When operating any type of battery, it is very important to avoid the possibility of their discharge voltage below the allowable level. Of course, for the batteries to different voltages, the value of this minimum level will also be different.
In the case of batteries with built-in charge-discharge (like a cell phone, for example) such a problem does not arise.
If the battery is used without such a special controller, then it is likely to discharge element below a critical level, and this can lead to significant loss of its capacity, if any, - to the impossibility of further operation.
In this article, your attention is invited simple circuit for automatically controlling the voltage on the element having the indication of the charge level reduction up to a critical value.
For example, using this scheme, together with a battery for a radio telephone (or Similarly, a voltage of 3.6 volts), it "will signal" voltage level of falling on it to 2.7... 2.9 volts. This allows enough time to disconnect the battery and make its charging.
scheme shown here allows you to adjust the value of the operation voltage. Therefore, it can be successfully applied to other types of batteries, with other values of the operating voltages.
If the battery voltage (or battery) is normal, the transistor T2 is locked and HL1 LED is off. When the voltage drops below a certain level, transistor T2 is opened and the LED will illuminate. The level of the battery voltage at which this happens, you can specify when setting up, tuning resistor selection in the base of the transistor T1.
Denomination resistor R3 depends on the operating voltage and battery type of the applied LED. Trimpot resistor may take from 30 to 100 ohms (depending on the required adjustment range). All other types of resistors MLT-0.125. Transistors can be any thin.
The scheme is extremely simple and quite universal. Its only drawback is that the LED lights up when the battery is almost depleted and the voltage on it a little. Therefore, care should be taken in the operation of the scheme, in order to prevent even greater battery discharge. LED, of course, better to choose a low-power, with as low as possible operating voltage and current.