3.7V Li-Ion Battery Charger Circuit using MCP73831 IC + PCB

Teacher’s Tip: The CC-CV (Constant Current – Constant Voltage) algorithm is the gold standard for charging lithium batteries. Initially, the charger provides a constant current until the battery reaches its nominal voltage (4.2V for most Li-Ion cells). Then, it maintains this constant voltage while the current gradually decreases until it reaches a predefined threshold, at which point charging is considered complete.

• Linear charge management controller:
• Integrated pass transistor
• Integrated current sensor
• Reverse Discharge Protection
• Exceptional precision: Preset voltage regulation with ±0.75% accuracy
• Voltage options: Four versions available: 4.20V, 4.35V, 4.40V, 4.50V
• Programmable charge current: Adjustable from 15 mA to 500 mA
• Selectable preconditioning: 10%, 20%, 40% or Disable
• Selectable charge termination control: 5%, 7.5%, 10% or 20%
• Status indication: Three-state output for visual indicators
• Integrated protections:
• Automatic shutdown
• Thermal Regulation
• Operating temperature range: -40°C to +85°C
• Compact package: 5 pins, SOT-23

The versatility of the MCP73831 allows its application in various electronic devices:

• Lithium-ion / lithium-polymer battery chargers
• Personal assistance devices (wearables)
• Cell phones and smartphones
• Digital cameras and camcorders
• MP3 players and portable music players
• Bluetooth headphones
• Universal USB chargers
• Portable speakers (Boom boxes)
• IoT projects and battery-powered devices

Lithium-ion batteries have revolutionized portable electronics, offering a much higher energy density than previous technologies. This means that, for the same weight and volume, a Li-Ion battery stores significantly more energy.

Unlike old NiCd batteries, Li-Ion batteries don’t suffer from the “memory effect“, which reduced charge capacity when batteries weren’t fully discharged before recharging. Additionally, they support hundreds of charge and discharge cycles with minimal capacity degradation.

However, lithium-ion batteries require special care during charging. They need to strictly follow the Constant Current and Constant Voltage (CC-CV) standard. Overcharging or improper handling can not only permanently damage the cell but also pose safety risks, including ignition or explosion in extreme cases.

The image above illustrates the CC-CV charging process. Note how the current remains constant during the first phase, while the voltage gradually increases. When the voltage reaches its peak (4.2V), it is kept constant while the current decreases to the cutoff point.

The standard regulation voltage for Li-Ion batteries is 4.2V, but different variants of the MCP73831 allow configuring other charging voltages. The version identification is done through the last digit in the IC’s nomenclature:

• MCP73831-2 = 4.2V (standard for most Li-Ion batteries)
• MCP73831-3 = 4.3V (for special high-capacity batteries)
• MCP73831-4 = 4.4V (for specific lithium-polymer batteries)
• MCP73831-5 = 4.5V (for special industrial applications)

One of the great advantages of the MCP73831 is the ability to program the charging current through a simple external resistor. In our circuit, we use the 2.2KΩ R3 resistor, which configures a charging current of approximately 450mA.

The formula provided by the manufacturer is incredibly simple:

• Rc = Charging resistor (in kΩ)
• Cc = Charging current (in mA)

Formula:

Cc = 1000 / Rc

Applying to our 2.2K resistor:

Cc = 1000 / 2.2 = ±450mA

It’s worth noting that the minimum charging current for this device is 15mA and the maximum is 500mA. For higher capacity batteries, you can adjust this value, but never exceed the limit specified by the battery manufacturer (generally 0.5C to 1C, where C is the battery capacity).

¿Le gustó este proyecto? Entonces le encantará explorar otros circuitos que hemos preparado. ¡Cada uno con sus particularidades y aplicaciones ideales!

• Simple 12V battery charger with automatic charging indicator + PCB
• Lithium (Li-Ion) Battery Charger using LP2951 IC + PCB
• 12 Volts Automatic Lead Acid Battery Charger Using LM350 IC with PCB
• How To Make Rechargeable Emergency LED Light Using LM350 IC with PCB
• USB 5V 4A Car Charger using 78S05 with PCB

In Figure 4 , we present our PCB designed specifically for this project. All components are of the SMD (Surface-Mount Device) type, which allows an extremely compact design, with dimensions of only 22.860 mm by 12.065 mm.

We are providing the PCB files in GERBER , PDF and JPEG formats, so you can manufacture the board at home or through a professional PCB manufacturing service.

Now that we know all the components and the schematic, let’s get to what matters: assembling our charger! Follow these steps to ensure a successful assembly:

1. Preparation: Make sure you have all the necessary tools: fine-tip soldering iron, solder, tweezers, solder flux (optional) and a magnifying glass or microscope if available.
2. Start with the smaller components: Begin by soldering the SMD resistors and capacitors. They are easier to position when there are no other components in the way.
3. IC Soldering: The MCP73831 is the most delicate component. Apply a small amount of flux to the soldering area, carefully position the IC, and use a drag soldering technique to connect the pins.
4. LEDs: Soldering the LEDs last allows you to easily check if they are working after assembly. Remember that SMD LEDs have polarity!
5. Battery connector: If using a JST connector, soldering it last makes it easier to connect the wires.
6. Visual inspection: Use a magnifying glass to check for solder bridges between pins or cold connections.
7. Initial test: Before connecting the battery, apply 5V to the input and check if the LEDs work correctly. The red LED should light up.
8. Final test: With everything verified, connect the battery and observe the charging process. The red LED should remain on during charging and change to green when complete.

After assembly, it’s essential to validate the charger’s operation to ensure safety and proper performance. Follow these testing procedures:

Test without Battery:

• Connect a 5V power source to the circuit input
• Measure the voltage at pin 3 (BATT) of the IC – it should be approximately 0V
• Check if the red LED lights up

Test with Discharged Battery:

• Connect a discharged Li-Ion battery (voltage below 3.5V)
• Measure the charging current – it should be close to 450mA
• Check if the red LED remains on

Cutoff Voltage Test:

• Monitor the battery voltage during charging
• Check if the voltage reaches approximately 4.2V
• Observe if the LED changes from red to green when charging is complete

Leakage Current Test:

• After full charge, measure the current consumed by the battery
• This current should be less than 1mA (ideally below 50µA)

If you encounter problems during assembly or operation of the charger, these tips may help:

The LED doesn’t light up:

• Check if the 5V power supply is present
• Confirm if the LEDs are soldered correctly and with the correct polarity
• Test the current limiting resistors (R1 and R2)

The battery doesn’t charge:

• Check the battery connections
• Measure the voltage at pin 3 (BATT) of the IC
• Test the programming resistor R3
• Check if there are shorts between the IC pins

Charging is very slow:

• Measure the actual value of resistor R3
• Check if the power supply can provide enough current
• Test the input voltage – it should be stable at 5V

The LED never changes to green:

• Check if the battery voltage reaches 4.2V
• Measure the charging current – it should decrease when the battery is almost full
• Test if the green LED is working (do a quick test with a 3V source)

Now that you’ve built your charger, how about exploring some interesting applications? Here are some ideas to take your projects to the next level:

DIY Power Bank:

Combine your charger with a boost module (voltage step-up) and an 18650 battery to create your own power bank. You can add a display to show the remaining charge and multiple USB output ports.

Uninterruptible Power Supply (UPS):

Use the charger along with a battery management module to create a small UPS for critical devices like routers, automation systems, or Raspberry Pi servers.

Multiple Charger:

Expand the project to charge multiple batteries simultaneously. You can use a microcontroller to monitor and manage each charging channel independently.

Smart Charging Station:

Add Bluetooth or Wi-Fi connectivity to monitor charging status remotely through an app. You can implement different charging profiles for specific battery types.

🤔 Frequently Asked Questions (FAQ)

To ensure your project is a success, we’ve compiled some of the most common questions on this topic. Check it out!

In this article, we explored in detail the design and construction of a Li-Ion battery charger using the MCP73831 IC. This project is not only extremely useful for a variety of electronic applications but also represents an excellent introduction to the world of power management circuits.

The simplicity of the project, combined with the robustness and safety offered by the MCP73831, makes this charger an essential tool for any electronics enthusiast. Whether to power your portable projects, create backup systems, or simply to recharge batteries safely and efficiently, this circuit will meet your needs with excellence.

We hope this guide has been useful and enlightening. Electronics is a fascinating field, and projects like this demonstrate how seemingly simple components can create elegant and practical solutions to everyday problems.