# Solar + 18650 Battery Power System

## System Overview

Complete solar-powered weather station with 18650 battery backup for continuous operation.

## Power System Components

### Battery System
- **18650 Battery**: 2500-3500mAh, 3.7V
- **Battery Holder**: 1x18650 with protection circuit
- **TP4056 Charging Module**: USB charging with protection
- **Battery Management System (BMS)**: Overcharge/discharge protection

### Solar System
- **Solar Panel**: 5V-6V, 1-2W (recommended 6V 2W)
- **Solar Charge Controller**: TP4056 or dedicated solar controller
- **Diode**: Schottky diode for reverse polarity protection
- **Mounting**: Adjustable angle for optimal sun exposure

### Power Regulation
- **3.3V Buck Converter**: High efficiency (>90%)
- **Voltage Divider**: For battery monitoring
- **Power Switch**: Manual power control
- **LED Indicators**: Charging status, battery level

## Circuit Design

### Solar Charging Circuit
```
Solar Panel (6V 2W)
    │
    ├───[Schottky Diode]───┐
    │                      │
    ▼                      ▼
TP4056 Charging Module    18650 Battery
    │                      │
    ├───[Protection]──────┘
    │
    ▼
3.3V Buck Converter
    │
    ▼
Wemos D1 Mini + Sensors
```

### Battery Monitoring Circuit
```
Battery (3.0V-4.2V)
    │
    ├───[100kΩ]───┐
    │             │
    ▼             ▼
   GND          A0 (Wemos)
    │             │
    └───[100kΩ]───┘
```

## Power Consumption Analysis

### Wemos D1 Mini Power Requirements
- **Active Mode**: 150mA @ 3.3V
- **WiFi Transmission**: 200mA peaks
- **Deep Sleep**: 0.02mA
- **Sensors**: 10mA total
- **Total Active**: ~160mA

### Solar Panel Requirements
- **Daily Energy Need**: 160mA × 24h = 3840mAh
- **Solar Efficiency**: ~70% (cloudy days, angle)
- **Required Solar**: 3840mAh ÷ 0.7 ÷ 6h = 914mAh
- **Recommended Panel**: 6V 2W (333mA @ 6V)

### Battery Runtime
- **Battery Capacity**: 2500mAh
- **Continuous Runtime**: 2500mAh ÷ 160mA = 15.6 hours
- **With Solar**: Continuous operation
- **Deep Sleep Runtime**: 2500mAh ÷ 0.02mA = 125,000 hours (~14 years)

## Hardware Components

### Required Parts
1. **18650 Battery** (2500mAh+) - 1x
2. **TP4056 Charging Module** - 1x
3. **Solar Panel** (6V 2W) - 1x
4. **3.3V Buck Converter** - 1x
5. **Schottky Diode** (1N5817) - 1x
6. **Resistors** (100kΩ × 2) - 2x
7. **Battery Holder** (1x18650) - 1x
8. **Power Switch** - 1x
9. **LEDs** (red/green) - 2x
10. **330Ω Resistors** (for LEDs) - 2x

### Optional Parts
- **Solar Charge Controller** (MPPT for better efficiency)
- **Battery Level Indicator** (LED bar graph)
- **Weatherproof Enclosure**
- **Solar Panel Mount**

## Assembly Instructions

### Step 1: Battery System
1. Connect 18650 to TP4056 module
2. Verify protection circuit is working
3. Test charging with USB power
4. Check battery voltage (3.0V-4.2V range)

### Step 2: Solar Integration
1. Connect solar panel to TP4056 input
2. Add Schottky diode for reverse protection
3. Test solar charging in sunlight
4. Verify charging LED operation

### Step 3: Power Regulation
1. Connect battery output to 3.3V buck converter
2. Set output voltage to 3.3V
3. Test with load (Wemos D1 Mini)
4. Verify efficiency (>90%)

### Step 4: Battery Monitoring
1. Build voltage divider (100kΩ + 100kΩ)
2. Connect to A0 pin
3. Calibrate reading with known voltage
4. Implement battery percentage calculation

### Step 5: System Integration
1. Connect all components
2. Add power switch
3. Add status LEDs
4. Test complete system

## Code Implementation

### Battery Monitoring
```cpp
#define BATTERY_PIN A0
#define BATTERY_VOLTAGE_DIVIDER 2.0

float getBatteryVoltage() {
  int adcValue = analogRead(BATTERY_PIN);
  float voltage = adcValue * (3.3 / 1023.0) * BATTERY_VOLTAGE_DIVIDER;
  return voltage;
}

int getBatteryPercentage() {
  float voltage = getBatteryVoltage();
  // Map 3.0V (0%) to 4.2V (100%)
  int percentage = map(voltage * 100, 300, 420, 0, 100);
  return constrain(percentage, 0, 100);
}
```

### Power Management
```cpp
void managePower() {
  int batteryLevel = getBatteryPercentage();
  
  // Low battery warning
  if (batteryLevel < 20) {
    // Enter deep sleep to conserve power
    Serial.println("Low battery - entering deep sleep");
    esp_sleep_enable_timer_wakeup(300000000); // 5 minutes
    esp_deep_sleep_start();
  }
  
  // Send battery status via MQTT
  String batteryPayload = "{\"battery\":" + String(batteryLevel) + "}";
  client.publish("weather/battery", batteryPayload.c_str());
}
```

## Solar Panel Sizing

### Location-Based Sizing
| Location | Peak Sun Hours | Recommended Panel |
|----------|----------------|-------------------|
| Sunny    | 6+ hours       | 6V 1W            |
| Moderate | 4-6 hours      | 6V 2W            |
| Cloudy   | 2-4 hours      | 6V 3W            |

### Panel Specifications
- **Voltage**: 6V nominal (5V-7V operating)
- **Current**: 167mA-500mA (1W-3W)
- **Efficiency**: >17% (monocrystalline)
- **Size**: 100mm x 70mm (1W) to 200mm x 140mm (3W)

## Battery Management

### Charging Stages
1. **Bulk Charge**: Constant current until 4.2V
2. **Absorption Charge**: Constant voltage at 4.2V
3. **Float Charge**: Maintenance at 4.0V

### Protection Features
- **Overcharge Protection**: Cuts off at 4.2V
- **Over-discharge Protection**: Cuts off at 2.5V
- **Short Circuit Protection**: Current limiting
- **Temperature Protection**: Thermal cutoff

### Battery Life Optimization
- **Depth of Discharge**: Keep above 20%
- **Temperature**: Operate 0°C-45°C
- **Charging**: Avoid overcharging
- **Storage**: Charge to 40% for long-term storage

## Expected Performance

### Daily Operation
- **Sunny Day**: Solar provides 100% of power needs
- **Cloudy Day**: Battery supplements solar power
- **Night Operation**: Battery provides 100% of power needs
- **Continuous Operation**: 24/7 operation possible

### Battery Life
- **Cycle Life**: 500-1000 cycles (2-3 years)
- **Calendar Life**: 3-5 years
- **Self-Discharge**: <2% per month
- **Temperature Impact**: High temperature reduces life

## Troubleshooting

### Common Issues
- **No Charging**: Check solar panel connection and sunlight
- **Low Battery**: Verify panel size and sun exposure
- **System Resets**: Check power supply stability
- **Inaccurate Reading**: Calibrate voltage divider

### Testing Procedures
1. **Solar Output**: Measure panel voltage in sunlight
2. **Charging Current**: Measure current to battery
3. **System Load**: Measure current draw
4. **Battery Voltage**: Verify monitoring accuracy

## Maintenance

### Regular Checks
- Clean solar panel surface
- Check battery connections
- Verify charging status
- Monitor battery health

### Seasonal Adjustments
- Adjust panel angle for sun position
- Increase panel size for winter months
- Check for weather damage
- Verify waterproofing

This solar + battery system enables truly autonomous weather station operation with minimal maintenance requirements.