Nowadays, lithium batteries are gaining more and more popularity. Especially finger type 18650 , at 3.7 V 3000 mA. I have no doubt that in another 3-5 years, they will completely replace nickel-cadmium. The truth remains an open question about their charging. If everything is clear with old batteries - collect them in a battery and through a resistor to any suitable power supply, then this trick does not work here. But how, then, to charge several pieces at once without using expensive branded balancing chargers?

Theory

For serial connection of batteries, usually positive electrical circuit connect the positive terminal of the first battery series connection. The positive terminal of the second battery is connected to its negative terminal, etc. The negative terminal of the last battery is connected to the minus of the unit. The resulting at serial connection accumulator battery has the same capacity as a single battery, and the voltage of such a battery is equal to the sum of the voltages of the batteries included in it. This means that if the batteries have the same voltage, then the battery voltage is equal to the voltage of one battery multiplied by the number of batteries in the battery.

The energy stored in the battery is equal to the sum of the energies of individual batteries (the product of the energies of individual batteries, if the batteries are the same), regardless of whether the batteries are connected in parallel or in series.

You cannot simply connect lithium-ion batteries to the power supply unit - you need to equalize the charging currents on each element (bank). Balancing is carried out when charging the battery, when there is a lot of energy and it can not be saved much and therefore, without any special losses, you can use the passive dissipation of "excess" electricity.

Nickel-cadmium batteries do not require additional systems, since each link, when its maximum charge voltage is reached, stops accepting energy. Signs of a full charge of Ni-Cd are an increase in voltage to a certain value, and then its drop by several tens of millivolts, and an increase in temperature - so that excess energy is immediately converted into heat.

The opposite is true for lithium batteries. Discharging to low voltages causes degradation of chemistry and irreversible damage to the element, with an increase in internal resistance. In general, they are not protected from overcharging, and a lot of excess energy can be wasted, thereby drastically reducing their service life.

If we connect several lithium cells in a row and feed them through the clamps at both ends of the block, then we cannot control the charge. individual elements... It is enough that one of them will have a slightly higher resistance or slightly lower capacitance, and this link will reach the charge voltage of 4.2 V much faster, while the rest will still have 4.1 V. And when the voltage of the entire package reaches charge voltage, it may be that these weak links are charged to 4.3 Volts or even more. With each such cycle, the parameters will deteriorate. In addition, Li-Ion is unstable and, when overloaded, can reach high temperatures and, therefore, explode.

Most often at the source output charging voltage a device called a "balancer" is placed. The simplest type of balancer is a voltage limiter. It is a comparator that compares the voltage on the Li-Ion bank with a threshold value of 4.20 V. When this value is reached, a powerful switch-transistor connected in parallel with the element opens slightly, passing through most of the charge current and converting energy into heat. In this case, the bank itself receives an extremely small part of the current, which, in practice, stops its charge, allowing the neighboring ones to recharge. The equalization of the voltages on the battery cells with such a balancer occurs only at the end of the charge when the cells reach the threshold value.

Simplified balancer diagram for battery

Here is a simplified schematic of the TL431 based current balancer. Resistors R1 and R2 set the voltage to 4.20 Volts, or others can be selected, depending on the type of battery. The reference voltage for the regulator is removed from the transistor, and already at the 4.20 V boundary, the system will begin to open the transistor to prevent the specified voltage from being exceeded. The smallest increase in voltage will cause a very rapid rise in the transistor current. During the tests, already at 4.22 V (excess by 20 mV), the current was more than 1 A.

In principle, any PNP transistor operating in the range of voltages and currents that interests us is suitable here. If the batteries need to be charged with 500mA. The calculation of its power is simple: 4.20 V x 0.5 A = 2.1 V, and the transistor must lose so much that it will probably require a little cooling. For a charging current of 1 A or more, the power dissipation increases accordingly, and it becomes more and more difficult to get rid of the heat. During the test, several different transistors were tested, in particular the BD244C, 2N6491 and A1535A - they all behave the same.

The voltage divider R1 and R2 should be chosen so as to obtain the desired limiting voltage. For convenience, here are some values ​​after applying which, we will get the following results:

• R1 + R2 = Vo
• 22K + 33K = 4.166V
• 15K + 22K = 4.204V
• 47K + 68K = 4.227V
• 27K + 39K = 4.230V
• 39K + 56K = 4.241V
• 33K + 47K = 4.255V

This is an analogue of a powerful zener diode loaded on a low-resistance load, the role of which is played here by diodes D2 ... D5. D1 microcircuit measures the voltage at the plus and minus of the battery and if it rises above the threshold, opens powerful transistor, passing through all the current from the charger. How it connects all this together and to the power supply - see below.

The blocks are really small, and you can safely install them directly on the element. It should only be borne in mind that the potential of the negative pole of the battery arises on the transistor case, and you must be careful when installing common heat sink systems - you must use the insulation of the transistor cases from each other.

Testing

Immediately 6 pieces of balancing blocks were needed to simultaneously charge 6 18650 batteries. The elements are visible in the photo below.

All the elements were charged exactly to 4.20 volts (the voltage was set by potentiometers), and the transistors became hot, although there was no additional cooling - charging with a current of 500 mA. Thus, we can safely recommend this method for simultaneous charging of several lithium batteries from a common voltage source.

Discuss the article SIMULTANEOUS CHARGING OF SEVERAL BATTERIES

Sent:

No, it will be not fishing bait, or even circus acrobats balancing under the dome. It will be about how to achieve a balance of battery parameters connected in series.

As you know, a battery cell is a fairly low-voltage device, so they are usually connected in packs in series. Ideally, if the parameters of all batteries are the same, we have a source with a voltage n-times greater than a single cell, and we can charge-discharge it as a single higher-voltage battery.

Alas, this will be the case only ideally. Each battery in this pack, like everything in this world, is unique, and it is impossible to find two completely identical ones, and their characteristics - capacity, leakage, state of charge, will change with time and temperature.

Of course, battery manufacturers try to choose the closest in terms of parameters, but there are always differences. And over time, such imbalances in characteristics can also increase.

These differences in cell characteristics cause batteries to perform differently and, as a result, total capacity a composite battery will be lower than its constituent cells, this time, and secondly, the resource of such a battery will also be lower, because it is determined by the "weakest" battery, which will wear out faster than others.
What to do?

There are two main criteria for assessing the degree of balancing of cells:
1. Voltage equalization on cells,
2. Equalization of the charge in the cells.

You can also achieve your goals in achieving these balancing methods in two ways:
1. Passive and
2. Active.

Let us explain what has been said.
With the balancing criteria, everything is clear, either we simply achieve equality of the cell voltages, or in some way calculate the battery charge and ensure that these charges are equal (while the voltages may differ).

There is nothing complicated with the implementation methods either. In the passive method, we simply convert energy into heat in the most charged battery cells, until the voltages or charges in them are equal.
In the active method, in any way, we transfer the charge from one cell to another, if possible with minimal losses. Modern circuitry easily realizes such abilities.

It is clear that dissipating is easier than pumping, and comparing voltages is easier than comparing charges.

Also, these methods can be used for both charging and discharging. Most often, of course, balancing is carried out when charging the battery, when there is a lot of energy and it can not be saved much and therefore, without any special losses, you can use passive dissipation of "excess" electricity.
When discharging, only active charge transfer is always used, but such systems are very rare, due to the greater complexity of the circuit.

Let's take a look at the practical implementation of the above.
When charging, in the simplest case, a device called a "balancer" is placed at the output of the charger.
Further, in order not to compose myself, I will simply insert a piece of text from an article from the site http://www.os-propo.info/content/view/76/60/. It is about charging lithium batteries.

"The simplest type of balancer is a voltage limiter. It is a comparator that compares the voltage on the LiPo bank with a threshold value of 4.20 V. When this value is reached, a powerful switch transistor connected in parallel with the LiPo bank opens slightly, passing through most of the charge current (1A or more) and converting energy into heat. In this case, the bank itself receives an extremely small part of the current, which, in practice, stops its charge, allowing the neighboring ones to recharge. In fact, the voltage equalization on the battery cells with such a balancer occurs only at the end of the charge when the cells reach the threshold value.

In such a scheme, the task of charging and equalizing a pair of different packs is really feasible. But such balancers in practice are only home-made. All proprietary microprocessor balancers use a different principle of operation.

Instead of dissipating the full charge currents at the end, the microprocessor balancer constantly monitors the voltages on the banks and gradually equalizes them throughout the entire charging process. To the bank charged more than others, the balancer connects in parallel some resistance (about 50-80 Ohm in most balancers), which passes a part of the charging current through itself and only slightly slows down the charge of this bank, without stopping it completely. Unlike a transistor on a radiator, capable of taking on the main charge current, this resistance provides only a small balancing current - about 100mA, and therefore such a balancer does not require massive radiators. It is this balancing current that is indicated in technical characteristics balancers and is usually no more than 100-300mA.

Such a balancer does not significantly heat up, since the process continues throughout the entire charge, and heat at low currents has time to dissipate without radiators. Obviously, if the charge current is significantly higher than the balancing current, then with a wide range of voltages across the banks, the balancer will not have time to equalize them until the moment the most charged bank reaches the threshold voltage."
End of quote.

An example of a working diagram of the simplest balancer is the following (taken from the site http://www.zajic.cz/).

Fig. 1. Simple circuit balancer.

In fact, this is a powerful zener diode, by the way, very accurate, loaded on a low-resistance load, the role of which is played here by diodes D2 ... D5. The D1 microcircuit measures the voltage at the plus and minus of the battery and if it rises above the threshold, it opens the powerful transistor T1, passing all the current from the charger through itself.

Fig. 2. Simple balancer circuit.

The second circuit works similarly (Fig. 2.), but in it all the heat is released in the transistor T1, which heats up like a "kettle" - the radiator can be seen in the picture below.

Figure 3 shows that the balancer consists of 3 channels, each of which is made according to the scheme in Figure 2.

Of course, the industry has long mastered such circuits, which are produced in the form of a complete microcircuit. Many companies produce them. As an example, I will use the materials of the article on balancing methods published on the RadioLotsman website http://www.rlocman.ru/shem/schematics.html?di=59991, which I will partially change or remove so as not to inflate the article.
Quote:
" Passive balancing method.
The simplest solution is to equalize the battery voltage. For example, the BQ77PL900 chip provides protection for battery packs with 5-10 batteries in series. The microcircuit is a functionally complete unit and can be used to work with the battery compartment, as shown in Figure 4. Comparing the cell voltage with the threshold, the microcircuit, if necessary, turns on the balancing mode for each of the cells.

Fig. 4. BQ77PL900 microcircuit, and the second analogue, where the internal structure is better visible (taken from here http://qrx.narod.ru/bp/bat_v.htm).

In Fig. 5 shows the principle of its operation. If the voltage of any battery exceeds a predetermined threshold, field-effect transistors are turned on and a load resistor is connected in parallel to the battery cell, through which the current bypasses the cell and no longer charges it. The rest of the cells continue to charge.
When the voltage drops, the field device closes and charging can continue. Thus, at the end of charging, all cells will have the same voltage.

When using a balancing algorithm that uses only voltage deviation as a criterion, incomplete balancing is possible due to the difference in the internal resistance of the batteries (see Fig. 6.). The fact is that a part of the voltage drops on this resistance when a current flows through the battery, which introduces an additional error in the spread of voltages during charging.
The battery protection IC cannot determine what caused the imbalance - the different capacity of the batteries or the difference in their internal resistances. Therefore, with this type of passive balancing, there is no guarantee that all batteries will be 100% charged.

The BQ2084 uses an improved version of balancing, also based on voltage variation, but to minimize the effect of variation in internal resistances, the BQ2084 balances towards the end of the charging process when the charging current is low.

Rice. 5. Passive method based on voltage balancing.

Rice. 6. Passive voltage balancing method.

Microcircuits of the BQ20Zхх family use the proprietary Impedance Track technology to determine the charge level, based on determining the state of charge of the batteries (SZB) and the capacity of the battery.

In this technology, for each battery, the charge Qneed is calculated, which is required to fully charge it, and then the difference? Q between the Qneed of all batteries is found. Then the microcircuit turns on the power switches, which discharge all cells to the level of the least charged, until the charges are equal

Due to the fact that the difference between the internal resistances of the batteries does not affect this method, it can be used at any time, both during charging and when the battery is being discharged. However, as mentioned above, it is stupid to use this method when discharging. energy is always lacking.

The main advantage of this technology is more accurate battery balancing (see Fig. 7) compared to other passive methods.

Rice. 7. Passive balancing based on SZB and capacity.

Active balancing

In terms of energy efficiency, this method is superior to passive balancing, because to transfer energy from a more charged cell to a less charged one, instead of resistors, inductors and capacitors are used, in which there are practically no energy losses. This method is preferred in cases where maximum operating time is required on a single charge.

The BQ78PL114 IC, manufactured with proprietary PowerPump technology, is TI's latest active battery balancing component and uses an inductive converter to transfer power.

PowerPump uses n-channel p-channel field-effect transistors and a choke that is located between the pair of batteries. The diagram is shown in Fig. 8. The field and choke constitute a buck / boost converter.

For example, if the BQ78PL114 detects that the top cell is charged more than the bottom one, then a signal is generated at the PS3 pin with a turn-on transistor Q1 with a frequency of about 200 kHz and a duty cycle of about 30%.

When Q2 is off, a standard buck circuit is obtained. pulse stabilizer, while the internal diode Q2 closes the inductance current during the closed state of the key Q1.

When pumping from the lower cell to the upper one, when only the Q2 key is opened, we also get a typical circuit, but already a step-up pulse stabilizer.

Keys Q1 and Q2, of course, should never be opened at the same time.

Rice. 8. Balancing by PowerPump technology.

In this case, the energy losses are small and almost all the energy flows from the highly charged to the low-charged bank. The BQ78PL114 microcircuit implements three balancing algorithms:
- by voltage at the battery terminals. This method is similar to the passive balancing method described above, but there is almost no loss;
- by voltage of open circuit. This method compensates for the difference in internal resistances of the batteries;
- by the state of charge of the battery (based on the prediction of the state of the battery). The method is similar to that used in the BQ20Zxx family of microcircuits with passive balancing for SZB and battery capacity. In this case, the charge that must be transferred from one battery to another is precisely determined. Balancing occurs at the end of the charge. This method achieves the best result (see Fig. 9.)

Rice. 9. Active balancing according to the algorithm of equalizing the state of charge of the battery.

Due to the high balancing currents, PowerPump technology is much more efficient than conventional passive power dissipation balancing. In the case of balancing the laptop battery pack, the balancing currents are 25 ... 50 mA. By choosing the value of the components, it is possible to achieve a balancing efficiency 12-20 times better than with the passive method with internal keys. Typical unbalance values ​​(less than 5%) can be achieved in just one or two cycles.

In addition, PowerPump technology has other advantages: balancing can occur in any operating mode - charging, discharging, and even when the battery giving energy has a lower voltage than the battery receiving energy." (End of partial citation.)

Let's continue the description of active methods of transferring charge from one cell to another with the following scheme, which I found on the Internet at the HamRadio website http://qrx.narod.ru/bp/bat_v.htm.

As a charge transfer circuit, not an inductive, but a capacitive storage is used. For example, so-called switched capacitor voltage converters are widely known. One of the most popular is the ICL7660 microcircuit (MAX1044 or domestic analogue KR1168EP1).

Basically, the microcircuit is used to obtain a negative voltage equal to its supply voltage. However, if for some reason the negative voltage at its output turns out to be greater in magnitude than the positive supply voltage, then the microcircuit will start pumping the charge "in the opposite direction", taking it from the minus and giving it to the plus, i.e. she is constantly trying to equalize these two tensions.

This property is used to balance two battery cells. The diagram of such a balancer is shown in Fig. 10.

Fig. 10. Balancer circuit with capacitive charge transfer.

A high-frequency microcircuit connects the capacitor C1 to either the upper battery G1 or the lower G2. Accordingly, C1 will be charged from a more charged one and discharged into a more discharged one, each time transferring some portion of the charge.
Over time, the battery voltages will become the same.

The energy in the circuit is practically not dissipated, the efficiency of the circuit can reach up to 95 ... 98% depending on the voltage on the batteries and the output current, which depends on the switching frequency and C1 capacity.

In this case, the actual consumption of the microcircuit is only a few tens of microamperes, i.e. is below the self-discharge level of many batteries, and therefore the microcircuit does not even need to be disconnected from the battery and it will constantly slowly perform work to equalize the voltage on the cells.

In reality, the pumping current can reach 30 ... 40mA, but the efficiency decreases. Usually ten mA. Also, the supply voltage can be from 1.5 to 10V, which means that the microcircuit can balance both ordinary Ni-Mh fingers and lithium batteries.

Practical note: in Figure 10. shows a circuit that balances batteries with a voltage of less than 3V, so its sixth leg (LV) is connected to output 3. To balance lithium batteries with a higher voltage, pin 6 must be left free, not connected anywhere.

Also, by this method it is possible to balance not only two, but also large quantity batteries. Figure 11. shows how to do this.

Fig. 11. Cascading charge transfer ICs.

Well, and finally, one more circuit solution that implements the capacitive transfer of charge from one battery to another.
If in the ICL7660 it was a multiplexer that could connect the capacitor C1 to only two sources, then taking a multiplexer with a large number of switching channels, (3, 4, 8), you can equalize the voltages on three, four or eight banks with one microcircuit. Moreover, banks can be connected as you like, both in series and in parallel. The main thing is that the supply voltage of the microcircuit is higher than the maximum voltage on the banks.

The circuit of the so-called "reversible voltage converter", described in the magazine "Radio" 1989, no. 8, is shown in Fig.12.

Fig. 12. Reversible voltage converter as a balancer on the 561KP1 .. multiplexer.

Up to four elements can be connected to the leveling device. Capacitor C2 is alternately connected to various elements, ensuring the transfer of energy from these elements and equalization of the voltage across them

The number of cells in the battery can be reduced. In this case, instead of the excluded elements, it is sufficient to connect a capacitor with a capacity of 10..20 µF.

The balancing current of such a source is very small, up to 2 mA. But since it works constantly, without disconnecting from the batteries, it fulfills its task - equalizing the charges of the cells.

In conclusion, I would like to note that the modern element base allows balancing the cells of a composite battery with virtually no loss and is already simple enough to cease to be something "cool" and inaccessible.

And therefore, a radio amateur who designs battery-powered devices, I think, should think about switching to active methods of pumping energy between banks in a battery, even if at least "in the old fashioned way", focusing on the equality of voltages between battery cells, and not the charges in them.

All articles on the site are allowed to be copied, but with the obligatory indication of a link to us.

Sometimes there is a need to charge a Li-Ion battery, consisting of several cells connected in series. Unlike Ni-Cd batteries, Li-Ion batteries require an additional control system that will monitor the uniformity of their charge. Charging without such a system will sooner or later damage the battery cells, and the entire battery will be ineffective and even dangerous.

Balancing is a charging mode that monitors the voltage of each individual cell in the battery pack and does not allow the voltage on them to exceed a set level. If one of the cells is charged before the rest, the balancer takes on excess energy and converts it into heat, preventing the charge voltage of a particular cell from exceeding.

For Ni-Cd batteries, there is no need for such a system, since each battery cell stops accepting energy when its voltage is reached. A sign of Ni-Cd charge is an increase in voltage to a certain value, followed by a decrease in it by several tens of mV and an increase in temperature, since excess energy is converted into heat.

Ni-Cd must be completely discharged before charging, otherwise a memory effect will occur, which will lead to a noticeable decrease in capacity, and it can only be restored after a few full charge / discharge cycles.

With Li-Ion batteries, the opposite is true. Discharging to too low voltages causes degradation and irreversible damage with an increase in internal resistance and a decrease in capacitance. Also, full-cycle charging will wear out the battery faster than recharging. The Li-Ion battery does not show any charging symptoms like Ni-Cd does, so the charger cannot detect when it is fully charged.

Material: ABS + Metal + Acrylic Lenses. LED lights...

Li-Ion is usually charged according to the CC / CV method, that is, at the first stage of charging, D.C., for example, 0.5 C (half of the capacity: so for a battery with a capacity of 2000 mAh, the charge current will be 1000 mA). Further, upon reaching the final voltage provided by the manufacturer (for example, 4.2 V), the charge is continued with a stable voltage. And when the charge current drops to 10..30mA, the battery can be considered charged.

If we have a battery of accumulators (several batteries connected in series), then we charge, as a rule, only through the terminals at both ends of the whole package. At the same time, we have no way to control the charge level of individual links.

It is possible that it will be so that one of the elements will have a higher internal resistance or a slightly lower capacity (as a result of battery wear), and it will reach a charge voltage of 4.2 V faster than the others, while the rest will only have 4.1 B, and the entire battery will not show a full charge.

When the battery voltage reaches the charge voltage, the weak cell may be charged to 4.3V or more. With each such cycle, such an element will wear out more and more, deteriorating its parameters, until this leads to the failure of the entire battery. Moreover, the chemical processes in Li-Ion are unstable and when the charge voltage is exceeded, the battery temperature rises significantly, which can lead to spontaneous combustion.

Simple balancer for li-ion batteries

What then is to be done? In theory, the easiest way is to use a zener diode connected in parallel to each battery cell. When the breakdown voltage of the zener diode is reached, it will begin to conduct current, not allowing the voltage to rise. Unfortunately, a 4.2V Zener diode is not easy to find, and 4.3V is already too much.

The way out of this situation can be the use of a popular one. True, in this case, the load current should not exceed more than 100 mA, which is very small for charging. Therefore, the current must be amplified using a transistor. This circuit, connected in parallel to each cell, will protect it from overcharging.

This is a slightly modified typical TL431 wiring diagram and can be found in the datasheet under the name “hi-current shunt regulator”.

Surely, every radio amateur faced a problem, connecting lithium batteries in series, noticed that one sits down quickly and the other still quite holds a charge, but because of the other dead, the entire battery does not give out the required voltage. This is due to the fact that when the entire battery pack is charged, they are not charged evenly, and some of the batteries gain full capacity and some do not. This leads not only to a quick discharge, but also to the failure of individual elements, due to the constant failure to charge.
It is quite simple to fix the problem, for each battery cell you need a so-called balancer, a device that, after fully charging the battery, blocks its further recharge, and bypasses the charging current past the cell with a control transistor.
The balancer circuit is quite simple, assembled on a precision controlled zener diode TL431A, and a direct conduction transistor BD140.