Charge cost question

Question

Suppose you get a new Samsung Galaxy S20 to take advantage of the new 5G network. Unfortunately, when you receive it, the 4000mAh battery is “dead”. Assume that the overall charging efficiency is 85%. How much would a complete charge cost if electricity costs $0.12/kW-hr ($). Assume the charger is a normal 5V and 12W USB.

Answer 1

The question is lacking of one important parameter: what is the nominal voltage of the battery. Without the voltage, with only the 4000mAh parameter it is not enough to calculate the total electricity need to charge the battery. The 5V 12W USB is irrelevant here since the question already assume the charging efficiency to be 85%, unless we assume the charging voltage is 5V and the charging process is at the full capacity of the USB charger (this assumption is pretty lame).

Anyway. under the lame assumption, the charger is working at its full capacity which is 12W / 5V = 2.4 A. It will take t= 4000mA /2.4A = 1.667 hour to full charge the battery. Consider the charging efficiency 85%, the charging time will be 1.667/0.85=1.96 hour. So the total electricity consumed is 12W × 1.96 Hour = 23.53 W-Hr. Therefore the total cost is 23.53 /1000 ×0.12=0.0028$, so it's less than 1 cent.