# Flat Battery Indicator

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.

#### Circuit diagram

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.