Power management: automatic sleep for battery conservation

[jctoledo]

Background

Currently, Blackbox is powered by an external USB powerbank. The long-term goal is to create a self-contained unit with an integrated battery - mount it in your car, forget about it, and it just works.

The problem: if you finish a track session and forget to turn it off, your battery dies. Since there's no connection to the vehicle's ignition, Blackbox needs to be smart about managing its own power.

The Goal

Blackbox should automatically sleep when the car hasn't moved for a while, and wake up when driving resumes - no user intervention required.

Measured Power Consumption

Real measurements from the current hardware:

State	Current	Notes
Full system active	160-185mA	WiFi AP + telemetry streaming
Without IMU	150-175mA	GPS adds ~35-45mA
Without GPS	125-140mA	IMU adds ~10-15mA
ESP32 only	100-130mA	WiFi dominates power draw

Key insight: The ESP32 with WiFi uses ~70% of total power. Turning off sensors saves little. The only real win is putting the ESP32 into deep sleep (~10-50µA) - a 3000x reduction.

Battery Life Estimates (2000mAh LiPo)

State	Current	Runtime
Active	~170mA	~12 hours
Full shutdown	~0mA	Forever (until next power-on)
Deep sleep (future)	~20-30µA	~1 year standby

Proposed Implementation

Phase 1: Timeout-Based Shutdown (No Additional Hardware)

Simple approach for the first PCB revision:

1. Detect stationary state using existing sensors (low acceleration, low gyro, low GPS speed)
2. After 20 minutes of no movement → full shutdown
3. User manually powers on when needed (button or power cycle)

ACTIVE ──[no movement 20min]──► SHUTDOWN (full power off)
                                    │
                    [manual button press or power cycle]
                                    │
                                    ▼
                                 ACTIVE

Pros: No hardware changes, simple to implement
Cons: Requires manual wake-up

Phase 2: Wake-on-Motion with LIS3DH (Future PCB)

Add a dedicated ultra-low-power accelerometer (~$1.50) that stays on and wakes the system:

1. Same stationary detection as Phase 1
2. After 20 minutes → deep sleep (ESP32 sleeping, LIS3DH watching)
3. LIS3DH detects motion → automatic wake

ACTIVE ──[no movement 20min]──► DEEP SLEEP (~20-30µA)
   ▲                                │
   │                    [LIS3DH motion interrupt]
   │                                │
   └────────────────────────────────┘

Why LIS3DH?

• Ultra-low power: 2-6µA (can watch for motion indefinitely)
• Configurable threshold: detect car door, engine start, driving
• Interrupt pin: wakes ESP32 instantly
• The WT901 IMU draws ~10-15mA and can't wake the ESP32 from sleep

Open Questions

Calibration on Wake

The IMU calibration currently runs at boot and requires the vehicle to be completely stationary for ~10 seconds. What happens if the system wakes while already driving?

Options to consider:

• Store calibration in flash (NVS) - reload last known biases on wake
• Skip calibration, accept degraded accuracy - until next stationary period
• Require stationary start - don't transition to active until stopped (bad UX)

EKF Convergence

After wake, the EKF state is reset. How quickly will it converge to accurate estimates?

• Position: Should lock on quickly once GPS has a fix (~2-30s depending on warm/cold start)
• Velocity: GPS provides this directly
• Yaw/heading: May drift until GPS velocity gives direction of travel

Is this acceptable, or do we need to persist EKF state across sleep cycles?

GPS Warm Start

The GPS backup battery keeps ephemeris data alive during sleep. Warm start is ~2 seconds vs cold start ~24 seconds. Need to verify the SparkFun NEO-M9N backup battery stays charged during deep sleep.

False Wakes (Phase 2)

In a busy parking lot, nearby cars or pedestrians might trigger the LIS3DH. Mitigations:

• Higher threshold (0.2g instead of 0.1g)
• Require sustained motion (multiple triggers within 1 second)
• Quick "is this real movement?" check after wake - go back to sleep if not

20-Minute Timeout

Is 20 minutes the right value? Considerations:

• Too short: annoying at gas stations, red lights in traffic
• Too long: wasted battery in parking lots
• Should this be user-configurable via the dashboard?

Hardware Requirements (Phase 2)

Component	Purpose	Cost
LIS3DH accelerometer	Wake-on-motion detection	~$1.50
Load switches (x2)	Cut power to IMU and GPS	~$1.60
Passives	Decoupling, pull-ups	~$0.20
Total		~$3.30

Summary

Phase	Hardware	Sleep Mode	Wake Method	Complexity
1	None	Full shutdown	Manual	Low
2	LIS3DH + load switches	Deep sleep (~25µA)	Automatic (motion)	Medium

Phase 1 is good enough for initial testing and proves the detection logic. Phase 2 delivers the "mount and forget" experience.

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Measurements taken 2025-01-11 with bench power supply. System: ESP32-C3 DevKitC-02 + WT901 IMU + SparkFun NEO-M9N GPS.