Summary of LITHIUM COIN CELL CHARGER FOR RECHARGEABLE COIN BATTERIES
This project is a USB-powered charger for rechargeable lithium coin cells (CR2016/CR2025/CR2032) using either the BQ21040 or MCP73831 charger IC (tested with BQ21040). It supports programmable charge current up to 800 mA via an external resistor (default 50 mA), provides status and power LEDs, input overvoltage protection, and integrates charge safety features such as thermal regulation and charge termination.
Parts used in the Lithium Coin Cell Charger:
- PCB with coin cell holder (for CR2016/CR2025/CR2032)
- BQ21040 charger IC (tested)
- MCP73831 charger IC (alternative option)
- USB input connector
- Programmable resistor R2 (current programming / monitor)
- Current sense FET (integrated on charger IC or board)
- D3 Power LED
- D1 Charge cycle/status LED
- Series resistor for STATUS LED or host pull-up resistor
- Input overvoltage protection components (as implemented on board)
- Passive components: resistors, capacitors as per charger IC reference design
This versatile charger has been designed to charge Lithium Coin Cell Rechargeable CR2016/CR2025/CR2032 Coin Batteries. Just insert the battery to the holder, and plug in to any USB port to recharge, D3 Power LED, D1 LED indicates the charge cycle. The board has been designed to use dual chips, either BQ21040 IC from Texas instruments or MCP73831 from Microchip however the board is tested with BQ21040 IC.
Programmable Charge Current using External Resistor up to 800 mA (default set to 50mA), follow formula bellow to set the desired current.
An external resistor R2 is used to Program the Output Current (50 to 800 mA) and can be used as a current monitor.
RISET = KISET / IOUT
Where
- IOUT is the desired fast charge current;
- KISET is a gain factor found in the electrical specification (1)
For greater accuracy at lower currents, part of the sense FET is disabled to give better resolution. Figure 1 shows the transition from low current to higher current. Going from higher currents to low currents, there is hysteresis and the transition occurs around 0.15 A.
For host monitoring, a pull-up resistor is used between the STATUS terminal and the VCC of the host and for a visual indication a resistor in series with an LED is connected between the STATUS terminal and a power source. First charge after Input supply applied LED will be ON and LED will be OFF when OVP/SLEEP condition.
BQ21040 – SINGLE-INPUT, SINGLE CELL LI-ION AND LI-POL BATTERY CHARGER
The bq21040 device is a highly integrated Li-Ion and Li-Pol linear battery charger device targeted at space limited portable applications. The device operates from either a USB port or AC adapter. The high input voltage range with input overvoltage protection supports low-cost unregulated adapters. The bq21040 has a single power output that charges the battery. A system load can be placed in parallel with the battery as long as the average system load does not keep the battery from charging fully during the 10 hour safety timer. The battery is charged in three phases: conditioning, constant current and constant voltage. In all charge phases, an internal control loop monitors the IC junction temperature and reduces the charge current if an internal temperature threshold is exceeded. The charger power stage and charge current sense functions are fully integrated. The charger function has high accuracy current and voltage regulation loops, charge status display, and charge termination.
Read more: LITHIUM COIN CELL CHARGER FOR RECHARGEABLE COIN BATTERIES
- What coin cell batteries does this charger support?
The charger is designed to charge Lithium Coin Cell Rechargeable CR2016, CR2025, and CR2032 batteries. - Which charger ICs can be used on this board?
The board is designed to use either the BQ21040 IC from Texas Instruments or the MCP73831 from Microchip, and it is tested with the BQ21040. - How is the charge current programmed?
The charge current is programmed using an external resistor R2; the default is set to 50 mA and can be adjusted up to 800 mA using the RISET = KISET / IOUT relationship. - What indicates the charger power and charge status?
D3 indicates power (power LED) and D1 indicates the charge cycle (status LED); the STATUS terminal can also be used with a pull-up resistor for host monitoring. - What happens to the LED on first charge and during OVP/SLEEP?
On the first charge after input supply is applied the LED will be ON; the LED will be OFF when an overvoltage protection or sleep condition occurs. - Does the charger include thermal and safety features?
Yes, the charger monitors IC junction temperature and reduces charge current if a temperature threshold is exceeded, and it uses a 10 hour safety timer for charge termination. - Can a system load be connected while charging?
Yes, a system load can be placed in parallel with the battery as long as the average system load does not prevent the battery from fully charging during the 10 hour safety timer. - What charge phases does the BQ21040 implement?
The BQ21040 charges the battery in three phases: conditioning, constant current, and constant voltage.
