3 Volt Lithium Cells are extensively used in low power applications such as Digital clocks, Toys etc. These are also used in Computer Mother boards as backup power source to keep the memory. The circuit described here is a handy tester to check the voltage level in the used Lithium cell before throwing it away.Even if it carries less than 2 volts, it can be used again in some very low power applications that require less than two volts.
The circuit uses an LED as a Zener diode to give 2.2 V reference voltage to compare the voltage level in the Lithium cell.
IC1 (TL071) is used as a Voltage comparator. The low noise JFET Operational Amplifier TL071 is used in the circuit since it requires very low input bias current. Its inverting input is connected to the junction between R2 and a Green LED. Here LED is used like a Zener diode to give 2.2 V (Forward voltage of Green LED) as reference voltage.
This LED has dual function. It act as a power on indicator as well as low value Zener which is not commonly available.
The non inverting input of IC1 receives voltage from the Lithium battery under test. When the non inverting input is not connected to the battery, output of IC1 will be low since its inverting input gets higher voltage than non inverting input. The low output from IC1 keeps T1 non conducting and the Green half of the Bicolour LED remains off.
At the same time the high voltage in the collector of T1 gives base bias to T2 and it conducts. The Red half of the bicolour LED lights.
When a Lithium battery is connected to test points, the non inverting input of IC1 gets higher voltage (more than 2.2 volts) and the output of the comparator swings high. This makes T1 conducting and Green half of Bicolour LED lights indicating that the battery under test is holding more than 2.2 volts. At the same time T2 turns off since its base goes to ground potential. Red LED remains off in this state. In short, Green LED lights if the battery holds more than 2.2 volts.