Single Lithium Cell Charger

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 catalogue 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.

Circuit diagram:
Single Lithium Cell-Charger-Circuit-diagram
Single Lithium Cell Charger Circuit diagram

The  charge  process  begins  with  a  trickle  charge current. When the cell terminal volt-age 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 I L of 1 A: the general formula is I L = 0.1 V / R1. In this example, the input voltage should be no greater than  5.3 V to ensure that the maximum allowable  power dissipation of the IC is not exceeded.  With a charge current of 0.5 A (R1 = 0.2 Ω), the  maximum allowable input voltage is 7.6 V.

The circuit offers a charge time limit and cell  temperature monitoring. The charge time  limit is set using JP2. If the jumper is not fitted  charging will always stop within three hours,  even if the cell has not reached its final volt- age. If the jumper is fitted to pull pin 13 to Vcc 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 volt-age 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 volt-age 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 deter-mined by a voltage divider comprising R2, R3  and an NTC thermistor, which is arranged to sense the temperature of the lithium cell and  which is wired in parallel with R3 via connector K2.

Pin 12 (CR) carries a reference voltage  of 2.85 V; so that charging is possible under  normal conditions the thermistor and the  voltage divider of which it forms a part must  be dimensioned so that the voltage on pin 7  lies within the comparator’s voltage window  when the cell is running at a safe temperature. The values shown for R2 and R3 will  allow charging as long as the resistance of the thermistor 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 [1].
Characteristics:

•   Designed for a single Li-ion cell
•   Suitable for all lithium chemistry cells with a final voltage of 4.1 V or 4.2 V (lithium-cobalt, lithium-manganese and lithium-polymer)
•   Configurable 4.1 V or 4.2 V final voltage
•   Input voltage from 4.5 V to 10 V (depending on charge current)
•   Charge current up to 1.2 A
•   Charge current configurable via shunt resistor
•   Linear regulator topology
•   Precharge function for deeply-discharged cells
•   Charge status indicated by two LEDs
•   Two package options: SSOP20 or QFN
Formulae are given in the datasheet [2] to  help  with  the  calculation  of  component  values in the voltage divider. Alternatively,  the TempSense Designer software [3] can be  used: it offers a graphical user interface and  a number of other features.
Author : Steffen Graf (Germany) - Copyright : Elektor

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