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Schematic diagram of the electrical circuit indicator of the discharge of lithium batteries. A simple indicator of the discharge of Li-ion batteries. Battery end indicator on LEDs

From the theory of rechargeable batteries, we remember that lithium batteries cannot be discharged below 3.2 volts per can, otherwise it loses its capacity and fails much faster. Therefore, the control of the minimum voltage level is very important for lithium batteries. Of course, in a mobile phone or laptop, the option of a critical discharge is excluded by a smart controller, but the battery for a Chinese flashlight can be killed very quickly, and then write on the forums what kind of shit the Chinese release. To prevent this from happening, I propose to assemble one of the simple circuits of the lithium battery discharge indicator.

An LED is used as an indication element in this circuit. A precision adjustable zener diode TL431 is used as a comparator. Recall TL 431 - an adjustable silicon zener diode with an output voltage that is set to any value from 2.5 to 36 volts using two external resistors. The circuit operation threshold is set by a voltage divider in the control electrode circuit. For a car battery, you need to choose other values ​​\u200b\u200bof the resistors.

LEDs are best to take bright blue, they are the most noticeable. Zener diode TL431 - used in many switching power supplies in the protection optocoupler control circuit and can be borrowed from there.

As long as the voltage is above a predetermined level, in our example 3.25 Volts, the zener diode operates in breakdown mode, therefore, the transistor is locked and all current flows through the green LED. As soon as the voltage on the li ion of the battery starts to decrease in the range from 3.25 to 3.00 volts, VT1 starts to unlock and the current flows through both LEDs.


When the battery voltage is 3V or less, only the red indicator is on. A serious disadvantage of the circuit is the difficulty in selecting zener diodes to obtain the desired response threshold, as well as in high current consumption from 1 mA.


The level of operation of the indicator is set by selecting the values ​​of the resistor R2 and R3.

Due to the use of field devices, the current consumption of the circuit is very small.


A positive voltage at the gate of the transistor VT1 is formed using a divider assembled on two resistances R1-R2. If its level is higher than the cutoff voltage of the field worker, it opens and lubricates the gate VT2 on the common wire, thereby blocking it.

At a given moment, as the li ion of the battery is discharged, the voltage from the divider is not enough to open VT1 and it is locked. A potential appears at the VT2 gate that is close to the supply level, therefore it opens and the LED lights up. The glow of which indicates the need to recharge the battery.

Discharge indicator on the TL431 chip

The threshold is set by the divider on the resistances R2-R3. With the ratings indicated in the figure, it is equal to 3.2 Volts. When this threshold is lowered on the battery, the microassembly will stop shunting the LED and it will light up.


If a battery consisting of several batteries connected in series is used, then the circuit above will have to be connected to each bank.

To set up the circuit, we connect an adjustable power source instead of the battery and by selecting R2 (R4) we achieve the indicator lighting up at the desired interval.

The indicator, in the role of which the LED is used, starts flashing as soon as the voltage on the battery drops below a controlled level. The detector circuit is based on a specialized microassembly MN13811, and the circuit is implemented on the basis of bipolar transistors Q1 and Q2.

If the MN13811-M chip is used, then when the battery voltage drops below 3.2V, the LED starts flashing. A huge plus of the circuit is that during monitoring, the circuit consumes less than 1 μA, and in flashing mode, about 20 mA. The device uses two bipolar transistors of different conductivity. Integrated circuits of the MN13811 series are available for different voltages, depending on the last letter, so if microassembly is required for a different response threshold, then you can use the same microcircuit, but with a different letter index.

"A comment was received with interesting suggestions for finalizing the design.

Since the battery discharge indicator (paragraph 3 of the commentary) is advisable to use on any autonomous electronic device, in order to avoid unexpected failures or equipment failure at the most inopportune moment when the battery is discharged, the manufacture of the discharge indicator is made in a separate article.

The use of a discharge indicator is especially important for most lithium batteries with a nominal voltage of 3.7 volts (for example, the 18650 batteries that are popular today and similar or common flat Li-ion batteries from phones that are replaced by smartphones), because. they really "dislike" a discharge below 3.0 volts and fail at the same time. True, most of them should have built-in emergency deep discharge protection circuits, but who knows what kind of battery you have in your hands until you open it (China is full of mysteries).

But most importantly, I would like to know in advance what charge is currently available in the battery used. Then we could connect the charger in time or put in a new battery without waiting for the sad consequences. Therefore, we need an indicator that will give a signal in advance that the battery will soon sit down completely. To implement this task, there are various circuit solutions - from circuits on a single transistor to fancy devices on microcontrollers.

In our case, it is proposed to make a simple indicator of the discharge of lithium batteries, which is easily assembled by hand. The discharge indicator is economical and reliable, compact and accurate in determining the controlled voltage.

Discharge indicator circuit


The circuit is made using the so-called voltage detectors. They are also called voltage monitors. These are specialized microcircuits designed specifically for voltage control. The indisputable advantages of circuits on voltage monitors are extremely low power consumption in standby mode, as well as its extreme simplicity and accuracy. To make the discharge indication even more noticeable and economical, we load the output of the voltage detector to a blinking LED or a flasher on two bipolar transistors.

The voltage detector (DA1) PS T529N used in the circuit connects the output (pin 3) of the microcircuit to a common wire, when the controlled voltage on the battery drops to 3.1 volts, this includes power to the high-duty pulse generator. At the same time, the super-bright LED starts flashing with a period: pause - 15 seconds, short flash - 1 second. This reduces the current consumption to 0.15 ma in pause, and 4.8 ma in flash. When the battery voltage is more than 3.1 volts, the indicator circuit is practically turned off and consumes only 3 uA.

As practice has shown, the indicated indication cycle is quite enough to see the signal. But if you wish, you can set a more convenient mode for you by selecting resistor R2 or capacitor C1. Due to the low current consumption of the device, a separate power supply switch for the indicator is not provided. The device is operational when the supply voltage drops to 2.8 volts.

Charger manufacturing

1. Complete set.
We purchase or select from available components for assembly in accordance with the scheme.

2. Assembling the circuit.
To check the performance of the circuit and its settings, we assemble the discharge indicator on a universal circuit board. For the convenience of observation (high pulse frequency), for the time of verification, we replace the capacitor C1 with a capacitor of a smaller capacity (for example, 0.47 microfarads). We connect the circuit to the power supply with the ability to smoothly adjust the constant voltage in the range from 2 to 6 volts.

3. Checking the circuit.
Slowly lower the supply voltage of the discharge indicator, starting from 6 volts. We observe on the display of the tester the voltage value at which the voltage detector (DA1) turns on and the LED starts flashing. With the correct selection of the voltage detector, the switching moment should take place in the region of 3.1 volts.


4. We prepare the board for mounting and soldering parts.
We cut out a piece necessary for mounting from a universal printed circuit board, carefully process the edges of the board with a file, clean and tin the contact tracks. The size of the board to be cut out depends on the parts used and their layout during installation. The dimensions of the board in the photo are 22 x 25 mm.

5. Mounting the debugged circuit on the working board
With a positive result in the operation of the circuit on the circuit board, we transfer the parts to the working board, solder the parts, and perform the missing wiring connections with a thin mounting wire. At the end of the assembly, we check the installation. The circuit can be assembled in any convenient way, including surface mounting.


6. Checking the working circuit of the discharge indicator
We check the performance of the discharge indicator circuit and its settings by connecting the circuit to the power supply, and then to the battery under test. When the voltage in the power circuit is less than 3.1 volts, the discharge indicator should turn on.



Instead of the voltage detector (DA1) PS T529N used in the circuit for a controlled voltage of 3.1 volts, it is possible to use similar microcircuits from other manufacturers, for example BD4731. This detector has an open collector output (as evidenced by the additional "1" in the designation of the microcircuit), and also independently limits the output current to 12 mA. This allows you to connect an LED directly to it, without limiting resistors.

It is also possible to use 3.08 volt detectors in the circuit - TS809CXD, TCM809TENB713, MCP103T-315E / TT, CAT809TTBI-G. It is desirable to specify the exact parameters of the selected voltage detectors in their datasheet.

Similarly, you can apply another voltage detector to any other voltage necessary for the operation of the indicator.

The decision on the second part of the question in paragraph 3 of the above comment - the operation of the discharge indicator only in the presence of illumination, has been postponed the following reasons:
- the operation of additional elements in the circuit requires additional energy from the battery, i.e. the efficiency of the scheme suffers;
- the operation of the discharge indicator during the day, most often, is useless, because. there are no “spectators” in the room, and by the evening the battery charge may end;
- the operation of the indicator in the dark is brighter and more efficient, and there is a power switch to quickly turn off the device.

The use of the domestic operational amplifier proposed in paragraph 2 of the comment was not considered, due to the debugging of the operating modes of the circuit for minimum currents, in the process of fine-tuning on the circuit board.

To solve the problem according to paragraph 1 of the commentary, I somewhat changed the scheme of the device “Night lamp with acoustic switch”. Why did I turn on the positive power bus of the acoustic relay through an inverter on VT3, controlled by a constantly working photo relay.

With two resistors, the breakdown voltage can be set between 2.5 V and 36 V.

I will give two schemes for using the TL431 as a battery charge / discharge indicator. The first circuit is for the discharge indicator, and the second for the charge level indicator.

The only difference is the addition of an npn transistor, which will turn on some kind of signaling device, for example, an LED or a buzzer. Below I will give a method for calculating the resistance R1 and examples for some voltages.

The zener diode works in such a way that it begins to conduct current when a certain voltage is exceeded on it, the threshold of which we can set using R1 and R2. In the case of a discharge indicator, the LED indicator should be lit when the battery voltage is less than necessary. Therefore, an npn transistor is added to the circuit.

As you can see, the adjustable zener diode regulates the negative potential, so a resistor R3 is added to the circuit, the task of which is to turn on the transistor when the TL431 is turned off. This resistor is 11k, selected by trial and error. Resistor R4 serves to limit the current on the LED, it can be calculated using.

Of course, you can do without a transistor, but then the LED will go out when the voltage drops below the set level - the circuit is below. Of course, such a circuit will not work at low voltages due to the lack of sufficient voltage and / or current to power the LED. This circuit has one disadvantage, which is the constant current consumption, in the region of 10 mA.

In this case, the charge indicator will be on constantly when the voltage is greater than what we have determined using R1 and R2. Resistor R3 serves to limit the current to the diode.

It's time for what everyone loves the most - math

I already said at the beginning that the breakdown voltage can be changed from 2.5V to 36V through the "Ref" input. And so, let's try to calculate something. Suppose that the indicator should light up when the battery voltage drops below 12 volts.

The resistance of the resistor R2 can be of any value. However, it is best to use round numbers (for ease of counting), such as 1k (1000 ohms), 10k (10,000 ohms).

Resistor R1 is calculated using the following formula:

R1=R2*(Vo/2.5V - 1)

Let's assume that our resistor R2 has a resistance of 1k (1000 ohms).

Vo is the voltage at which breakdown should occur (in our case 12V).

R1 \u003d 1000 * ((12 / 2.5) - 1) \u003d 1000 (4.8 - 1) \u003d 1000 * 3.8 \u003d 3.8k (3800 Ohms).

That is, the resistance of the resistors for 12V is as follows:

And here is a small list for the lazy. For resistor R2=1k, resistance R1 will be:

  • 5V - 1k
  • 7.2V - 1.88k
  • 9V - 2.6k
  • 12V - 3.8k
  • 15V - 5k
  • 18V - 6.2k
  • 20V - 7k
  • 24V - 8.6k

For low voltage, for example, 3.6V, resistor R2 should have a higher resistance, for example, 10k, since the current consumption of the circuit will be less.


nik34 sent:


Charge indicator based on the old Li-Ion battery protection board.

An easy solution for indicating the end of the 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, etc. analogues). The task of this controller is to disconnect the battery from the load when the battery is completely discharged and disconnect the battery from charging when it reaches 4.25V.

Here is the last effect and you can use. For my purposes, an LED that will light up when the charge is complete is quite suitable.

Here is a typical scheme for switching on this mikruha and a scheme into which it must be converted. The whole alteration consists in soldering the mosfets and soldering the LED.

Take the red LED, it has less 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 panel, but it does not allow the battery to be discharged 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 algorithm of work will be as follows: our SB, when illuminated, gives a voltage to the lipolka and until the native charge controller on the battery works at a voltage of about 4.3V. As soon as the cutoff is triggered and the battery is turned off, the voltage on the diode jumps above 4.3V and our circuit, in turn, tries to protect its battery, which no longer exists and, giving a command to the same non-existent mosfet, lights the LED.

Having removed the SB from the light, the voltage on it 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 decorate as you like, since the controller's handkerchief is miniature, no more than 3-4 mm wide, here is an example:



Our magic mikruha is 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, it's easy.


TL431- a three-legged microcircuit, which is often called a "controlled zener diode", because with its help you can get any voltage in the range of 2.5 ... 36 volts. In addition, it can be used as a comparator for a voltage of 2.5 volts:

- if the input is less than 2.5 volts, the current does not flow through the output transistor of the microcircuit;
- if the input is more than 2.5 volts - the transistor is open, and the current flows through it.



It looks a lot like a transistor in the key mode, doesn't it? And even the load - the same indicator LEDs - can be turned on in the same way as in a transistor switch.


Ready scheme for 7 volts(for two Li-ion batteries connected in series, where 8.4 volts when fully charged); to improve accuracy R2 can be made from permanent 47k and tuning on 10k. Conclusion 1, drawing an analogy with n-p-n transistor - "base", pin 2 - "emitter", pin 3 - "collector" (conditionally, of course, a zener diode is not a transistor). As long as the voltage on the “base” is higher than 2.5 volts, the microcircuit is open, and current flows through it. As the battery discharges, the voltage decreases, and as soon as less than 2.5 volts goes from the divider, the transistor of the microcircuit will close, and the current will flow through the LED.

If desired, you can assemble the same circuit on resistors 10k And 5k6- it will work, but it will become a little more gluttonous. So to save money, it is better to take larger resistors. I repeat: discharge indicator batteries should not be hard on it discharge.

R3 sets the current through the load LED and the output transistor of the microcircuit. It is selected at least according to the desired brightness of the glow.


The red LEDs need a low voltage to turn on (starting at 1.5 V), so they can glow even when TL431, in theory, is open and shunts them. The solution is to put a second LED or diode in series 1N4007. Or use LEDs with a higher turn-on voltage - green, blue, white.