One rechargeable battery of mass 15.0 g delivers to a CD player an average current of 18.0 mA at 1.60 V for 2.40 h before the battery needs to be recharged. The recharger maintains a potential difference of 2.09 V across the battery and delivers a charging current of 13.5 mA for 4.20 h.
(a) What is the efficiency of the battery as an energy storage device?