How to Use 18650 7.4V 2S Li-ion Battery: Examples, Pinouts, and Specs

The 18650 7.4V 2S Li-ion Battery, manufactured by Espressif, is a rechargeable lithium-ion battery pack consisting of two 18650 cells connected in series. This configuration provides a nominal voltage of 7.4V, making it suitable for a wide range of applications requiring reliable and portable power sources. With its high energy density, long cycle life, and compact size, this battery pack is commonly used in portable electronics, robotics, electric vehicles, and DIY projects.

• Powering portable electronic devices (e.g., handheld tools, flashlights)
• Robotics and IoT devices
• Electric vehicles and e-bikes
• Backup power supplies
• DIY electronics projects

The battery pack typically includes two or three wires for connection. Below is the pin configuration:

1. Connection: Connect the red wire to the positive terminal of your circuit and the black wire to the negative terminal. If a BMS wire is present, it can be used for monitoring or balancing purposes.
2. Charging: Use a dedicated Li-ion battery charger designed for 2S (7.4V) configurations. Ensure the charger has overcharge protection and does not exceed 8.4V.
3. Discharging: Avoid discharging the battery below 6.0V to prevent damage to the cells. Use a low-voltage cutoff circuit or a BMS for protection.
4. Mounting: Secure the battery pack in your project using a holder or adhesive to prevent physical damage.

• Safety: Always use a Battery Management System (BMS) to protect against overcharging, over-discharging, and short circuits.
• Temperature: Operate the battery within the recommended temperature range (typically 0°C to 45°C for charging and -20°C to 60°C for discharging).
• Storage: Store the battery at ~50% charge in a cool, dry place if not in use for extended periods.
• Polarity: Double-check the polarity before connecting the battery to avoid damage to your circuit or the battery.

The 18650 7.4V 2S Li-ion Battery can be used to power an Arduino UNO via its VIN pin. Below is an example circuit and code to read the battery voltage using a voltage divider.

• Connect the battery's positive terminal to the VIN pin of the Arduino.
• Connect the battery's negative terminal to the GND pin of the Arduino.
• Use a voltage divider (e.g., 10kΩ and 20kΩ resistors) to scale the battery voltage for reading via an analog pin.

// Define the analog pin connected to the voltage divider
const int voltagePin = A0;

// Define the voltage divider ratio (e.g., 10kΩ and 20kΩ resistors)
const float voltageDividerRatio = 3.0;

// Define the reference voltage of the Arduino (5V for most boards)
const float referenceVoltage = 5.0;

void setup() {
  Serial.begin(9600); // Initialize serial communication
}

void loop() {
  // Read the raw analog value from the voltage divider
  int rawValue = analogRead(voltagePin);

  // Convert the raw value to the actual battery voltage
  float batteryVoltage = (rawValue * referenceVoltage / 1023.0) * voltageDividerRatio;

  // Print the battery voltage to the Serial Monitor
  Serial.print("Battery Voltage: ");
  Serial.print(batteryVoltage);
  Serial.println(" V");

  delay(1000); // Wait for 1 second before the next reading
}

1. Battery Not Charging

   • Cause: Faulty charger or damaged BMS.
   • Solution: Verify the charger output voltage and ensure it matches the battery's charging requirements. Check the BMS for damage.

2. Battery Drains Quickly

   • Cause: Over-discharge or aging cells.
   • Solution: Avoid discharging below 6.0V. Replace the battery if the capacity has significantly degraded.

3. Battery Overheats

   • Cause: Excessive current draw or faulty BMS.
   • Solution: Ensure the load does not exceed the battery's maximum current rating. Check the BMS for proper operation.

4. Arduino Not Powering On

   • Cause: Incorrect wiring or insufficient voltage.
   • Solution: Verify the battery connections and ensure the voltage is within the Arduino's operating range (7–12V via VIN).

Q: Can I use this battery without a BMS?
A: While it is possible, it is not recommended. A BMS ensures safe operation by protecting against overcharging, over-discharging, and short circuits.

Q: How do I know when the battery is fully charged?
A: The battery is fully charged when the charger output reaches 8.4V and the charging current drops to near zero.

Q: Can I connect multiple 2S packs in parallel?
A: Yes, but ensure all packs are at the same voltage level before connecting them to avoid current surges.

Q: What happens if I over-discharge the battery?
A: Over-discharging can permanently damage the cells and reduce their capacity. Always use a low-voltage cutoff circuit or a BMS.

This concludes the documentation for the 18650 7.4V 2S Li-ion Battery. For further assistance, refer to the manufacturer's datasheet or contact Espressif support.