Suppose I have a 20 AmpHour battery at 4V (as in, the actual capacity of the battery is 80 Watt hours). And I'm using a 10V 2A charger to charge it (this is a 20 Watt charger). How can the charge time be calculated?
Is it 4 hours?
Note: The charge time is definitely not 20Ah/2A = 20h as a lot of people state, since these numbers are close to the actual values for most fast-charging smartphones today, and the charge time should be of the order of 1 hr).
Note 2: Please explain using the fundamentals of physics, instead of obscure formulae.
Since the charger is at 10V and the battery is at 4V, how does this affect the charging?