How do you optimize BLE for battery life on a wearable device?

BLE power optimization is a multi-layered problem that spans radio parameters, firmware architecture, and hardware design. The single most impactful parameter is the connection interval — increasing it from 7.5 ms to 500 ms can reduce radio power consumption by 50x or more, since the radio only wakes for a brief window at each connection event. For a wearable that sends heart rate data once per second, a connection interval of 500 ms with slave latency of 0 is sufficient and keeps average radio current well under 50 microamps. Pair this with slave latency — setting slave latency to 9 means the peripheral can sleep through 9 consecutive connection events, waking only on the 10th, effectively turning a 100 ms connection interval into a 1-second wake cycle while still allowing the peripheral to respond within 100 ms if it has urgent data.

At the firmware level, minimize the time the radio is active per connection event. Use Data Length Extension (DLE) to send up to 251 bytes per PDU instead of the default 27 bytes — this amortizes the radio wake-up overhead over more data, dramatically improving energy per byte. Batch sensor readings and send them in a single burst rather than one reading per connection event. Prefer Notifications over Indications for data that can tolerate occasional packet loss, since Indications require an application-layer acknowledgment that extends the radio-on time. On the advertising side, increase the advertising interval to the maximum acceptable for your use case (1-2 seconds is common for wearables) and minimize scan response data to avoid the extra TX/RX cycle when a scanner requests it.

At the system level, ensure the MCU enters the deepest sleep mode between connection events — on Nordic nRF52, this is System ON sleep with RAM retention at approximately 1.5 microamps. Disable unused peripherals, use the lowest possible clock source for the RTC (32.768 kHz crystal consumes roughly 0.5 microamps), and configure the DC-DC converter if available (nRF52 internal DC-DC reduces radio current from 5.3 mA to 3.7 mA at 0 dBm TX). Measure actual power consumption with a tool like the Nordic Power Profiler or a shunt resistor and oscilloscope — calculated estimates often miss wake-up transients, crystal settling times, and flash read currents that add 10-30% overhead. A well-optimized BLE wearable can run for months on a 100 mAh coin cell, but a poorly configured one (short connection interval, no slave latency, excessive advertising) might drain the same battery in days.