Battery charging Basics - 2

The main challenge for any system in a battery powered system is the battery withstanding time. For example if you have the following system:

The battery calculations are done this way:

Let us assume, the following conditions for the above circuit:

Switcher output = 1V

Load Current = 1A

So, total power = 1W

For a given efficiency of switcher of 80%, Input power = Output Power/Efficiency = 1/0.8 = 1.25W

For a power of 1.25W at the input, current  = Power/Voltage = 1.25/7.4 = 0.169A

So, a 7.4V battery with 3200mAh battery capacity supplying 0.169A to load can withstand for,

3.2/0.169 ~ 19 hours