and
• a controller which supplies the current to the power load while controlling the electromotive voltages of the first to third battery cells to be substantially equal by using the first to third voltage sensors, and the cell balancing circuit, thereby determining looseness of the connection.