The low battery indicator is designed to provide a quick warning of a low battery, which will help protect you from many problems. The proposed circuit is quite simple, and the whole adjustment is to set the threshold for operation with a variable resistor to turn on the LED display.
In order to simplify the home-made design as much as possible, information on the degree of battery discharge comes by the principle of an LED column, that is, the higher the voltage on the batteries, the more LEDs light up. The lower level is indicated by a red LED (the upper one according to the diagram), the lower green LED indicates the maximum voltage. The complete absence of glow indicates a strong critical discharge of the battery.
The design is based on four comparators of the operational amplifier LM324, each of them controls a certain voltage level.
The reference voltage of 5 volts for all four comparators comes from the zener diode and resistance R6.
If at the direct input of the op-amp the potential is less than the potential at its inverse input, a low logic level is present at the output of the comparator and the LED does not light. If the reference voltage exceeds the potential at the opposite input, the comparator switches and the LED lights up. Each comparator has its own personal level, which is adjusted by the resistance of the divider on the resistors R1-R5.
A variant of this design, but already on the operational amplifier LM 339 is suitable for batteries with an output voltage of 6 or 12 volts.
In the arsenal of domestic microcircuits there is a series of КР1171, which are specially designed to control the reduction of the supply voltage. So we use it to control the voltage in the battery.
Low current consumption in off mode allows you to embed this design in devices with continuous monitoring of the battery voltage. In this case, the indicator can be connected to the power switch of the device, directly to the battery terminals. To convert this indicator circuit to a different voltage, it is enough to use the corresponding chip in the КР1171 series and select the resistor R1 for the new voltage. The only exception is the KR1171SP20 chip, because its threshold level is 2V, and the generator on the K561LA7 chip does not work.
To achieve a minimum size, you can use a miniature emitter instead of a speaker. Using resistance R6, you can adjust the sound volume.
This design is designed for battery voltage from 6 to 24 volts.
The circuit consists of a voltage divider on resistors R1 R2, the first transistor responds to a voltage decrease below the specified value, and the electronic switch on the second transistor, through the drain circuit, triggers a super-bright LED.
When the circuit is connected to the rechargeable battery, the voltage of which must be controlled; a voltage of positive polarity, regulated by resistor R2, appears on the gate of the first transistor. If it is higher than the threshold, the transistor is open, the resistance of its channel is not higher than a dozen Ohms, so the voltage at the drain of the second transistor VT2 tends to zero and it is closed, the LED does not light, respectively, indicating that the battery voltage is normal. When the voltage drops to the threshold level, at which the voltage at the gate of the first transistor becomes lower than the threshold, it closes, the resistance of its channel increases sharply and the voltage at the drain tends to the value of the supply voltage. At the same time, the transistor switch opens and the LED lights up, indicating an unacceptable degree of battery discharge.
Schmitt trigger is built on transistors VT2, VT3, on VT1 - a module for inhibiting its operation. The VTL collector circuit includes an HL1 indicator located on the dashboard. When hot, the indicator filament has a resistance in the region of 50 Ohms. The resistance of the cold indicator thread is several times lower. Therefore, the VT3 transistor can withstand a surge current in the collector circuit to a level of 2.5 A.
The voltage of the on-board network minus the voltage at the Zener diode VD2 through the divider R5-R6 goes to the base VT2. If it is higher than 13.5 V, the Schmitt trigger switches and the transistor VT3 is closed, and HL1 is not lit.
nik34 sent:
Charge indicator based on an old Li-Ion battery protection board.
An easy solution to indicate the end of charge of a LiIon or LiPo battery from a solar battery can be made from ... any dead LiIon or LiPo battery :)
They use a six-legged charge controller on a specialized mikruha DW01 (JW01, JW11, K091, G2J, G3J, S8261, NE57600 and other analogs). The task of this controller is to disconnect the battery from the load when the battery is completely discharged and to disconnect the battery from charging when it reaches 4.25V.
Here is the last effect and you can use it. For my purposes, an LED that will light up at the end of the charge is quite suitable.
Here is a typical scheme for including this mikruhi and a scheme into which it needs to be redone. The whole alteration consists in soldering mosfets and soldering the LED. 
Take the red LED, it has a lower ignition voltage than other colors.
Now we need to connect this circuit after the traditional diode, which also traditionally steals from 0.2V (Schottky) to 0.6V from the solar battery, but it does not allow the battery to discharge to the solar panel after dark. So, if you connect the circuit to the diode, then we get an indication of undercharging the battery by 0.6V, which is quite a lot.
Thus, the operation algorithm will be as follows: our SB in illumination gives a stick to lipolka and until the native charge controller on the battery works at a voltage of about 4.3V. As soon as the cut-off is triggered and the battery is turned off, a voltage above 4.3V jumps on the diode and our circuit, in turn, tries to protect its battery, which is no longer there, and giving a command to the non-existent mosfet, it also lights up the LED.
After removing the voltage on it from the SB light, the voltage will drop and the LED will turn off, stopping eating precious milliamps. The same solution can be used with other chargers, it is not necessary to go in cycles in the solar battery :)
You can issue it as you like, the benefit of the controller's scarf is miniature, no more than 3-4 mm wide, here is an example:
Our magic mikruha on the left, two mosfets in one case on the right, they must be removed and soldered to the board in accordance with the LED circuit.
That's all, use it, the benefit is simple.
In modern practice, there are still cars on which there is neither an on-board computer, nor a board with a battery charge indicator. Movement without an indicator is fraught with a complete engine stop and the inability to start it in the future.
The battery charge indicator performs two functions: it shows charging the battery current from the generator and informatively the amount of battery charge. There are several ways to eliminate this defect in the car. One of them is the easiest, do-it-yourself device showing battery charging.
Available sources have many suggestions for making the digital current circuit of such a device. It has a fairly simple look. To do this, you need skills in soldering radio components and a desire to assemble a device with your own hands. Choose LED, Zener diode, breadboard and resistors. The diagram of the battery charge indicator is shown in the figure below.
Principle of operation
The LED indicator due to the presence of three colors of LEDs can show different phases of charging current. Start charging. Working middle. Warning of the end of the process. This circuit gives us the ability to control the entire battery life cycle.
Soldering parts with your own hands is not difficult, but first make a check with a tester. If all the details are working, you can make an assembly according to the scheme. By calling the tester the LED output. We determine the low voltage output from six to eleven volts.
This is a red LED. Eleven to thirteen volts is yellow. More than thirteen - there will be a green LED. The circuit has a simple set of parts and works reliably.
Interesting! The battery produces a certain voltage on the LED. He lights up. So we determine the beginning and end of the battery charge.
If you don’t have any components, then you need to look on the Internet for similar schemes and modify the device with your own hands. The circuit will also show a reliable indication of the battery charge.
For a car, it is important that the circuit does not work all the time, but only when the driver is driving. It is recommended that after completing the work with your own hands, mount the resulting device under the steering wheel and connect it to the ignition switch. In this case, the indicator will work only when the car ignition is on.
We see that after finishing work, you can create a battery indicator convenient and necessary for reliable operation of the car with your own hands. The cost of such a product will not be high.
Important! The reliability of the indicator and the convenience of its placement can effectively eliminate the incompleteness of designers - car manufacturers.
On the one hand, any device, whether it is a vehicle or simple kitchen utensils, seems perfect and refined from a technical point of view. Not requiring the intervention of human thought and competent hands.
On the other hand, there will always be competent "Kulibins" for whom this device does not seem perfect and requires improvement and technical refinement.
This is the basis for progressive technological progress. It seems to be a simple, but at the same time vital visual indication of the process of charging the car’s battery, not designed by the designers, found its simple development by simple admirers of the world of science and technology.
Successful starting of a car engine depends largely on the state of charge of the battery. Regularly checking the voltage at the terminals with a multimeter is inconvenient. It is much more practical to use a digital or analog indicator located next to the dashboard. The simplest indicator of the battery charge can be made by yourself, in which five LEDs help to monitor the gradual discharge (charge) of the battery.
Scheme
The considered circuit diagram is a simple device that displays the battery charge level at 12 volts. Its key element is the LM339 chip, in the housing of which 4 of the same type of operational amplifier (comparator) are assembled. General view of LM339 and pin assignment is shown in the figure. 
Direct and inverse comparator inputs are connected through resistive dividers. 5 mm indicator LEDs are used as a load.
The VD1 diode protects the microcircuit against accidental polarity reversal. The Zener diode VD2 sets the reference voltage, which is the standard for future measurements. Resistors R1-R4 limit the current through the LEDs.
Principle of operation
The battery charge indicator circuit works on LEDs as follows. A voltage of 6.2 volts stabilized by a resistor R7 and a zener diode VD2 is applied to a resistive divider assembled from R8-R12. As can be seen from the circuit, between each pair of these resistors, reference voltages of different levels are formed, which are fed to the direct inputs of the comparators. In turn, the inverse inputs are interconnected and connected through the resistors R5 and R6 to the terminals of the battery (battery).
In the process of charging (discharging) the battery, the voltage at the inverse inputs gradually changes, which leads to alternating switching of the comparators. Consider the operation of the operational amplifier OP1, which is responsible for indicating the maximum charge level. We set the condition if the charged battery has a voltage of 13.5V, then the last LED starts to light up. The threshold voltage at its direct input, at which this LED lights up, is calculated by the formula:
U OP1 + \u003d U CT VD2 - U R8,
U ST VD2 \u003d U R8 + U R9 + U R10 + U R11 + U R12 \u003d I * (R8 + R9 + R10 + R11 + R12)
I \u003d U CT VD2 / (R8 + R9 + R10 + R11 + R12) \u003d 6.2 / (5100 + 1000 + 1000 + 1000 + 10000) \u003d 0.34 mA,
U R8 \u003d I * R8 \u003d 0.34 mA * 5.1 kOhm \u003d 1.7 V
U OP1 + \u003d 6.2-1.7 \u003d 4.5V
This means that when the potential reaches more than 4.5 volts at the inverse input, the comparator OP1 switches and a low voltage level appears at its output, and the LED lights up. Using these formulas, you can calculate the potential at the direct inputs of each operational amplifier. The potential at the inverse inputs is found from the equation: U OP1- \u003d I * R5 \u003d U BAP - I * R6.
Circuit board and assembly details
The printed circuit board is made of single-sided foil textolite measuring 40 by 37 mm, which can be downloaded. It is designed for mounting DIP elements of the following type:
- mLT-0.125W resistors with an accuracy of at least 5% (E24 series)
R1, R2, R3, R4, R7, R9, R10, R11– 1 kOhm,
R5, R8 - 5.1 kOhm,
R6, R12 - 10 kOhm; - diode VD1 any low-power with a reverse voltage of at least 30V, for example 1N4148;
- zener diode VD2 low-power with a stabilization voltage of 6.2V. For example, KS162A, BZX55C6V2;
- lEDs LED1-LED5 - indicator type AL307 of any color glow.
This circuit can be used not only to control the voltage on 12 volt batteries. Recounting the values \u200b\u200bof the resistors located in the input circuits, we get an LED indicator for any desired voltage. To do this, you need to set the threshold voltages at which the LEDs will turn on, and then use the formulas for recalculating the resistances given above.
Read the same