1. LiR Battery
   • Choose the specific model of your LiR coin cell battery to determine its voltage and capacity ratings.
2. Charging IC
   • Select a suitable charging integrated circuit (IC) that is capable of handling the specific requirements of the LiR battery, such as charging current and voltage. For coin cell batteries, a constant current/constant voltage (CC/CV) charger IC is typically used.
3. Micro USB or USB-C Connector
   • Use this for the input power source. It will allow you to connect your charger to a USB port or power adapter.
4. Protection Circuitry
   • Integrate over-voltage, over-current, and thermal protection to ensure safe charging.
5. LED Indicators
   • Include LEDs to indicate charging status (e.g., charging, fully charged).

• Charging Voltage: Typically 4.2V for Li-ion batteries.
• Charging Current: Determined by the battery's capacity. A common rule of thumb is to charge at 0.5C to 1C of the battery's capacity (where C is the battery capacity in mAh).

1. Power Source Connections
   • Connect the USB connector to the charging IC.
2. Charging IC Configuration
   • Connect the charging IC outputs to the battery terminals, ensuring it is configured for the correct voltage and current.
3. Protection and Status Indicators
   • Incorporate additional components like resistors, capacitors, diodes, and LEDs for protection and status indication.

1. Select a Charging IC: Choose an IC from manufacturers like Texas Instruments, Microchip, or Analog Devices.
2. Create a Schematic: Use Flux to draft a schematic diagram.
3. Prototype and Test: Build a prototype on a breadboard or a custom PCB to test the functionality.

1. Cheapest IC manufacturer for prototyping

• Microchip Technology: Known for providing a variety of reasonably priced ICs suitable for prototyping.
• Diodes Incorporated: Offers affordable single-chip charging solutions.
• Texas Instruments (TI): They provide a range of low-cost charging ICs, especially for small projects.
• ON Semiconductor: Known for budget-friendly solutions in the power management field.

• MCP73831: A simple linear charger from Microchip, often used for low-current applications. It is cost-effective and widely available.
• BQ21040: A simple, cost-effective linear battery charger IC from Texas Instruments.
• AP3608A: From Diodes Incorporated, it is a low-cost single-cell lithium-ion battery charger with a basic feature set.

• Input Voltage Range: 3.75V to 6.0V
• Charge Voltage: 4.2V, suitable for single-cell lithium-ion batteries
• Charging Current: Up to 500mA, configurable with an external resistor
• Small Package: Comes in SOT-23 package (very compact)

1. Connect the Power Input:
   • Connect the VCC pin of the MCP73831 to your USB power source (5V).
2. Connect the Battery:
   • Connect the BAT pin to the positive terminal of the LiR battery.
3. Ground Connections:
   • Connect the GND pin to the system ground.
4. Programming the Charging Current:
   • Use a resistor (RPROG) connected from the PROG pin to ground to set the charging current.
   • For example, to set a charging current of 100mA, use a resistor value of approximately 10kΩ.
5. Status Indicator:
   • Connect an LED with a series resistor from the STAT pin to ground to indicate the charging status.

• Connect USB connector 5V to MCP73831:VCC.
• Connect MCP73831:BAT to the positive terminal of the LiR battery.
• Connect MCP73831:GND to ground.
• Use a resistor RPROG (e.g., 10kΩ) connected to MCP73831:PROG to set the charging current.
• Connect an LED in series with a resistor from MCP73831:STAT to ground for charging status indication.

1. Power source 4,5-5V 2. Ok 3. Ok 4. Charging current designed for a 5-10 mAh capacity. Choose charging current and resistor accordingly. 5. Led On=charging, led off=charged 6. Add all necessary additional components .

1. Power Source:
   • Input voltage range of 4.5V to 5V is suitable.
   • Use a micro USB or USB-C connector for easy compatibility with standard USB power sources.
2. Charging Current for 5-10 mAh Capacity:
   • For a small battery capacity like 5-10 mAh, a lower charging current is prudent. It’s common to use a 0.5C rate.
   • Assuming a capacity of 10 mAh, a charging current of 5 mA would be appropriate.
   • According to the MCP73831 datasheet, the charging current $I_{CHARGE}$ is given by: $I_{CHARGE} = \dfrac{V_{PROG}}{R_{PROG}} \times 1000$
   • With $V_{PROG} = 1V$ and $I_{CHARGE} = 5 \, \text{mA}$, the resistor $R_{PROG}$ is: $R_{PROG} = \dfrac{1V}{5mA} \times 1000 = 200 \, \text{k}\Omega$
3. LED Indicator:
   • Use a simple red LED to indicate charging status.
   • Connect the LED in series with a resistor to the MCP73831's STAT pin.
   • The LED turns on during charging and off when charged.
4. Additional Components and Connections:
   • Bypass Capacitor: Use a 1µF ceramic capacitor between VCC and GND for stability.
   • LED Resistor: Use a 220Ω resistor in series with the LED, assuming a typical forward voltage of 2V for the LED.

• Power Source:
  • Connect a USB connector's VBUS to MCP73831:VCC.
  • Connect the USB's ground to MCP73831:GND and battery negative terminal.
• Battery Connection:
  • Connect MCP73831:BAT to the positive terminal of the LiR battery.
• Charging Current Resistor:
  • Connect a 200kΩ resistor from MCP73831:PROG to ground.
• LED Indicator:
  • Connect an LED cathode to MCP73831:STAT with the anode connected to VCC via a 220Ω resistor.
• Bypass Capacitor:
  • Place a 1µF capacitor between MCP73831:VCC and GND.

graph TD;
    USB_Power[USB VBUS $4.5-5V$] --> VCC[MCP73831 V<sub>CC</sub>];
    GND[Ground] --> GND1[MCP73831 GND];
    GND --> GND2[Battery Negative];
    GND --> GND3[Capacitors' GND];

    VCC --> C1[$1\mu F$ Capacitor];
    C1 --> GND3;

    PROG[MCP73831 PROG] --> R_PROG[$200k\Omega$];
    R_PROG --> GND1;

    STAT[MCP73831 STAT] --> L1[LED Cathode];
    L1 --> R_LED[$220\Omega$];
    R_LED --> VCC;

    BAT[MCP73831 BAT] --> Battery_Pos[LiR Battery Positive];

    BATTERY[LiR Battery] --> |Positive| Battery_Pos;
    GND2 --> |Negative| BATTERY;

• USB Power: Connects directly to the VCC pin of the MCP73831.
• Ground Connections: All grounds (GND) are connected together.
• Capacitor: The $1\mu F$ capacitor stabilizes the input voltage.
• Charging Current Resistor (R_PROG): Sets the charging current to $5mA$ using a $200k\Omega$ resistor.
• LED Indicator: Connected from STAT pin through a $220\Omega$ resistor to indicate the charging status.
• Battery Connections: Connects BAT pin to the positive terminal of the battery.

1. Add Components:
   • Search for and place the following components:
     • MCP73831
     • USB connector (Micro USB or USB-C)
     • LiR battery symbol
     • Resistors (200kΩ and 220Ω)
     • Ceramic capacitor (1µF)
     • LED for the charge indicator
2. Make Connections:
   • Power Input: Connect the USB connector's VBUS to MCP73831:VCC.
   • Ground: Connect all the ground symbols together. This includes the USB connector ground, battery ground, and MCP73831 GND pin.
   • Battery: Connect MCP73831:BAT to the positive terminal of the LiR battery.
   • Charging Current Resistor: Connect a 200kΩ resistor from MCP73831:PROG to ground.
   • LED Indicator: Connect an LED cathode to MCP73831:STAT, and the LED anode to VCC via a 220Ω resistor.
   • Capacitor: Connect a 1µF capacitor between MCP73831:VCC and GND.

graph TB;
    subgraph Power Source
        USB[USB Connector]
    end

    subgraph MCP73831 Charger IC
        VCC[MCP73831 V<sub>CC</sub>]
        GND[MCP73831 GND]
        BAT[MCP73831 BAT]
        PROG[MCP73831 PROG]
        STAT[MCP73831 STAT]
    end

    subgraph Battery
        Battery_Pos[LiR Battery +]
        Battery_Neg[LiR Battery -]
    end

    subgraph LED Indicator
        LED[LED]
        R_LED[$220\Omega$ Resistor]
    end

    subgraph Stabilization
        C1[$1\mu F$ Capacitor]
    end

    subgraph Current Set
        R_PROG[$200k\Omega$ Resistor]
    end

    %% Connections
    USB -->|VBUS| VCC
    GND -- Ground --> |Ground| GND
    Battery_Pos -->|Positive| BAT
    Battery_Neg -->|Negative| GND

    VCC --> C1
    C1 --> GND

    PROG --> R_PROG
    R_PROG --> GND

    STAT --> LED
    LED --> R_LED
    R_LED --> VCC

• Power Source: The USB connector delivers power to the MCP73831.
• MCP73831 Charger IC: This is the core component that regulates charging.
• Battery: The Li-ion rechargeable battery is connected to the BAT and GND pins.
• LED Indicator: Shows the charging status. The LED turns on during charging and off when the battery is fully charged.
• Stabilization: A $1\mu F$ bypass capacitor is connected across VCC and GND to stabilize the voltage supply.
• Current Set Resistor: A $200k\Omega$ resistor sets the charging current to $5 \, \text{mA}$.