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

18650 Battery (2500mAh+) - 1x
TP4056 Charging Module - 1x
Solar Panel (6V 2W) - 1x
3.3V Buck Converter - 1x
Schottky Diode (1N5817) - 1x
Resistors (100kΩ × 2) - 2x
Battery Holder (1x18650) - 1x
Power Switch - 1x
LEDs (red/green) - 2x
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

Connect 18650 to TP4056 module
Verify protection circuit is working
Test charging with USB power
Check battery voltage (3.0V-4.2V range)

Step 2: Solar Integration

Connect solar panel to TP4056 input
Add Schottky diode for reverse protection
Test solar charging in sunlight
Verify charging LED operation

Step 3: Power Regulation

Connect battery output to 3.3V buck converter
Set output voltage to 3.3V
Test with load (Wemos D1 Mini)
Verify efficiency (>90%)

Step 4: Battery Monitoring

Build voltage divider (100kΩ + 100kΩ)
Connect to A0 pin
Calibrate reading with known voltage
Implement battery percentage calculation

Step 5: System Integration

Connect all components
Add power switch
Add status LEDs
Test complete system

Code Implementation

Battery Monitoring

#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

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

Bulk Charge: Constant current until 4.2V
Absorption Charge: Constant voltage at 4.2V
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

Solar Output: Measure panel voltage in sunlight
Charging Current: Measure current to battery
System Load: Measure current draw
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.

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