The first power measurements for the Firefly Ice prototype PCBA look really good. Very low power means small batteries can be used while still getting great battery life.
Sleep Mode
Average power consumption for the PCBA with the real time clock and watchdog timer running (everything else in sleep mode) was measured at 2.7uA.
Plus Accelerometer
Average power consumption for the PCBA with the real time clock, watchdog timer, and accelerometer running (everything else in sleep mode) was measured at 6.0uA. The accelerometer was running at 25Hz in low power mode.
Battery Life
To relate the 6.0uA to battery life:
- A CR2032 240mAh coin cell battery would last 4.5 years.
- A 40mAh rechargeable lithium polymer battery would last 9 months.
Of course a typical application will need to use other devices on the PCBA, such as the Bluetooth 4.0 low energy radio, LEDs, CPU, etc.
Activity Monitoring Scenario
Consider a simple activity monitoring scenario. The radio sleeps 90% of the time, is in discovery mode 5% of the time, and connected 5% of the time, adding 2.4uA. The CPU usage averages 10,000 instructions per second adding 2.8uA. Two LEDs are illuminated for 30 seconds a day adding 3.5uA. This along with the CPU and accelerometer usage of 6.0uA, brings the total average current to 14.7uA. The CR2032 battery would last about 1.8 years and the 40mAh battery would last about 3.5 months.
Balance
Two of the primary design challenges for body worn sensors are size and battery life. Those are opposing forces – smaller size means smaller batteries and lower battery life. One of the keys to a good design is to find the right balance for your application. For certain applications where the duty cycles are low it is certainly possible, with a good design, to have a very low overall average current which enables long life when using small batteries.
