Arduino UNO Lithium Battery Charge Indicator – Extended Example

This example demonstrates how to estimate the charge level of a lithium-ion battery using an Arduino UNO. The program calculates:

Battery Capacity: User-defined in Watt-hours (Wh)
Discharge Load: Approx. 10% of capacity rating
Estimated Discharge Time: Based on current charge level

⚠️ Critical Lithium Battery Safety Warning

Lithium-ion batteries pose significant fire and explosion risks when improperly charged, discharged, or short-circuited. They must never be connected directly to an Arduino without a proper battery protection circuit.

Use a BMS (Battery Management System) for overcharge, over-discharge, and short-circuit protection.
Never exceed 4.2 V per cell – overcharging can cause thermal runaway.
Avoid deep discharge below 3.0 V – this permanently damages lithium cells.
Use protected charger modules such as TP4056 with protection circuitry.
Never leave lithium batteries charging unattended.

Failure to follow proper safety practices can result in fire, injury, or property damage.

Detailed Steps

Battery Voltage Measurement: Voltage is read using the ADC and scaled.
Charge Level Calculation: Voltage is mapped to battery capacity percentage.
Discharge Time Calculation: Based on a load equal to 10% of Wh capacity.

Arduino Code


/*
 * Arduino UNO Lithium Battery Charge Indicator
 * Measures battery voltage and estimates charge level and discharge time.
 * Capacity parameter in Watt-hours (Wh) and discharge load based on 10% of Wh rating.
 * © 2024 Peter I. Dunne - All rights reserved
 * Released under the Mozilla Public License
 */

const int batteryPin = A0;        // Battery voltage to A0
const float adcMaxVoltage = 5.0;  // ADC reference voltage
const float maxADCValue = 1023.0; // 10-bit resolution

// Battery parameters
const float batteryCapacityWh = 2.0;   // Example: 2.0 Wh
const float dischargeLoadFactor = 0.20; // Load = 20% of Wh (0.20 = 20%)

void setup() {
    Serial.begin(115200);
}

void loop() {
    // Read voltage
    float batteryVoltage = analogRead(batteryPin) * (adcMaxVoltage / maxADCValue);

    // Charge estimation (needs calibration for real use)
    float minVoltage = 3.0;
    float maxVoltage = 4.2;
    float chargeLevel = ((batteryVoltage - minVoltage) / (maxVoltage - minVoltage)) * 100.0;

    if (chargeLevel < 0) chargeLevel = 0;
    if (chargeLevel > 100) chargeLevel = 100;

    // Power load
    float dischargeLoadWatts = batteryCapacityWh * dischargeLoadFactor;

    // Remaining time
    float totalHours = (batteryCapacityWh * (chargeLevel / 100.0)) / dischargeLoadWatts;
    int hours = (int)totalHours;
    int minutes = (totalHours - hours) * 60;

    Serial.print("Voltage: ");
    Serial.print(batteryVoltage, 2);
    Serial.print(" V, Charge: ");
    Serial.print(chargeLevel, 2);
    Serial.print(" %, Load: ");
    Serial.print(dischargeLoadWatts, 2);
    Serial.print(" W, Time Left: ");

    if (hours < 10) Serial.print("0");
    Serial.print(hours);
    Serial.print(":");
    if (minutes < 10) Serial.print("0");
    Serial.println(minutes);

    delay(1000);
}

How It Works

Voltage Measurement: ADC scales the measured voltage.
Charge Percentage: Linearly based on calibrated min/max voltage.
Discharge Time: Battery capacity ÷ calculated load.