The application functionality and the amount of time a device is expected to spend in each state is defined here.
Once you know the battery characteristics, you can calculate the battery life. The
following table defines the symbols used to characterize properties for each state.
| Variable | Description |
|---|---|
| Qtotal | Battery charge |
| Qpoll | Average charge of a single data poll operation |
| Qapp | Average charge of a single application operation |
| Qnet | Average charge of application data over network transfer |
| Ttotal | Lifetime, battery life |
| Tpoll | Average period for data request action |
| Tapp | Average period for application activity |
| Tnet | Average period for sending application data over the network |
| tpoll | Average time for a single data request action |
| tapp | Average application activity time |
| tnet | Average time spent on the single application data over network transfer |
| tactive | Total time spent in an active state |
| Ipoll | Average current for a single data request action |
| Iapp | Average current for a single application activity |
| Inet | Average current for sending application data |
| Isleep | Average sleep current |
Total time spent in active states is given in the following equation.
Total sleep time is given by the following equation.
Total charge consumed by data requests is given in the following equation.
The following equation shows the total charge that is consumed by application activity.
Total charge consumed by an application data network transfer is shown in the following
equation.
Total charge consumed in sleep state is given in the following equation.
The following equation calculates the total charge consumed by all components.
The following equation gives the total charge represented by the current and time
characteristics.
Battery life as a ratio of total consumed charge (or battery charge) over the average
current of the system is shown in the following equation.
Because the time of an active operation is very short compared to its period and the three
orders of magnitude difference between the sleep current and active operations current, we
can approximate the battery life expression with current values equation as follows.
Working with current values for short events is more difficult than operating on charge
values. The charge value is equal to the operation current integrated over the time duration
as presented in the following figure showing a simplified battery life expression.
With this approximation, the battery life can be estimated for any application by measuring the average charge for active operations and their period. Ratios of these together with the average sleep current give the total application average current. Dividing the battery charge by the application average current results in the estimated battery life.