Thread Sleepy End Devices (SEDs) achieve multi-year battery life by spending >99% of their time asleep. Understanding the power breakdown reveals why sleep current dominates total consumption.
Formula: Average current = (I_active × t_active + I_sleep × t_sleep) / (t_active + t_sleep)
Worked Example - Matter Door Sensor:
Given: - Active current: 15 mA (transmitting sensor event) - Sleep current: 5 µA (maintaining mesh membership) - Active time: 100 ms per event, 20 events/day - Sleep time: remainder of day
Calculate daily active time: \[t_{\text{active}} = 100\,\text{ms} \times 20 = 2\,\text{s/day}\] \[t_{\text{sleep}} = 86{,}400 - 2 = 86{,}398\,\text{s/day}\]
Calculate average current: \[I_{\text{avg}} = \frac{(15\,\text{mA} \times 2\,\text{s}) + (0.005\,\text{mA} \times 86{,}398\,\text{s})}{86{,}400\,\text{s}}\] \[I_{\text{avg}} = \frac{30 + 432}{86{,}400} = 5.35\,\mu\text{A}\]
With a 225 mAh CR2032 battery: \[\text{Battery life} = \frac{225\,\text{mAh}}{0.00535\,\text{mA}} = 42{,}056\,\text{hours} \approx 4.8\,\text{years}\]
Interpretation: Active transmissions contribute only 30 mA·s/day vs 432 mA·s/day from sleep current – just 6.5% of total consumption. In practice, CR2032 self-discharge and temperature effects reduce this to roughly 3–4 years. This is why reducing sleep current from 5 uA to 1 uA (achievable with advanced Thread implementations) has a larger impact than optimizing active current.
