Li-ion and Li-polymer batteries in our designs. Circuit for protecting batteries from deep discharge on the ne7555 chip Protection against battery discharge

DEVICE for protecting 12v batteries from deep discharge and short circuit with automatic disconnection of its output from the load.

CHARACTERISTICS
The voltage on the battery at which the shutdown occurs is 10± 0.5V. (I got exactly 10.5 V)
The current consumed by the device from the battery when turned on is no more than 1 mA
The current consumed by the device from the battery when turned off is no more than 10 µA
The maximum permissible direct current through the device is 5A. (30 Watt light bulb 2.45 A - Mosfit without radiator +50 degrees (room +24))
The maximum permissible short-term (5 sec) current through the device is 10A
Turn-off time in case of short circuit at the device output, no more than - 100 μs

OPERATING ORDER OF THE DEVICE

THE DEVICE OPERATES AS follows:

Spare parts

2. Any field-effect transistor, select according to A and B. I took RFP50N06 N-channel 60V 50A 170 deg 3. Resistors 3 for 10 Ω, and 1 for 100 Ω

5. Zener diode 9.1 V

Soldering iron + tin + alcohol rosin + wire cutters + wiring + multimeter + load, etc. and so on

Soldered using the tin-nozzle method. I don’t want to poison the board. There is no layout.

Load 30 Watt, Current 2.45 A, the field worker heats up to +50 degrees (room temperature +24). No cooling needed.

I visited a load of 80 watts... VAH-VAH. Temperature over 120 degrees. The tracks began to turn red... Well, you know, you need a radiator, Well-soldered tracks.

Communities › Electronic Crafts › Blog › Protecting the Battery from Deep Discharge…
Tags: battery protection, battery, 12v, 12v, 12v, 12v, protection, recorder, mosfit. Protecting the Battery from deep discharge... The circuit is not mine. I’ll just repeat... Use where necessary... Recorders, tape recorders, etc. ... DEVICE for protecting 12v batteries from deep discharge and short circuit with automatic disconnection of its output from the load. CHARACTERISTICS Battery voltage...

Hi all. I recently assembled an electronic switch based on a field-effect transistor that automatically turns off the battery when discharged to a specified voltage. That is, this device is capable of monitoring the decrease in voltage on the battery and disconnecting it from the load in time so that it does not go to zero and deteriorate. For example, if you forgot to turn off the flashlight.

Battery protection device diagram

For lead-acid batteries with a voltage of 12 V, the minimum permissible voltage during discharge is approximately 9 V. It is at this voltage that the load must be disconnected from the battery in order to prevent it from being deeply discharged. It is convenient to control the battery voltage using the TL431 parallel stabilizer chip. This chip contains a built-in error amplifier and a precision voltage reference. To switch the load, it is recommended to use a MOSFET transistor, which can provide a very low on-state voltage drop. The scheme is extremely simple, I used it myself for several years, having assembled it using a hinged installation, and only recently made a “boxed” version:

In this version, the switch is for 6/12V batteries, P1 is selected and then replaced with permanent ones. For 6 V – the threshold is 4.8..5 V, for 12 V – 9.6..10 V, respectively. You can set your P1 for other cutoff voltages if desired. For convenience, I added an indicator - LED.

In view of the shortage of powerful P-channel field-effect transistors, and even “Logic Level”, the circuit can be converted to an N-channel one, instead of the P-channel one, installing a low-power P-N-P transistor of the KT316 type, and this can be used to switch the powerful N-channel one key. But in this case, it is not the “plus”, but the “minus” of the load that will be disconnected.

A radiator is not required for load currents up to several amperes - this is accurate, verified. In general, for installation in a car, where currents reach tens of amperes, everything is easy to calculate. We multiply the resistance of the open field switch by the squared current.

And although the transistor does not heat up at all, I still installed it on a small radiator, just to be on the safe side. There was just one case when, during the process of recharging the battery, I touched a field worker - it was noticeably hot. While figuring out what was going on, I found out that the 431st stabilizer had failed, and the key was “stuck” in linear mode, never fully opening – which was why it was heating up. Why the stabilizer burned out remains a mystery, it was soldered, maybe it had already happened before. All other elements of the circuit remained intact.

Battery deep discharge protection
Protection against deep battery discharge Hello everyone. I recently assembled an electronic switch based on a field-effect transistor that automatically turns off the battery when discharged to a specified voltage. That is

A device for protecting 12v batteries from deep discharge and short circuit with automatic disconnection of its output from the load.

CHARACTERISTICS

The battery voltage at which the shutdown occurs is 10± 0.5V. (I got exactly 10.5 V) The current consumed by the device from the battery when turned on is no more than 1 mA. The current consumed by the device from the battery when turned off is no more than 10 µA. The maximum permissible direct current through the device is 5A. (30 Watt light bulb 2.45 A - Mosfit without radiator +50 degrees (room +24))

The maximum permissible short-term (5 sec) current through the device is 10A. Turn-off time in case of short circuit at the device output, no more than - 100 μs

OPERATING ORDER OF THE DEVICE

Connect the device between the battery and the load in the following sequence:
- connect the terminals on the wires, observing the polarity (orange wire + (red), to the battery,
- connect to the device, observing the polarity (the positive terminal is marked with a + sign), the load terminals.

In order for voltage to appear at the output of the device, you need to briefly short-circuit the negative output to the negative input. If the load is powered by another source besides the battery, then this is not necessary.

THE DEVICE OPERATES AS follows:

When switching to battery power, the load discharges it to the response voltage of the protection device (10± 0.5V). When this value is reached, the device disconnects the battery from the load, preventing further discharge. The device will turn on automatically when voltage is supplied from the load side to charge the battery.

If there is a short circuit in the load, the device also disconnects the battery from the load. It will turn on automatically if a voltage of more than 9.5V is applied from the load side. If there is no such voltage, then you need to briefly bridge the output negative terminal of the device and the negative terminal of the battery. Resistors R3 and R4 set the response threshold.

Spare parts

1. Mounting board (optional, can be mounted)
2. Any field-effect transistor, select according to A and B. I took RFP50N06 N-channel 60V 50A 170 deg
3. Resistors 3 for 10 com, and 1 for 100 com
4. Bipolar transistor KT361G
5. Zener diode 9.1 V
Add. You can use terminals + Mikrik for starting. (I didn’t do it myself because it will be part of another device)
6. You can have an LED at the input and output for clarity (Select a resistor, solder in parallel)

Soldering iron + tin + alcohol rosin + wire cutters + wiring + multimeter + load, etc. and so on. Soldered using the tin-nozzle method. I don't want to poison the board. There is no layout. Load 30 Watt, Current 2.45 A, the field worker heats up at +50 degrees (room temperature +24). No cooling needed.

I tried a load of 80 watts... VAH-VAH. Temperature over 120 degrees. The tracks began to turn red... Well, you know, you need a radiator, Well-soldered tracks.

Protection of batteries from deep discharge
Battery protection from deep discharge A device for protecting 12v batteries from deep discharge and short circuit with automatic disconnection of its output from the load. CHARACTERISTICS

How often do we forget to turn off the load from the battery... Have you ever thought about this question... But it often happens that the battery seems to be working and working, but then something has dried up... We measure the voltage on it, and there is 9-8V, or even less. Torba, you can try to restore the battery, but it doesn’t always work.
For this reason, a device was invented that, when the battery is discharged, will disconnect the load from it and prevent deep discharge of the battery, because it is no secret that batteries are afraid of deep discharge.
To be honest, I thought many times about a device for protecting the battery from deep discharge, but it was never my destiny to try everything. And over the weekend I set a goal to make a small protection circuit

Battery protection circuit from full discharge

Any Start and Stop buttons without fixing

Let's look at the diagram. As you can see, everything is built on two op-amps switched on in comparator mode. LM358 was taken for the experiment. And so let's go...
The reference voltage is formed by the R1-VD1 chain. R1 is a ballast resistor, VD1 is a simple 5V zener diode, it can be used for higher or lower voltages. But not more than and not equal to the voltage of a discharged battery, which by the way is equal to 11V.

A comparator was assembled on the first op-amp, comparing the reference voltage with the battery voltage. The voltage to the 3rd leg is supplied from the battery through a resistor divider, which creates the compared voltage. If the voltage on the divider is equal to the reference one, a positive voltage appears on the first leg, which opens the transistors, which are installed as an amplifier stage, so as not to load the output of the op-amp.

Everything is set up simply. We supply 11V to the Out terminal. It is on this leg, because the diode drops by 0.6V and then you will have to rebuild the circuit. A diode is needed so that when you press the start button, the current does not go to the load, but supplies voltage to the circuit itself. By selecting resistors R2R6, we catch the moment when the relay turns off, the voltage on the 7th leg disappears, and on the 5th leg the voltage should be slightly less than the reference

When the first comparator has been built, we apply 12V voltage, as expected, to the Vcc terminal and press Start. The circuit should turn on and operate without problems until the voltage drops to 10.8V, the circuit should turn off the load relay.

Press Stop, the voltage on the 5th leg will disappear and the circuit will turn off. By the way, it is better not to set C1 to a higher value, since it will take a long time to discharge and you will have to hold the STOP button longer. By the way, I haven’t yet figured out how to force the circuit to turn off immediately if there is a good capacitance on the load itself, which will take longer to discharge, although you can throw a ballast resistor on the condenser itself

At the second Op, it was decided to assemble an indicator indicating when the battery is almost discharged and the circuit should turn off. It is configured in the same way... We supply 11.2V to Out and select R8R9 to ensure that the red LED lights up
This completes the setup and the circuit is fully operational...

Good luck everyone with your repetition...
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Author of the Article: Admin check

Deep discharge battery protection device
How often do we forget to turn off the load from the battery. Then we measure the voltage on it, and there it is 9-8V. Khan to him Here is a device that will prevent the battery from completely discharging

I needed to protect the battery from deep discharge. And the main requirement for the protection circuit is that after the battery is discharged, it turns off the load and cannot turn it on on its own after the battery has built up a little voltage at the terminals, without a load.

The circuit is based on the 555th timer, connected as a single pulse generator, which, after reaching the minimum threshold voltage, will close the gate of transistor VT1 and turn off the load. The circuit will be able to turn on the load only after disconnecting and reconnecting the power.

Fee (No need to mirror):

SMD Board (Need mirroring):

All SMD resistors are 0805. The MOSFET package is D2PAK, but DPAK is also possible.

When assembling, you should pay attention to the fact that there is a jumper under the chip (in the board with DIP components) and the main thing is not to forget about it!

The circuit is configured as follows: resistor R5 is set to the top position according to the circuit, then we connect it to a power source with a voltage set on it, at which it should turn off the load. If you believe Wikipedia, then the voltage of a completely discharged 12-Volt battery corresponds to 10.5 Volts, this will be our load disconnect voltage. Next, rotate the R5 regulator until the load is turned off. Instead of the IRFZ44 transistor, you can use almost any powerful low-voltage MOSFET, you just need to take into account that it must be designed for a current 2 times greater than the maximum load current, and the gate voltage must be within the supply voltage.

If desired, the trimming resistor can be replaced with a constant one with a nominal value of 240 kOhm, and in this case resistor R4 must be replaced with 680 kOhm. Provided that the threshold of the TL431 is 2.5 Volts.

The current consumption of the board is about 6-7 mA.

Any Start and Stop buttons without fixing

Good luck everyone with your repetition...
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A simple charger with an LED charging indicator, green battery is charging, red battery is charged.

There is short circuit protection and reverse polarity protection. Perfect for charging Moto batteries with a capacity of up to 20A/h; a 9A/h battery will charge in 7 hours, 20A/h in 16 hours. The price for this charger is only 403 rubles, free delivery

This type of charger is capable of automatically charging almost any type of 12V car and motorcycle batteries up to 80A/H. It has a unique charging method in three stages: 1. Constant current charging, 2. Constant voltage charging, 3. Drop charging up to 100%.
There are two indicators on the front panel, the first indicates the voltage and charging percentage, the second indicates the charging current.
Quite a high-quality device for home needs, the price is just RUR 781.96, free delivery. At the time of writing these lines number of orders 1392, grade 4.8 out of 5. Eurofork

Charger for a wide variety of 12-24V battery types with current up to 10A and peak current 12A. Able to charge Helium batteries and SA\SA. The charging technology is the same as the previous one in three stages. The charger is capable of charging both automatically and manually. The panel has an LCD indicator indicating voltage, charging current and charging percentage.

A good device if you need to charge all possible types of batteries of any capacity, up to 150Ah

If any product has become unavailable, please write in the comment at the bottom of the page.
Author of the Article: Admin check

There are two things that batteries really don't like: overcharging and overdischarging. And if the first problem is successfully solved by modern chargers (except for the simplest rectifiers), then with a discharge below a critical level things are worse - almost never battery-powered devices provide protection against overdischarge. An accidental discharge cannot be ruled out - when you simply forgot to turn off the device and it discharges, discharges... To solve this problem, a simple low-voltage circuit disconnect module is offered for self-assembly. This circuit is quite simple and can be applied to any lithium or lead-acid battery. Naturally, the shutdown threshold can be adjusted according to the battery.

Battery protection unit diagram

How it works. When the reset button is pressed, positive voltage is applied to the gate of the N-channel MOSFET power transistor.

If the voltage at the output of Zener diode U1 is higher than 2.5 volts, as determined by the voltage divider consisting of R4, R5 and R6, the cathode of U1 is connected to its anode, making it negative with respect to its emitter, R2 limits the base current to a safe value and provides sufficient current to operate U1. And transistor Q1 will keep the circuit open even when you release the reset button.

If the voltage at U1 drops below 2.5 volts, the zener diode turns off and pulls up the positive voltage at the emitter of R1, turning it off. Resistor R8 also turns off the field-effect transistor, resulting in the load being disconnected. Moreover, the load will not be turned on again until the reset button is pressed.

Most small FETs are rated for only +/- 20 volts at the gate source voltage, meaning the block circuit is suitable for no more than 12 volt devices: if higher operating voltages are required, additional circuit elements will need to be added to maintain safety fieldworker's work. An example of using such a circuit: a simple solar battery charge controller shown in the photo.

If a lower voltage than 9 volts (or higher than 15) is required, it will be necessary to recalculate the values of resistors R4 and R6 to change the adjustment range.

You can put almost any silicon PNP transistor with a rating of at least 30 volts and any N-channel MOSFET with a rated voltage of at least 30 volts and a current more than 3 times that which you are going to switch into the circuit. Feedthrough resistance of a fraction of Ohm. For the prototype, the F15N05 was used - 15 amps, 50 volts. For high currents, transistors IRFZ44 (50 A Max.) and PSMN2R7-30PL (100 A Max.) are suitable. You can also connect several field-effect transistors of the same type in parallel as needed.

This device should not remain connected to the battery for a long time, since it itself consumes several milliamps due to the LED and the current consumption of U1. When turned off, its current consumption is negligible.

It's no secret that Li-ion batteries do not like deep discharge. This causes them to wither and wither, and also increase internal resistance and lose capacity. Some specimens (those with protection) can even plunge into deep hibernation, from where it is quite problematic to pull them out. Therefore, when using lithium batteries, it is necessary to somehow limit their maximum discharge.

To do this, special circuits are used that disconnect the battery from the load at the right time. Sometimes such circuits are called discharge controllers.

Because The discharge controller does not control the amount of discharge current; strictly speaking, it is not a controller of any kind. In fact, this is an established but incorrect name for deep discharge protection circuits.

Contrary to popular belief, the built-in batteries (PCB boards or PCM modules) are not designed to limit the charge/discharge current, or to timely disconnect the load when fully discharged, or to correctly determine the end of charging.

Firstly, Protection boards, in principle, are not capable of limiting the charge or discharge current. This should be handled by the memory department. The maximum they can do is turn off the battery when there is a short circuit in the load or when it overheats.

Secondly, Most protection modules turn off the li-ion battery at a voltage of 2.5 Volts or even less. And for the vast majority of batteries, this is a very strong discharge; this should not be allowed at all.

Third, The Chinese are riveting these modules in the millions... Do you really believe that they use high-quality precision components? Or that someone tests and adjusts them before installing them in batteries? Of course, this is not true. When producing Chinese motherboards, only one principle is strictly observed: the cheaper, the better. Therefore, if the protection disconnects the battery from the charger exactly at 4.2 ± 0.05 V, then this is more likely a happy accident than a pattern.

It’s good if you got a PCB module that will operate a little earlier (for example, at 4.1V). Then the battery simply won’t reach ten percent of its capacity and that’s it. It is much worse if the battery is constantly recharged, for example, to 4.3V. Then the service life is reduced and the capacity drops and, in general, may swell.

It is IMPOSSIBLE to use the protection boards built into lithium-ion batteries as discharge limiters! And as charge limiters too. These boards are intended only for emergency battery disconnection in case of emergency situations.

Therefore, separate circuits for limiting charge and/or protecting against too deep discharge are needed.

We looked at simple chargers based on discrete components and specialized integrated circuits in. And today we’ll talk about the solutions that exist today to protect a lithium battery from too much discharge.

To begin with, I propose a simple and reliable Li-ion overdischarge protection circuit, consisting of only 6 elements.

The ratings indicated in the diagram will result in the batteries being disconnected from the load when the voltage drops to ~10 Volts (I made protection for 3 series-connected 18650 batteries in my metal detector). You can set your own shutdown threshold by selecting resistor R3.

By the way, the full discharge voltage of a Li-ion battery is 3.0 V and no less.

A field chip (like the one in the diagram or something similar) can be dug out from an old computer motherboard; usually there are several of them at once. TL-ku, by the way, can also be taken from there.

Capacitor C1 is needed for the initial startup of the circuit when the switch is turned on (it briefly pulls the gate T1 to minus, which opens the transistor and powers the voltage divider R3, R2). Further, after charging C1, the voltage required to unlock the transistor is maintained by the TL431 microcircuit.

Attention! The IRF4905 transistor indicated in the diagram will perfectly protect three lithium-ion batteries connected in series, but is completely unsuitable for protecting one 3.7 Volt bank. It is said how to determine for yourself whether a field-effect transistor is suitable or not.

The downside of this circuit: in the event of a short circuit in the load (or too much current consumed), the field-effect transistor will not close immediately. The reaction time will depend on the capacitance of capacitor C1. And it is quite possible that during this time something will have time to burn out properly. A circuit that instantly responds to a short load under load is presented below:

Switch SA1 is needed to “restart” the circuit after the protection has tripped. If the design of your device provides for removing the battery to charge it (in a separate charger), then this switch is not needed.

The resistance of resistor R1 must be such that the TL431 stabilizer reaches operating mode at a minimum battery voltage - it is selected in such a way that the anode-cathode current is at least 0.4 mA. This gives rise to another drawback of this circuit - after the protection is triggered, the circuit continues to consume energy from the battery. The current, although small, is quite enough to completely drain a small battery in just a couple of months.

The diagram below for self-made monitoring of the discharge of lithium batteries is free from this drawback. When the protection is triggered, the current consumed by the device is so small that my tester does not even detect it.

Below is a more modern version of the lithium battery discharge limiter using the TL431 stabilizer. This, firstly, allows you to easily and simply set the desired response threshold, and secondly, the circuit has high temperature stability and clear shutdown. Clap and that's it!

Getting TL-ku today is not a problem at all, they are sold for 5 kopecks per bunch. Resistor R1 does not need to be installed (in some cases it is even harmful). Trimmer R6, which sets the response voltage, can be replaced with a chain of constant resistors with selected resistances.

To exit the blocking mode, you need to charge the battery above the protection threshold, and then press the S1 “Reset” button.

The inconvenience of all the above schemes is that to resume operation of the schemes after going into protection, operator intervention is required (turn SA1 on and off or press a button). This is the price to pay for simplicity and low power consumption in lock mode.

The simplest li-ion overdischarge protection circuit, devoid of all disadvantages (well, almost all) is shown below:

The principle of operation of this circuit is very similar to the first two (at the very beginning of the article), but there is no TL431 microcircuit, and therefore its own current consumption can be reduced to very small values - about ten microamps. A switch or reset button is also not needed; the circuit will automatically connect the battery to the load as soon as the voltage across it exceeds a preset threshold value.

Capacitor C1 suppresses false alarms when operating on a pulsed load. Any low-power diodes will do; it is their characteristics and quantity that determine the operating voltage of the circuit (you will have to select it locally).

Any suitable n-channel field-effect transistor can be used. The main thing is that it can withstand the load current without straining and be able to open at low gate-source voltage. For example, P60N03LDG, IRLML6401 or similar (see).

The above circuit is good for everyone, but there is one unpleasant moment - the smooth closing of the field-effect transistor. This occurs due to the flatness of the initial section of the current-voltage characteristic of the diodes.

This drawback can be eliminated with the help of modern element base, namely with the help of micro-power voltage detectors (power monitors with extremely low power consumption). The next circuit for protecting lithium from deep discharge is presented below:

MCP100 microcircuits are available in both DIP packages and planar versions. For our needs, a 3-volt option is suitable - MCP100T-300i/TT. Typical current consumption in blocking mode is 45 µA. The cost for small wholesale is about 16 rubles/piece.

It’s even better to use a BD4730 monitor instead of the MCP100, because it has a direct output and, therefore, it will be necessary to exclude transistor Q1 from the circuit (connect the output of the microcircuit directly to the gate of Q2 and resistor R2, while increasing R2 to 47 kOhm).

The circuit uses a micro-ohm p-channel MOSFET IRF7210, which easily switches currents of 10-12 A. The field switch is fully open already at a gate voltage of about 1.5 V, and in the open state it has negligible resistance (less than 0.01 Ohm)! In short, a very cool transistor. And, most importantly, not too expensive.

In my opinion, the last scheme is the closest to the ideal. If I had unlimited access to radio components, I would choose this one.

A small change in the circuit allows you to use an N-channel transistor (then it is connected to the negative load circuit):

BD47xx power supply monitors (supervisors, detectors) are a whole line of microcircuits with response voltages from 1.9 to 4.6 V in steps of 100 mV, so you can always choose them to suit your purposes.

A small retreat

Any of the above circuits can be connected to a battery of several batteries (after some adjustment, of course). However, if the banks have different capacities, then the weakest of the batteries will constantly go into a deep discharge long before the circuit operates. Therefore, in such cases, it is always recommended to use batteries not only of the same capacity, but preferably from the same batch.

And although such protection has been working flawlessly in my metal detector for two years now, it would still be much more correct to monitor the voltage on each battery personally.

Always use your personal Li-ion battery discharge controller for each jar. Then any of your batteries will serve you happily ever after.

How to choose a suitable field-effect transistor

In all of the above schemes for protecting lithium-ion batteries from deep discharge, MOSFETs operating in switching mode are used. The same transistors are usually used in overcharge protection circuits, short-circuit protection circuits, and in other cases where load control is required.

Of course, in order for the circuit to work as it should, the field-effect transistor must meet certain requirements. First, we will decide on these requirements, and then we will take a couple of transistors and use their datasheets (technical characteristics) to determine whether they are suitable for us or not.

Attention! We will not consider the dynamic characteristics of FETs, such as switching speed, gate capacitance and maximum pulsed drain current. These parameters become critically important when the transistor operates at high frequencies (inverters, generators, PWM modulators, etc.), however, discussion of this topic is beyond the scope of this article.

So, we must immediately decide on the circuit that we want to assemble. Hence the first requirement for a field-effect transistor - it must be the right type(either N- or P-channel). This is the first.

Let's assume that the maximum current (load current or charge current - it doesn't matter) will not exceed 3A. This leads to the second requirement - a field worker must withstand such current for a long time.

Third. Let's say our circuit will protect the 18650 battery from deep discharge (one bank). Therefore, we can immediately decide on the operating voltages: from 3.0 to 4.3 Volts. Means, maximum permissible drain-source voltage U ds should be more than 4.3 Volts.

However, the last statement is true only if only one lithium battery bank is used (or several connected in parallel). If, to power your load, a battery of several batteries connected in series is used, then the maximum drain-source voltage of the transistor must exceed the total voltage of the entire battery.

Here is a picture explaining this point:

As can be seen from the diagram, for a battery of 3 18650 batteries connected in series, in the protection circuits of each bank it is necessary to use field devices with a drain-to-source voltage U ds > 12.6V (in practice, you need to take it with some margin, for example, 10%).

At the same time, this means that the field-effect transistor must be able to open completely (or at least strongly enough) already at a gate-source voltage U gs of less than 3 Volts. In fact, it is better to focus on a lower voltage, for example, 2.5 Volts, so that there is a margin.

For a rough (initial) estimate, you can look in the datasheet at the “Cut-off voltage” indicator ( Gate Threshold Voltage) is the voltage at which the transistor is on the threshold of opening. This voltage is typically measured when the drain current reaches 250 µA.

It is clear that the transistor cannot be operated in this mode, because its output impedance is still too high, and it will simply burn out due to excess power. That's why The transistor cut-off voltage must be less than the operating voltage of the protection circuit. And the smaller it is, the better.

In practice, to protect one can of a lithium-ion battery, you should select a field-effect transistor with a cutoff voltage of no more than 1.5 - 2 Volts.

Thus, the main requirements for field-effect transistors are as follows:

transistor type (p- or n-channel);
maximum permissible drain current;
the maximum permissible drain-source voltage U ds (remember how our batteries will be connected - in series or in parallel);
low output resistance at a certain gate-source voltage U gs (to protect one Li-ion can, you should focus on 2.5 Volts);
maximum permissible power dissipation.

Now let's look at specific examples. For example, we have at our disposal the transistors IRF4905, IRL2505 and IRLMS2002. Let's take a closer look at them.

Example 1 - IRF4905

We open the datasheet and see that this is a transistor with a p-type channel (p-channel). If we are satisfied with this, we look further.

The maximum drain current is 74A. In excess, of course, but it fits.

Drain-source voltage - 55V. According to the conditions of the problem, we have only one bank of lithium, so the voltage is even greater than required.

Next, we are interested in the question of what the drain-source resistance will be when the opening voltage at the gate is 2.5V. We look at the datasheet and don’t immediately see this information. But we see that the cutoff voltage U gs(th) lies in the range of 2...4 Volts. We are categorically not happy with this.

The last requirement is not met, so discard the transistor.

Example 2 - IRL2505

Here is his datasheet. We look and immediately see that this is a very powerful N-channel field device. Drain current - 104A, drain-source voltage - 55V. So far everything is fine.

Check the voltage V gs(th) - maximum 2.0 V. Excellent!

But let's see what resistance the transistor will have at a gate-source voltage = 2.5 volts. Let's look at the chart:

It turns out that with a gate voltage of 2.5V and a current through the transistor of 3A, a voltage of 3V will drop across it. In accordance with Ohm's law, its resistance at this moment will be 3V/3A=1Ohm.

Thus, when the voltage on the battery bank is about 3 Volts, it simply cannot supply 3A to the load, since for this the total load resistance, together with the drain-source resistance of the transistor, must be 1 Ohm. And we only have one transistor that already has a resistance of 1 ohm.

In addition, with such an internal resistance and a given current, the transistor will release power (3 A) 2 * 3 Ohm = 9 W. Therefore, you will need to install a radiator (a TO-220 case without a radiator can dissipate somewhere around 0.5...1 W).

An additional alarm bell should be the fact that the minimum gate voltage for which the manufacturer specified the output resistance of the transistor is 4V.

This seems to hint that the operation of the field worker at a voltage U gs less than 4 V was not envisaged.

Considering all of the above, discard the transistor.

Example 3 - IRLMS2002

So, let's take our third candidate out of the box. And immediately look at its performance characteristics.

N-type channel, let's say everything is in order.

Maximum drain current - 6.5 A. Suitable.

The maximum permissible drain-source voltage V dss = 20V. Great.

Cut-off voltage - max. 1.2 Volts. Still alright.

To find out the output resistance of this transistor, we don’t even have to look at the graphs (as we did in the previous case) - the required resistance is immediately given in the table just for our gate voltage.