# Electronic – How to test the voltage of a battery without a multimeter? (Analogue components)

analogbatteriesvoltage

I am trying to make a subsystem for my project that gives a low battery warning when the batteries are close to dying. How do I implement this?

Many thanks

If you want something simple and your battery voltage is maybe too small for using a zener diode, you can use some bipolar transistors for turning off the an LED in case the battery voltage is insuficient.

Consider the following circuit. If the battery voltage is high enough the LED will turn on with a current set by $$\R_{LED}\$$. The point at which the led is turned off can be regulated through the pot $$\R_3\$$.

EDIT #1

Here is an updated version whose threshold can be more linearly controlled via a pot too:

The value of $$\R_3\$$ can be roughly calculated to turn off the LED for a given battery voltage threshold $$\V_{BAT,TH}\$$:

$$R_3=(V_{BAT,TH}-V_Z-V_{BE})\dfrac{R_2}{V_{BE}}$$

For example:

$$R_3=(3V-2.1V-650mV)\dfrac{100k\Omega}{650mV}\approx 38k\Omega$$