A cell phone charger uses 4.83 joules per second when plugged into an outlet, but only 1.31 joules per
second actually goes into the cell phone battery. The remaining joules are lost as heat. That’s why the battery
feels warm after it has been charging for a while. How efficient is the charger?

Well, that depends on how you look at it.Efficiency =        (amount of power that does what you want) / (total power supplied).Power to charge battery . . . 1.31 wattsPower changed to heat . . .  3.52 watts Total power supplied . . . . . . 4.83 wattsUsing the cell-phone as a heater, efficiency of the charger in supplying heat = 3.52 / 4.83 =  72.9 % .Using the cell-phone as cell-phone,efficiency of the charger in charging the battery = 1.31 / 4.83 =  27.1 % .The charger is more efficient as a heater than it is as a battery charger.