energy loss Questions

Two identical capacitors $C_1$ and $C_2$ of equal capacitance are connected as shown in the circuit. Terminals $a$ and $b$ of the key $k$ are connected to charge capacitor $C_1$ using a battery of emf $V$ volt. Now disconnecting $a$ and $b$ terminals, terminals $b$ and $c$ are connected. Due to this, what will be the percentage loss of energy?