Using the BQ24002 from Texas Instruments it is possible to build a simple and small charger module for single lithium-ion (Li-ion) cells. The device is available in a SSOP20 package and so does not require heroic assembly and soldering skills. Individual cells are becoming available from the main catalog suppliers, but a much cheaper option is to rescue cells from defunct notebook batteries. In most cases only a couple of cells are faulty and the others can still look forward to a long and useful life. A single cell is ideal for any equipment that needs a 3.3 V power supply, and will generally give a good operating life. The charger circuit requires a 5 V input, which can readily be obtained from a USB port or from any 5 V power supply. The charge process begins with a trickle charge current. When the cell terminal voltage is sufficiently high the charger switches to a higher constant charge current. Charging is terminated when the cell voltage reaches a preset limit (the ‘final voltage’). The charger described here is suitable for cells with a final voltage of 4.1 V or 4.2 V, configured using jumper JP1: pin 9 is taken to ground to select 4.1 V or to VCC to select 4.2 V. It is important never to exceed the maximum permissible cell voltage: if in doubt, consult the manufacturer’s specifications for the definitive value.
The charge current is determined and monitored by input shunt resistor R1. A value of 0.1 Ω gives a charge current IL of 1 A: the general formula is IL = 0.1 V / R1. In this example (the supply voltage) the time limit is four and a half hours, and if pin 13 is pulled to ground the time limit is six hours. If the final voltage is reached early, charging will of course cease before expiry of the time limit. The LEDs allow the charge process to be monitored. Red LED D1 lights during charging and flashes to indicate that a fault has been detected. When the cell is more than 90 % charged the red LED is extinguished and the green LED lights. Pin 7 (APG/THM) is the input to 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 is over 1.5 V or below 0.56 V the IC regards this as a fault and aborts the charging process. Charging can only occur if the voltage on the pin lies between the two thresholds. The window comparator can be used either to monitor the IC’s supply voltage or to monitor the temperature of the lithium cell.
In the circuit shown we have used the input in a temperature monitoring configuration: the voltage on pin 7 is determined by a voltage divider comprising R2, R3 and an NTC thermistor, which is arranged to sense the temperthermistor lies between 4.8 kΩ (upper temperature limit) and 26.6 kΩ (lower temperature limit). Using a typical 10 kΩ thermistor (such as the Vishay 2381 640 63103) this means that charging will occur as long as the cell temperature is between approximately 5 °C and approximately 43 °C. A 12 kΩ thermistor from the same series gives an upper limit of 48 °C: this is the arrangement used in Texas Instruments’ evaluation module . Formulae are given in the datasheet  to help with the calculation of component values in the voltage divider. Alternatively, the TempSense Designer software  can be used: it offers a graphical user interface and a number of other features.