Feature: Battery Backup

Complexity: 🟡 Medium Hardware Required: 🏗️ Major (Battery, charging circuit) User Value: ⭐⭐⭐ Essential

Overview

Implement battery backup to maintain access control functionality during power outages. This ensures security is maintained and authorized users can still enter during emergencies.

Benefits

Continuous operation during power outages
Security maintained 24/7
Emergency access preserved
System status preservation
Compliance with security standards

Power Requirements Analysis

Current Consumption Estimates

ESP32-C3 SuperMini: ~100mA (WiFi active), ~20mA (idle), 43μA (deep sleep)
PN532 RFID Reader: ~50mA active, ~10mA standby
JQ6500 MP3 Player: ~20mA idle, ~200mA playing with speaker
Relays (4x): ~80mA when active
Total: ~50mA idle, ~350mA peak

Battery Runtime Calculations

With deep sleep enabled (43μA base):

2000mAh battery: ~1000 hours deep sleep, ~40 hours idle, ~6 hours active
5000mAh battery: ~2500 hours deep sleep, ~100 hours idle, ~14 hours active
10000mAh battery: ~5000 hours deep sleep, ~200 hours idle, ~28 hours active

Implementation Checklist

Phase 1: Power Management System

Create PowerController class:

class PowerController {
private:
    gpio_num_t batteryPin;
    gpio_num_t chargerPin;
    gpio_num_t powerGoodPin;
    float batteryVoltage;
    bool onBattery;
    esp_adc_cal_characteristics_t adc_chars;
public:
    void begin();
    float getBatteryVoltage();
    uint8_t getBatteryPercent();
    bool isOnBattery();
    bool isCharging();
    void enablePowerSaving();
    void calibrateADC();
};

ESP32-C3 ADC calibration
Power source detection
Automatic switchover

Phase 2: Battery System Hardware

Phase 3: Power Switching

Automatic Switchover:

// Power path using Mini360 buck converter
12V Vehicle -----> Mini360 (5V) ----+
                                    |
                                  Diode
                                    |
Battery Pack ----> Boost (5V) ---Diode----> System

// Or use power management IC like LTC4412

Zero-downtime switching
Reverse polarity protection
Automotive transient protection

Phase 4: ESP32-C3 Power Optimization

Deep Sleep Implementation:

void enterDeepSleep(uint64_t sleep_time_us) {
    // Configure wake sources
    esp_sleep_enable_timer_wakeup(sleep_time_us);
    esp_sleep_enable_ext0_wakeup(GPIO_NUM_0, 0); // Wake on button

    // Power down peripherals
    esp_wifi_stop();
    esp_bt_controller_disable();

    // Enter deep sleep (43μA)
    esp_deep_sleep_start();
}

Dynamic Power Management:

void adaptivePowerMode() {
    if (getBatteryPercent() < 20) {
        // Disable WiFi/BLE
        WiFi.mode(WIFI_OFF);
        btStop();

        // Reduce CPU frequency
        setCpuFrequencyMhz(80); // From 160MHz

        // Enable light sleep
        esp_pm_config_esp32c3_t pm_config = {
            .max_freq_mhz = 80,
            .min_freq_mhz = 10,
            .light_sleep_enable = true
        };
        esp_pm_configure(&pm_config);
    }
}

Phase 5: Battery Monitoring

ESP32-C3 ADC Usage:

float readBatteryVoltage() {
    // Use ESP32 ADC with calibration
    uint32_t adc_reading = 0;

    // Multisampling for accuracy
    for (int i = 0; i < 64; i++) {
        adc_reading += adc1_get_raw(ADC1_CHANNEL_0);
    }
    adc_reading /= 64;

    // Convert to voltage with calibration
    uint32_t voltage = esp_adc_cal_raw_to_voltage(
        adc_reading, &adc_chars);

    // Account for voltage divider
    return (voltage / 1000.0) * DIVIDER_RATIO;
}

Low battery actions:
- 30%: Disable WiFi/BLE
- 20%: Reduce scan frequency
- 10%: Audio alerts only
- 5%: Deep sleep between scans
- 2%: Save state and shutdown

Phase 6: Solar Charging Option

Solar Integration:

// MPPT solar charge controller
Solar Panel -> CN3791 MPPT -> Battery
                   |
                   +-> System Power

5W-10W solar panel
MPPT efficiency tracking
Weather-resistant mounting

Phase 7: Emergency Power Mode

Critical Battery Response:

void emergencyPowerMode() {
    // ESP32-C3 specific optimizations

    // Disable all non-essential peripherals
    esp_wifi_stop();
    btStop();

    // Keep only RFID functional
    audio.disable();
    digitalWrite(8, LOW); // Blue LED off

    // Use ULP for ultra-low power monitoring
    if (batteryPercent < 2) {
        // Save state to NVS
        preferences.putBytes("last_state", &system_state, sizeof(system_state));

        // Enter hibernation (5μA)
        esp_sleep_pd_config(ESP_PD_DOMAIN_RTC_PERIPH, ESP_PD_OPTION_OFF);
        esp_sleep_pd_config(ESP_PD_DOMAIN_RTC_SLOW_MEM, ESP_PD_OPTION_OFF);
        esp_sleep_pd_config(ESP_PD_DOMAIN_RTC_FAST_MEM, ESP_PD_OPTION_OFF);
        esp_deep_sleep_start();
    }
}

Hardware Design

ESP32-C3 Power Architecture

Vehicle 12V -> Mini360 (5V) -> System 5V Bus
                                    |
                                    +-> ESP32-C3 (3.3V internal LDO)
                                    +-> Relays (5V)
                                    +-> PN532 (3.3V/5V tolerant)
                                    +-> JQ6500 (3.3V-5V)

Battery -> Protection -> Boost/Buck -> 5V Bus

Battery Management System

Battery + ----[Fuse]---- BMS IC ---- Charge +
              |            |
              +--[Temp]----+
              |
Battery - ----+--------- BMS IC ---- Charge -

Testing Checklist

Safety Considerations

Automotive-grade components
Temperature monitoring (ESP32 internal sensor)
Overcurrent protection
Reverse polarity protection
ESD protection on all inputs
Conformal coating for moisture

Power Consumption Optimization

Mode Comparison

| Mode | ESP32-C3 | PN532 | JQ6500 | Total | |——|———-|——–|———|——–| | Deep Sleep | 43μA | Off | Off | 43μA | | Light Sleep | 0.8mA | 10mA | Off | 11mA | | Idle | 20mA | 10mA | 20mA | 50mA | | Active | 100mA | 50mA | 200mA | 350mA |

Cost Analysis

Basic System (24-hour backup)

18650 battery holder: $3
18650 batteries (2x): $10
TP4056 charger: $2
Protection circuit: $3
Boost converter: $5
Total: ~$23

Advanced System (7-day backup with solar)

LiFePO4 pack (10Ah): $45
CN3791 MPPT charger: $8
10W solar panel: $25
Power management: $10
Enclosure/mounting: $12
Total: ~$100

Power Bank Solution

20000mAh USB power bank: $30
USB-C PD trigger board: $5
Backup switching circuit: $5
Total: ~$40

Future Enhancements

Supercapacitor for instant backup
Energy harvesting from vehicle alternator
Wireless charging capability
Cloud-based power monitoring
Predictive battery health analytics
Integration with vehicle CAN bus