Battery charge indicator. Simple battery charge and discharge indicator

The most surprising thing is that the battery level indicator circuit does not contain any transistors, microcircuits, or zener diodes. Only LEDs and resistors connected in such a way that an indication of the level of the applied voltage is provided.

Indicator scheme

I assembled the circuit on a universal circuit board, soldering the output of the elements to each other. For a better perception, I took LEDs of different colors.
Such an indicator can be made not only for six LEDs, but for example, for four.
You can use the indicator not only for the battery, but to create a level indication on the music speakers. By connecting the device to the output of the power amplifier, parallel to the column. In this way, critical levels for the speaker system can be monitored.
It is possible to find other applications of this, in truth, a very simple scheme.

Not all cars have an indicator that displays the battery charge level. The motorist must independently monitor this indicator, periodically checking it with a voltmeter, after disconnecting the battery from the machine's electrical network. However, a simple electronic device will allow you to get approximate figures without leaving the cabin.

The choice of circuit and components

Finished construction

Structurally, a self-made battery charge control indicator consists of an electronic unit, on the body of which there are three LEDs: red, blue and green. The choice of color may be different - it is important that when one of them is activated, the information received is correctly interpreted.

Due to the small size of the device, you can use an ordinary prototyping board. The optimal scheme of the device is preselected. You can find several models, but the most common and therefore workable version of the battery charge indicator is shown in the figure.

Diagram of the board and its components

Before installing the components, it is necessary to arrange them on the printed circuit board according to the diagram. Only then can you cut it to the desired size. It is important that the indicator has a minimum size. If you plan to install it in a housing, you should take into account its internal dimensions.

This circuit is designed to control the operation of a car battery with a mains voltage of 6 to 14 V. For other values of this parameter, the characteristics of the components should be changed. Their list is shown in the table.

An LED indicator of the charge level of a conventional or rechargeable battery, where all thresholds are set using potentiometers, can be assembled according to the diagram given in this material. A huge plus is that it works with batteries from 3 to 28 V.

Low battery indicator circuit

The light emitting diode indicators themselves come in various types and colors, the recommended ones are shown on the diagram itself. Due to differences in forward voltage drop, current limiting resistors must be adjusted for best performance and uniformity. In the R18-R22 scheme, the same resistance is offered - note that these resistors do not have to be equal in the end. However, if they are all the same color, one resistor value will be enough.

LED color - charge level

Red: 0 to 25%
Orange : 25 - 50%
Yellow : 50 - 75%
Green : 75 - 100%
Blue: >100% voltage

Here the LM317 acts as a simple 1.25 V reference. The minimum input voltage must exceed the output voltage by a couple of volts. Minimum input voltage = 1.25V + 1.75V = 3V. Although the LM317 has a minimum datasheet load of 5mA, not a single instance has been found that does not function at 3.8mA. It is the resistor R5 (330 ohms) that provides the minimum load.

During the tests, the charge level of 4.5 V of the battery was estimated, it is for it that the voltages on the diagram are given. The setting is as follows: first, the response voltages of each comparator must be determined in accordance with the level of battery discharge, then the voltage must be divided according to the division factor of the voltage divider. So, for a 4.5 V battery, it looks like this:

Threshold voltage

4.8V 1.12V
4.5V 1.05V
4.2 0.98V
3.9V 0.91V

Operation of the battery status indicator

The LM317 U3 chip is a 1.25 volt voltage reference. Resistors R5 and R6 form a voltage divider, which reduces the battery voltage to a level that is close to the reference voltage. The U2A element is an amplifier, so no matter how much current this node draws, the voltage remains stable. Resistors R8 - R11 provide high resistance to the comparator inputs. U1 consists of four comparators that compare the reference voltage of the potentiometers with the battery voltage. Op-amp LM358 U2B - also works as a kind of comparator that controls the lower-order LED.

At the limiting voltage values, the LEDs may not shine clearly, as a rule, flickering occurs between two adjacent LEDs. To prevent this, a small amount of positive feedback voltage is added across R14 - R17.

Indicator testing

If testing is done directly from the battery, please note that reverse polarity protection is not provided. It is better to initially connect the power circuits through a 100 ohm resistor in order to limit possible malfunctions. And after determining that the polarity is correct, this resistor can be removed.

Simplified version of the indicator

For those who want to build a simpler device, the U2 chip, all diodes and some resistors can be eliminated. We advise you to start with this version, and then, after making sure that it works normally, collect the full version of the battery discharge indicator. Good luck with your launch!

The operation of the device is based on the initial turn-on voltage of the LED. Any LED is a semiconductor device that has a limiting voltage point, only after exceeding which it starts to work (shine). Unlike an incandescent lamp, which has almost linear current-voltage characteristics, the LED is very close to the characteristic of a zener diode, with a sharp current slope with increasing voltage.
If you connect the LEDs in a circuit in series with resistors, then each LED will turn on only after the voltage exceeds the sum of the LEDs in the circuit for each segment of the circuit separately.
The voltage threshold for opening or starting to light up the LED can range from 1.8 V to 2.6 V. It all depends on the specific brand.
As a result, each LED lights up only after the previous one lights up.
Assembling the battery level indicator

I assembled the circuit on a universal circuit board, soldering the output of the elements to each other. For a better perception, I took LEDs of different colors.
Such an indicator can be made not only for six LEDs, but for example, for four.
You can use the indicator not only for the battery, but to create a level indication on the music speakers. By connecting the device to the output of the power amplifier, parallel to the column. In this way, critical levels for the speaker system can be monitored.
It is possible to find other applications of this, in truth, a very simple scheme.

Watch the video of the operation and assembly of the level indicator