Battery charging Basics - 3

In the previous scenario, we have seen battery life time before it must be replaced or before it must be recharged. The condition was that there is a constant current drawn by the load for an hour. In a real time scenario, that is not the case. There will be a sudden current drawn by the load and it remains in sleep for a long period of time. This can be a case in latest IoT (Internet of things) products.

The battery calculations are done this way:

Let us assume, the following conditions for the circuit shown in the previous post:

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, in addition, let us assume a condition when the load is drawing current of 0.169A every 10 minutes in a given hour. Considering this scenario, the current of 0.169A will be drawn 7 times an hour and negligible current for the other time.

So, in such case average current drawn by circuit from load is, 0.169*(7/(7+6)) = 0.091A

So, a 7.4V battery with 3200mAh battery capacity supplying 0.091A to load for 1 hour can withstand for, 

3.2/0.091 ~ 35 hours 

The commonly used  terminology here is the duty cycle which is calculated by the formula, T_ON/ T_ON+ T_OFF