Lithium-ion batteries are sensitive to bad treatment. Fire, explosions, and other hazardous condition may occur when you charge the cell below the margin that the manufacturer defines. Modern battery chargers can manage the hazardous conditions and deny operation when illegal situations occur. This fact doesn’t mean, however, that all cells are bad. In most cases, you can replace the discharged battery and increase your device’s lifetime. Figure 1 shows the circuit for testing battery packs.
When the supply voltage is lower than 2.6V, no current drives the base of the transistor. LED1 lights up, and LED2 is off. When the voltage exceeds 2.6V, the transistor begins to short LED1, turning it off and lighting LED2. This condition indicates that the battery is below the allowed limit for recharging. The voltage margins highly depend on the type or color of the chosen LEDs. A standard red LED has a forward voltage of 1.7V; a green LED, about 2.1 or 2.2V.
The circuit in this design uses red LEDs with forward voltages of approximately 1.6V at 2 mA. Other LEDs may require a simple redesign, mostly resulting in the requirement for a Schottky diode instead of the 1N4148 in this circuit.