Simple circuit indicates health of lithium-ion batteries

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. Even white or blue LEDs with 3V or more forward voltage make sense for certain applications.

Lower-value resistors increase the brightness of the LEDs but increase the supply current, as well. Table 1 shows how this indicator provides three states of operation. Although this simple device draws little current, you cannot expect a long battery life if you use the device as a permanent display, especially when it is in storage. Although a fully charged 32-Ahr cell will expire after about a year, an empty battery of the same size but slightly higher than the allowed margin for charging will expire after one or two days.

You can build an array of indicators in one test module. By connecting to the measuring/balancing port of the pack, you can easily inspect a whole pack with one view. Adding zener diodes in series to the LEDs also makes this circuit a simple indicator for higher voltage levels.