Utilizing the BQ24002 from Texas Instruments allows the construction of a compact and uncomplicated charger module designed for single lithium-ion (Li-ion) cells. Packaged in SSOP20, it eliminates the need for advanced assembly and soldering skills. While individual cells are available from major catalog suppliers, a more economical approach is salvaging cells from non-functional notebook batteries. In most instances, only a couple of cells are defective, leaving others with extended potential usage. A single cell serves as an excellent power source for devices requiring 3.3 V, ensuring a robust and enduring performance.
For the charger circuit, a 5 V input suffices, easily accessible from a USB port or any 5 V power supply. The charging process initiates with a trickle charge current. As the cell terminal voltage reaches a specific threshold, the charger transitions to a higher constant charge current. Charging concludes upon reaching a predetermined limit, denoted as the ‘final voltage.’ This charger configuration is compatible with cells featuring a final voltage of either 4.1 V or 4.2 V, determined by jumper JP1: connecting pin 9 to ground selects 4.1 V, while connecting it to VCC selects 4.2 V. Adhering to the manufacturer’s specifications for the maximum permissible cell voltage is crucial, and if uncertain, referencing the specifications provides the definitive value.
The charge current is established and monitored through the input shunt resistor R1, with a value of 0.1 Ω yielding a charge current (IL) of 1 A according to the formula IL = 0.1 V / R1. In this specific case (with the supply voltage), the time limit is set at four and a half hours. Pulling pin 13 to ground extends the time limit to six hours. If the final voltage is attained before the designated time, the charging process naturally halts. The LEDs serve as indicators for monitoring the charge progress. The red LED D1 illuminates during charging and flashes if a fault is detected. When the cell surpasses 90% charge, the red LED goes off, and the green LED lights up.
Pin 7 (APG/THM) serves as the input for a window comparator with a lower threshold of 0.56 V and an upper threshold of 1.5 V. If the voltage on this pin exceeds 1.5 V or falls below 0.56 V. The IC recognizes it as a fault and terminates the charging procedure. Charging can only proceed if the voltage on the pin falls between these two thresholds. The window comparator can be configured either to monitor the IC’s supply voltage or to track the temperature of the lithium cell.
In the presented circuit, we employ the input in a temperature monitoring setup. The voltage on pin 7 is determined by a voltage divider consisting of R2, R3, and an NTC thermistor. This arrangement senses the cell temperature and alters the voltage on the pin. The thermistor’s resistance ranges from 4.8 kΩ (upper temperature limit) to 26.6 kΩ (lower temperature limit). Using a standard 10 kΩ thermistor (like the Vishay 2381 640 63103), charging occurs within a cell temperature range of approximately 5 °C to about 43 °C.
With a 12 kΩ thermistor from the same series, the upper temperature limit extends to 48 °C, as adopted in Texas Instruments’ evaluation module . The datasheet  provides formulas to assist in calculating component values for the voltage divider. Alternatively, the TempSense Designer software  can be utilized, offering a user-friendly interface and various features.