NiMH Battery Charger Circuit: What You Need to Know

Introduction to NiMH Batteries and Chargers

Nickel-Metal Hydride (NiMH) batteries have gained popularity as a rechargeable power source for various electronic devices due to their high energy density, low self-discharge rate, and environmental friendliness compared to other battery types. To effectively utilize NiMH batteries, it is crucial to understand the fundamentals of NiMH battery charger circuits and their essential components.

In this comprehensive article, we will delve into the world of NiMH battery charger circuits, covering topics such as the basic principles of NiMH charging, the components required to build a charger circuit, safety considerations, and frequently asked questions.

Understanding NiMH Battery Charging Principles

Charging Stages

NiMH batteries undergo three main stages during the charging process:

1. Constant Current (CC) Stage: In this stage, the charger supplies a constant current to the battery, typically ranging from 0.1C to 1C (where C is the battery’s capacity in amp-hours). The battery voltage gradually increases during this stage.

2. Constant Voltage (CV) Stage: Once the battery reaches its peak voltage (usually around 1.4V to 1.6V per cell), the charger switches to a constant voltage mode. The current begins to decrease as the battery approaches full charge.

3. Trickle Charge Stage: After the battery is fully charged, the charger maintains a low current (usually 0.02C to 0.05C) to compensate for the battery’s self-discharge and keep it at full capacity.

Charge Termination Methods

To prevent overcharging and ensure the longevity of NiMH batteries, charger circuits employ various charge termination methods:

1. Negative Delta V (-ΔV): This method detects a slight decrease in battery voltage (-5mV to -10mV per cell) that occurs when the battery reaches full charge.

2. Temperature Cutoff: NiMH batteries experience a temperature rise during charging, especially near the end of the charge cycle. By monitoring the battery temperature, the charger can terminate charging when a predefined temperature threshold is reached.

3. Timer-Based Cutoff: A simple yet effective method is to set a timer based on the expected charging time. The charger will stop charging once the timer expires.

Key Components of a NiMH Battery Charger Circuit

To build a functional NiMH battery charger circuit, you will need the following components:

1. Power Supply

A stable power supply is essential to provide the necessary charging current and voltage. The power supply should have a higher voltage than the fully charged battery voltage. For example, a 12V power supply is suitable for charging a 7.2V or 8.4V NiMH battery pack.

2. Charge Controller IC

Charge controller ICs simplify the design of NiMH Charger circuits by integrating various features such as constant current and constant voltage regulation, charge termination, and safety functions. Some popular charge controller ICs include:

• TP4056: A compact linear charger IC for single-cell lithium-ion batteries, which can be adapted for NiMH charging with minor modifications.
• MCP73831: A highly integrated linear charge management controller for NiMH and lithium-ion batteries, offering programmable charge current and charge termination.
• MAX712: A switch-mode charger IC designed specifically for NiMH and NiCd batteries, providing fast charging and charge termination.

3. Current Sensing Resistor

To monitor and control the charging current, a current sensing resistor is placed in series with the battery. The value of the resistor determines the maximum charging current and can be calculated using Ohm’s law: R = V / I, where V is the desired voltage drop across the resistor and I is the charging current.

4. Temperature Sensor

For chargers employing temperature-based charge termination, a temperature sensor, such as a thermistor or a dedicated temperature monitoring IC (e.g., DS18B20), is used to monitor the battery temperature during charging.

5. Indicator LEDs

Indicator LEDs provide visual feedback on the charging status. Typically, a red LED indicates that the battery is charging, while a green LED signifies a fully charged battery.

Safety Considerations

When designing and using NiMH battery charger circuits, safety should be a top priority. Consider the following safety measures:

1. Overcharge Protection: Implement charge termination methods to prevent overcharging, which can lead to battery damage, reduced capacity, and potential safety hazards.

2. Short-Circuit Protection: Include short-circuit protection in the charger circuit to prevent damage to the components and the battery in case of accidental short-circuits.

3. Reverse Polarity Protection: Use a diode or a MOSFET in series with the battery to prevent reverse current flow if the battery is connected with incorrect polarity.

4. Thermal Management: Ensure proper heat dissipation from the charger circuit components, especially the charge controller IC and the current sensing resistor. Use heat sinks or cooling fans if necessary.

5. Fuses: Incorporate fuses in the charger circuit to protect against overcurrent conditions and prevent damage to the components and the battery.

NiMH Battery Charger Circuit Design Examples

Single-Cell NiMH Charger using TP4056

         +--------+
         |  12V   |
         | Power  |
         | Supply |
         +---+----+
             |
             |
         +---+----+
         | TP4056 |
         | Charge |
         |   IC   |
         +---+----+
             |
             |
         +---+----+
         |  NiMH  |
         | Battery|
         | (1.2V) |
         +--------+

In this simple single-cell NiMH charger, the TP4056 IC is used to control the charging process. The TP4056 is primarily designed for lithium-ion batteries but can be adapted for NiMH charging by adjusting the charge termination voltage. The charging current is set by the value of the current sensing resistor connected to the PROG pin of the TP4056.

Multi-Cell NiMH Charger using MAX712

         +--------+
         |  12V   |
         | Power  |
         | Supply |
         +---+----+
             |
             |
         +---+----+
         | MAX712 |
         | Charge |
         |   IC   |
         +---+----+
             |
             |
         +---+----+
         |  NiMH  |
         | Battery|
         |  Pack  |
         | (7.2V) |
         +--------+

For charging multi-cell NiMH battery packs, the MAX712 switch-mode charger IC is a suitable choice. The MAX712 supports fast charging and provides charge termination based on -ΔV, temperature, or timer methods. The charging current and termination thresholds can be set using external resistors and capacitors.

Frequently Asked Questions (FAQ)

1. Q: Can I use a lithium-ion battery charger for charging NiMH batteries?
   A: While some lithium-ion battery chargers can be adapted for NiMH charging, it is not recommended without proper modifications. NiMH batteries have different charging requirements, such as lower peak voltage and charge termination methods, compared to lithium-ion batteries.

2. Q: What is the recommended charging current for NiMH batteries?
   A: The charging current for NiMH batteries typically ranges from 0.1C to 1C, where C is the battery’s capacity in amp-hours. For example, a 2000mAh NiMH battery can be charged with a current of 200mA (0.1C) to 2A (1C). Higher charging currents may be used for fast charging, but it is essential to ensure proper charge termination and temperature monitoring.

3. Q: How long does it take to fully charge a NiMH battery?
   A: The charging time depends on the battery capacity and the charging current. As a general rule, charging a NiMH battery at 0.1C takes approximately 12-14 hours, while charging at 0.5C takes around 2-3 hours. Fast charging at 1C can fully charge the battery in about 1 hour.

4. Q: What should I do if my NiMH battery charger circuit is not working properly?
   A: If your NiMH battery charger circuit is not functioning as expected, follow these troubleshooting steps:

5. Check the power supply voltage and ensure it is within the specified range.
6. Verify that the battery is connected with the correct polarity.
7. Inspect the circuit for any loose connections or damaged components.
8. Ensure that the current sensing resistor value is correct for the desired charging current.

9. Check the charge controller IC datasheet for any specific requirements or troubleshooting guidelines.

10. Q: Can I leave a NiMH battery connected to the charger after it is fully charged?
    A: Most NiMH battery charger circuits include a trickle charge feature that maintains the battery at full charge after the main charging process is complete. However, it is generally recommended to remove the battery from the charger once it is fully charged to prevent overcharging and extend the battery’s lifespan.

Conclusion

NiMH battery charger circuits play a crucial role in ensuring the safe and efficient charging of NiMH batteries. By understanding the charging principles, selecting the appropriate components, and implementing safety measures, you can design and build reliable NiMH charger circuits for various applications.

When designing your NiMH battery charger circuit, consider factors such as the battery capacity, desired charging current, charge termination methods, and safety features. Choosing the right charge controller IC and properly sizing the current sensing resistor are critical steps in the design process.

Remember to prioritize safety by incorporating overcharge protection, short-circuit protection, reverse polarity protection, thermal management, and fuses in your charger circuit design.

By following the guidelines and examples provided in this article, you can confidently develop efficient and safe NiMH battery charger circuits for your projects.