AN92584 Datasheet, PDF (23/42 Pages) Ramtron International Corporation – Designing for Low Power and Estimating Battery Life for BLE Applications

Designing for Low Power and Estimating Battery Life for BLE Applications
4.1.2
Data and Event Processing Block
The sensor data requires processing to derive meaningful and actionable information. You may also need to
compress the data to reduce the over-the-air bandwidth required to transfer the data to the peer device.
Sometimes, a BLE device may use more than one sensor. For example, a heart-rate monitor device may measure
the battery level in the device to detect a low-battery condition, in addition to heart-rate sensing. A BLE touch mouse
may have an optical sensor, a touch sensor, a scroll key, a trackpad, and multiple buttons to detect key presses. In
such systems, you need to detect activities in multiple sensors, process the data from multiple sensors, and send the
data from multiple sensors over the BLE link. Each sensor’s scan rate can be asynchronous to the others and the
BLE link. Efficient management of these multiple asynchronous activities is an important factor that contributes to the
current consumption.
4.1.3
BLE Connectivity Block
The sensor data is sent to a Central device through BLE notifications or through Read requests from the Central
device. The time taken to process the notifications and send the sensor data to the peer device efficiently will
determine the current consumption.
4.2 Battery Life
Most BLE devices are battery operated. A long battery life is typically the most important requirement for a battery-
operated device. A long battery life helps reduce the maintenance cost of replacing the batteries often. It ensures
high availability of the devices in the deployed environment. It also allows using a lower-capacity battery for a given
lifetime of device usage, reducing the battery cost and allowing the devices to be smaller (See Table 13).
Table 13. CR Battery Comparison
Type
CR1025
CR1220
CR1616
CR1620
CR1632
CR2032
CR2450
Capacity (mAh)
30
35–40
50–55
75–78
140
225
610–620
Size (d x h, mm)
10 × 2.5
12.5 × 2.0
16 × 1.6
16 × 2.0
16 × 3.2
20 × 3.2
24.5 × 5.0
The battery life is a function of the time that the device is used over the lifetime of the battery and the average current
consumption of the system during its use. A device usage profile is the time pattern of a typical user using the BLE
device through a normal day or a week—how long the device is used for the intended function, how long the device is
idle, how long the device is switched OFF, and so on.
Based on the usage profile, the device can be put through different low-power states to reduce current consumption.
The duration that the device is put in a specific state is determined by the usage profile and the application
requirements. The current consumed in each state is determined by the system architecture, low-power modes used,
and the current consumed by the different parts of the system in that state. Therefore, it is important to determine the
usage profile and a current profile in various system states to estimate the battery life.
An illustration of the usage and current profile for an activity-monitoring (sleep and activity levels) device is shown in
Figure 12. A user uses this device through the day; the user may switch it OFF intermittently. The current consumed
varies accordingly.
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Document No. 001-92584 Rev. *A
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