MustafaMulla29/stride-pedometer

docs/power-budget.md

# Battery-life estimate

The design target is **at least four weeks from a 100 mAh cell**. With 80 mAh treated as usable capacity, four weeks allows an average battery current of **119 µA**. These are planning estimates, not measurements.

| Contribution | Assumption | Average battery current |
|---|---|---:|
| Always-on electronics and occasional processing | BMA400 counting, MCU standby, gauge sleep, charger low-power mode, regulator and leakage allowance | 20 µA |
| BLE advertising | 10 mA battery current for 5 ms every 2 s | 25 µA |
| Display | 30 mA from the battery, twenty 5-second views per day | 34.7 µA |
| **Total example** | | **79.7 µA** |

80 mAh / 0.0797 mA = approximately **42 days**. A 3–6 week engineering target is reasonable for this usage, subject to measurements, battery quality, temperature, firmware, display content, and radio duty cycle.

The OLED figure is battery-side current including conversion loss. It varies with brightness and lit pixels. Displaying a sparse step count at moderate contrast uses less energy than lighting the whole screen.

| Scenario | Average current | Idealized life from 80 mAh usable |
|---|---:|---:|
| Example above | 79.7 µA | 41.8 days |
| Gauge remains in 50 µA normal mode instead of 9 µA sleep | 120.7 µA | 27.6 days |
| Heavier BLE traffic adds 200 µA | 279.7 µA | 11.9 days |
| OLED stays on continuously at 30 mA | about 30 mA | about 2.7 hours |

Do not promise weeks with an always-on OLED. The high-voltage boost must be disabled and the display logic rail switched off between views. For an always-visible display, revise the design around a memory LCD, segmented LCD, or e-paper display with an appropriate controller.

The largest risks to the estimate are waking the MCU on every step, short BLE connection intervals, a fuel gauge that never enters sleep, leaving charger LP high, holding CE low through its pullup on battery, enabled debug circuitry, GPIO back-power into the OLED, and unmeasured leakage. Use an instrument capable of integrating brief milliamp radio peaks and measuring microamp sleep current.

Manufacturer typical values used to inform the standby allowance: [BMA400](https://www.bosch-sensortec.com/media/boschsensortec/downloads/datasheets/bst-bma400-ds000.pdf), [CC2340R5](https://www.ti.com/lit/ds/symlink/cc2340r5.pdf), [BQ27427](https://www.ti.com/lit/ds/symlink/bq27427.pdf), [BQ25150](https://www.ti.com/lit/ds/symlink/bq25150.pdf), and [TPS7A02](https://www.ti.com/lit/ds/symlink/tps7a02.pdf). Actual system current must be measured.