This small circuit was developed to monitor the battery in a model hovercraft. The lift in the model is produced by an electric motor driving a fan. To avoid the possibility of discharging the rechargeable battery pack too deeply, the design lights a conspicuous LED mounted on the model when a preset threshold voltage is reached. The circuit only uses a few components, which helps keep the total weight of the model down.
The circuit connects to the model only across the two points where the voltage to be monitored can be measured. These also supply power to the circuit. The best place to connect the circuit is not at the battery terminals, but rather at the motor connections.
The circuit is suitable for use with nominal battery voltages of 4.8 V to 9.6 V (four to eight 1.2 V cells). For example, if there are six cells in the battery, its nominal terminal voltage will be 7.2 V. A discharge threshold voltage of around 1 V per cell is appropriate, which means that for six cells the threshold is 6 V. We now need to set the voltage UZ across the adjustable Zener diode D1 (an LM431) to about 0.5 V less than the threshold voltage at which we want LED D2 to light. This voltage is controlled by the choice of the value of resistor R1. As indicated in the circuit diagram, this is done with the help of a trimmer potentiometer (R1.A) with a fixed resistor (R1.B) in series. Using the suggested values (10 kΩ for both the potentiometer and the fixed resistor) allows the discharge threshold voltage to be set between about 5.5 V and 8 V. For lower or higher voltages R1.B should be made correspondingly smaller or larger.
Once the desired value of UZ has been set the total resistance (R1.A plus R1.B) can be measured and a single fixed-value resistor of this value substituted at R1. In the example mentioned of a six-cell battery, a voltage of 7.2 V will appear at the emitter of T1 when the battery is charged. At its base is UZ, which should be 5.5 V (6 V – 0.5 V) in the case of a discharge threshold voltage of 6 V. As long as the battery voltage remains at least 0.5 V higher than UZ, T1 will conduct and T2 will block, with the result that LED D2 will not light. If the battery voltage should fall below about 6 V (UZ + 0.5 V), T1 will block, T2 will conduct and LED D2 will light.
To ensure stable operation of the circuit R6 provides a small amount of switching hysteresis. By adjusting the resistor value between 100 kΩ and 220 kΩ the amount of hysteresis can be varied.
The current drawn by the circuit itself is less than 5 mA (as measured with a battery voltage of 7.2 V). When the LED lights an additional 10 mA (the LED current) is drawn, for a total of around 15 mA. The adjustable Zener diode can be replaced by a fixed Zener with a voltage 0.5 V less than the desired threshold. Resistors R1 and R2 can then be dispensed with. A flashing LED can be used for D2 (without series resistor R7). An acoustic alarm can be provided by replacing D2 and R7 by a DC buzzer with a suitable operating voltage.