Build a 2 Cell Lithium Ion Charger Circuit – 3.6V to 8.2V

Description

This circuit was constructed to power a pair of series Lithium cells (3.6 volts each, 1 Amp Hour capacity) utilized in a portable transistor radio.

The charging system operates by supplying brief current bursts through a series resistor and then monitoring the battery voltage to determine if another pulse is necessary. The current flow can be altered by modifying the series resistor or adjusting the input voltage. When the battery level is low, the current pulses are delivered in close succession, resulting in a moderately constant current flow. As the batteries approach full charge, the pulses become spaced more widely, and the complete charge state is indicated by the LED flashing at a reduced pace.

A TL431, a band gap voltage reference (2.5 volts), is employed on pin 6 of the comparator, ensuring that the comparator’s output switches low, triggering the 555 timer when the voltage at pin 7 falls below 2.5 volts. The 555 output activates the two transistors, and the batteries charge for approximately 30 milliseconds. Upon the conclusion of the charge pulse, the battery voltage is measured and divided down using the combination of a 20K ohm, 8.2K ohm, and 620 ohm resistor, so that when the battery voltage reaches 8.2 volts, the input to pin 7 of the comparator slightly surpasses 2.5 volts, and the charging process ceases.

This circuit can be employed to charge different kinds of batteries, such as Ni-Cad, NiMh, or lead acid, however, the termination voltage requires adjustment by modifying the 8.2K and 620 ohm resistors so that the input to the comparator remains at 2.5 volts when the terminal battery voltage is achieved.

For instance, to charge a 6 volt lead acid battery to a limit of 7 volts, the current through the 20K resistor will be (7-2.5)/ 20K = 225 microamps. Consequently, the combination of the remaining two resistors (8.2K and 620) must equal R=E/I = 2.5/ 225 uA = 11,111 ohms. But this is not a standard value, so you could use a 10K in series with a 1.1K, or other values that total 11.11K

Caution must be exercised to prevent overcharging the batteries. I would advocate using a large capacitor in place of the battery to test the circuit and verify it terminates at the appropriate voltage.

Circuit diagram