In [1], a diagram of a simple indicator of the state of a battery of batteries was presented. The author used it for a ship model, but the scope of this scheme, of course, can be much wider.The main conditions for achieving long battery life are, in particular, limiting the maximum battery discharge current and the absence of deep
discharge. Even the simplest battery voltage indicator helps to fulfill the last condition (Fig . 1) .
The operation of the circuit is based on the use of the properties of the integral parametric voltage stabilizer D1 of type TL431 and its analogues (LM431, KA431, etc.). The fact is that at a voltage of less than 2.5 V (if the battery is discharged), the output transistor of the IC D1 is in a non-conductive state and the current flowing through this chip does not exceed 1 mA. The voltage drop across the resistor R3 (U e ) is small, and the transistor T1 is locked. Accordingly, the base current through the elements R5 unlocked transistor T2. A light on the D2 LED indicates that the device’s battery is low.
If the VT1 battery is in the working (charged) state, then at the control transition of the MS D1 the voltage reaches 2.5 V, and the output transistor of this microcircuit is unlocked. The current flowing through D1 and R3 will increase. The increase in current will lead to an increase in the voltage drop across the resistor R3 (U e ), the opening of the transistor T1 and the locking of the transistor T2. LED D2 is off.
Depending on the supply voltage of the VT1 source, a voltage threshold is set, below which the LED D2 begins to glow . The extinction voltage D2 in this circuit is made slightly higher than the beginning of the glow of D2. This is achieved by connecting the lower output circuit with the collector of the transistor T2, and not with the "minus" of the circuit power source.
To be able to more accurately set the moment of ignition of the indicator LED D 2, the author of the publication [1] recommends using Fig. 1 chain of tuning resistance R1.1 with a nominal value of 10 kOhm and a limiting resistor R1.2 with a nominal value of 10 kOhm.
In principle, instead of the integral parallel voltage regulator D1, a standard low-power zener diode can be used. Its type and stabilization voltage depend on the supply voltage of the circuit, or rather, on the rated voltage of the used VT1 battery.
Literature
Wolfgang Fritz. Flat Battery Indicator // - 2015. - No. 7/8. - P.30-31.
Prepared by Andrey Nikolaev, Zaporozhye