Consider a capacitor  $C$ that is charged by a battery of emf $V$ to a potential $V$. The capacitor is then disconnected from the battery and then connected again with it but now with its polarity reversed. Then the 

Initial charge on the battery = CV , Final charge on the battery  = CV .

So change in energy stored in the battery = 0

Total charge flown through the battery = CV + CV = 2CV

Work done by the battery $W_b = Charge flown through the battery \times emf =2C{V}^2$