Fast battery charger

Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells

 Comments on the article

The device described in the article is designed for accelerated charging of Ni-Cd and Ni-MH batteries with an exponentially decreasing current. Its advantages include the ability to select a charging time ranging from 45 minutes to 3 hours, ease of manufacture and setup, no heating of the batteries at the end of charging, the ability to visually monitor the charging process, automatic recovery of the process when the power supply is turned off and then turned back on, and ease of use. The device can be used as a stand for measuring the charging and discharging characteristics of batteries.

When charging with a large constant current (0.5 E or more, where E is the battery capacity), the battery begins to heat up after 75...80% charge, and Ni-MH batteries heat up more than Ni-Cd batteries [1]. After the battery is fully charged, the temperature increases rapidly [1], and if this process is not stopped in time, it ends in the battery igniting or exploding. The recommended charging termination temperature is +45 °C [2]. However, this criterion is only suitable as an emergency criterion: the combination of overcharging and overheating reduces the battery capacity and, therefore, shortens its service life.

Reaching a certain voltage on the battery is also not a satisfactory criterion for the end of the process. The fact is that its value corresponding to a full charge is not known in advance, since it depends on the temperature and the "age" of the battery. An error of a few millivolts leads to the fact that the battery charging never ends or ends too soon [3].

When charging with a constant current, it is easy to control the charge - it is directly proportional to the duration of the process. In particular, its value can be set equal to the nominal capacity of the battery. But over time, its capacity decreases and at the end of its service life it is approximately 80% of the nominal value. Therefore, limiting the charge to the nominal capacity does not guarantee the absence of overcharging and overheating of the batteries and, therefore, cannot be the only criterion for the end of charging.

The most difficult criterion for the end of the process is the moment when the battery voltage reaches a maximum and then begins to decrease. The maximum voltage on the battery corresponds to a full charge, but in [2] it is shown that it is a consequence of heating the battery during the charge recovery process. The maximum value is very small, especially for Ni-MH batteries (about 10 mV), so ADCs or voltage-to-frequency converters are used to detect it [2]. When charging a battery, the maximum voltage of its different elements is reached at different times, so it is advisable to control each of them separately. In addition, there are batteries with an anomalous charging characteristic in which this maximum is absent. In other words, monitoring only the voltage is not enough; it is also necessary to control both the temperature and the amount of charge passed through the battery.

Thus, when charging a battery with a large constant current, it is necessary to control each of its elements according to several criteria, which complicates the charger. Only charging with a low current (not more than 0,2E) does not cause emergency overheating of the batteries even with a large recharge. In this case, the state of each element does not need to be monitored, the charger turns out to be very simple, but its disadvantage is also obvious - a long charging time.

There are chargers in which the initially large charging current decreases over time [4-6]. In this case, it is also not necessary to monitor the status of each battery element. But in these devices there is no control of the amount of charge, and the achievement of a certain voltage is used as a criterion for full charge, which, as mentioned above, is not satisfactory.

In [7] a charger is described in which the battery is charged as a capacitor from a constant voltage source through a resistor. In this case, the charging current should theoretically decrease exponentially over time with a time constant equal to the product of the equivalent battery capacity and the resistance of this resistor. In practice, the dependence of the charging current on time differs from exponential, since the equivalent capacitance and output resistance of the source change during the charging process. But even if we neglect this difference, the most important parameter - the charging time constant - is unknown, as a result of which it is impossible to control the charge passed through the battery. Therefore, charging ends again when a certain voltage is reached...

In the proposed device, the charging current in the form of an exponentially decreasing pulse is chosen because it is easy to implement using the simplest RC circuit. It ends naturally, eliminating the need for a timer to turn off the batteries after a predetermined time, and the charge is limited even if the batteries are in the charger for a long time. It is essential that the charging current is generated by a current generator, so its value and form do not depend on the voltage on the batteries, or on the nonlinearity of their charging characteristics.

During charging, the current through the batteries I decreases exponentially:

I = l0exp(-t/T0), (1)

where t is time; l0 - initial charging current; T0 is the charging time constant. In this case, each battery receives a charge q, which is estimated by the expression

q = I0T0[1 - exp(-t/T0)] = (I0 - I)T0. (2)

Graphs of dependences of I and q on time t are presented in fig. one.

It can be seen that during the time 0Т0,95 the charge reaches a value of 0I0Т0 and then approaches the value I0Т0. It is recommended to select the values ​​of I0 and TXNUMX according to the formulas

I0 \u0d nE, T1 \u1.2,3,4d 3 h / n, where n \uXNUMXd XNUMX. (XNUMX)

The most convenient value is n = 1. The initial charging current in this case is equal to the electrical capacity E, the charging time is 3 hours (You can practically leave the batteries in the charger overnight, and by morning they will be fully charged). If this charging time is too long, the value of n is increased. At n = 2 it will be 1,5 hours with an initial charging current of 2E. This mode is suitable for Ni-Cd and Ni-MH batteries. Increasing n to 3 reduces the charging time to 1 hour, but the initial charging current increases to 4E. Finally, when n = 45, the charging time is reduced to 4 min, and the initial charging current is increased to XNUMXE.

Values ​​of n equal to 3 and 4 are acceptable for Ni-Cd batteries, since their internal resistance is low (less than 0,1 Ohm). As for Ni-MH batteries, their internal resistance is several times higher, so a high current can heat them up at the beginning of charging, which is unacceptable. Values ​​of n greater than 4 are not recommended. You can choose I0 5% greater than that determined by formula (3). Then the exact charging time will be 3 h/n, and a further 5% recharge is insignificant.

The principle of operation of the device is illustrated in Fig. 2.

A capacitor with a capacitance C1, pre-charged to a voltage U0l, is discharged through a current amplifier A1 with an input resistance Rin and a current gain Ki. The current in the input circuit of the amplifier Iin |P is determined by the expression

lin = U0exp(-t/RinC1)/Rin. (four)

The current in the output circuit of the amplifier I = Kilin charges the battery GB1:

I = KlU0exp(-t/RinC1)/Rin = SU0exp(-t/RinC1), (5)

where S = Ki/Rin is the slope of the amplifier gain, if it is considered as a voltage-to-current converter.

Comparing (2) and (5), we have

Т0 = RinC1, I0 = KU0/Rin = SU0.(6)

It is convenient to choose U0 = 1 V, C1 = 1000 μF, then from (3) it follows that Rin = 3,6 MOhm/n, S = nE, Ki = SRin = 3600000E. (7)

For example, with E = 1 Ah and n = 1 the following parameters should be: Rin = 3,6 MOhm, S = 1 A/V, K = 3600000 = 131 dB.

The schematic diagram of the device is shown in fig. 3.

The current amplifier is assembled using op-amp DA2.1 and transistors VT2 and VT3. The op-amp supply voltage is stabilized by the DA1 microcircuit. The node on transistor VT1 controls the magnitude of this voltage. When it is normal, this transistor is open, current flows through the winding of relay K1, the contacts of relay K1.1 are closed, the LED HL1 lights up, signaling the normal operation of the device. Switch SA1 selects the charging mode: direct current (when its contacts are closed) or exponentially decreasing (when they are open). Resistors R2 and R3 form a voltage divider. The voltage across the motor of variable resistor R3 determines the charging current. In the “Constant” mode, this voltage is supplied to the non-inverting input of the op-amp through resistor R1 and the closed contacts of relay K1.1. Its output current is amplified by transistors VT2, VT3 and is set so that the voltages on resistors R11 and R5 become the same. The current gain K, = R5/R11 and with the ratings indicated on the diagram is approximately equal to 107, and the slope of the voltage conversion b tok S=1/R11=ZA/V.

In the "Decreasing" mode (the contacts of the SA1 switch are open), the capacitor C2 with a capacity of 1000 μF is discharged through the resistor R5 with a time constant selected by formula (3). The exponentially decreasing current through this capacitor is amplified by the op-amp DA2.1 and transistors VT2, VT3 and charges the batteries connected to the X1 connector ("Output"). Diode VD2 prevents them from discharging when the supply voltage is turned off. Ammeter PA1 is used to control the current value of the charging current. Capacitor C5 prevents self-excitation of the device. Resistors R4, R8-R10 - current limiting. They protect the op-amp and transistor VT2 in emergency situations, for example, when the resistor R11 breaks or the transistor VT3 breaks down, preventing the failure of other elements.

When the power is turned off in charging mode with a decreasing current, transistor VT1 closes and the relay opens contacts K1.1, preventing further discharge of capacitor C2. The HL1 LED goes out, signaling a power failure. When power is restored, transistor VT1 opens, relay K1 closes contacts K1.1 and battery charging automatically continues from the current value at which it was interrupted. The HL1 LED lights up again, signaling that charging has resumed. By pressing the SB1 button, you can briefly stop charging when removing the charging characteristics. In this case, capacitor C4 prevents network noise from penetrating the op-amp input.

The device is assembled on a universal printed circuit board and housed in a housing measuring 310x130x180 mm. AA size batteries are placed in a trough on the top cover of the case. The contact sockets are made in the form of pieces of tape made of tinned sheet metal, which are pressed against the batteries by a spring from a standard compartment for an AA-size element. No current flows through the spring. It should be noted that commercially available plastic compartments are only suitable for currents not exceeding 500 mA. The fact is that the current flowing through the contact springs heats them up, and the batteries also heat up. Already at a current of 1 A, the springs heat up so much that they melt the wall of the plastic housing of the compartment, making its further use impossible.

Transistor VT3 is installed on a finned heat sink with a surface area of ​​600 cm2, diode VD2 is installed on a plate heat sink with an area of ​​50 cm2. Resistor R11 is made up of three MLT-1 resistors connected in parallel with a resistance of 1 Ohm. All high-current connections are made with pieces of copper wire with a cross-section of 3 mm2, which are soldered directly to the terminals of the corresponding parts.

The K1446UD4A (DA2) op amp can be replaced with a K1446UD1A chip or another of these series, but from the two op amps, you need to choose the one with the lower bias voltage. The second op-amp can be used as part of a temperature-sensitive bridge [8] for emergency shutdown of batteries in case of overheating during charging with direct current (no overheating of batteries was observed when charging with decreasing current). In the case of using other types of op-amp, it should be borne in mind that in this design its power supply is unipolar, so it must be operational at zero voltage at both inputs.

The KR1157EN601A (DA1) microcircuit is replaceable by the stabilizer of this series with index B, as well as by the K1157EN602 series microcircuit, however, the latter has a different "pinout" [9].

Transistor VT1 - any of the KP501, VT2 series must have a static base current transfer coefficient h21E of at least 100. Transistor KT853B (VT3) is different in that its h21E exceeds 1000. Transistors of other types can be used as VT2, VT3, but the overall gain the current must exceed 100.

Capacitor C2, which sets the charging time constant T0, must have a stable capacitance, not necessarily equal to the nominal one indicated in the diagram, since the required value of T0 is set during setup by selecting resistor R5. The author used a Jamicon oxide capacitor with a large voltage reserve (25 times).

Relay K1 is a reed switch EDR2H1A0500 from ECE with a voltage and operating current of 5 V and 10 mA, respectively. A possible replacement is a domestically produced relay KUTS-1 (passport RA4.362.900).

The PA1 ammeter must be designed for the maximum charging current (in the author’s version, the M4200 device for the current is used). Fuse FU1 is a self-resetting MF-R300 from BOURNS [10].

Setting up the device comes down to setting the required value of the charging time constant T0, selected according to formula (3). The resistance of resistor R5 is chosen equal to Rin according to formula (7), assuming that the capacitance of capacitor C2 is exactly equal to 1000 μF. Instead of batteries, a digital ammeter is included. Before turning on the power, both when charging batteries and when setting up the device, the variable resistor R3 slider is moved to the lower (according to the diagram) position and the contacts of switch SA1 are closed (this is necessary to discharge capacitor C2). Then turn on the power and, by moving the slider of resistor R3, set the initial current l0 to about 1 A. Next, SA1 is moved to the “Decreasing” position. After time T1 (approximately equal to T0), current i1 is measured. The adjusted resistance value of resistor R5* is calculated using the formula R5* = R5[ln(l0/I1)]. Finally, install resistor R5 with a resistance equal to this adjusted value.

Before charging, batteries must be discharged to a voltage of 1...1,1 V in order to prevent them from overcharging and the manifestation of the memory effect [2]. If the batteries become hot during discharging, then before charging they should be cooled to ambient temperature (0...+30 °C [2]). Before connecting the batteries to the charger, you need to make sure that it is de-energized, the resistor R3 slider is in the lower (according to the diagram) position, and SA1 is in the “Constant” position. Next, observing the polarity, install the batteries, turn on the power and, using variable resistor R3, set the initial current l0 according to formula (3). After this, SA1 is switched to the “Decreasing” position, and after the ZTO time, the batteries are ready for use.

To power the device, you need a voltage source from 8 to 24 V, which can be unstabilized. You can charge from one to ten cells at the same time. The minimum supply voltage, taking into account ripple, should be 2 V per cell plus 4 V (but within the specified limits).

The device can be used as a stand for taking not only charging, but also discharging characteristics of batteries. In the latter case, the battery under test must be connected to the device in reverse polarity. The voltage on its electrodes must be constantly monitored with a voltmeter. It should not be allowed to change its polarity, so as not to cause emergency destruction of the battery. For this reason, it is not recommended to discharge a battery of several series-connected cells in this way, as it is possible to miss the moment of failure of the cell with the smallest capacity.

Literature

1. New types of accumulators ("Abroad"). - Radio, 1998, No. 1, p. 48, 49.
3. A little about charging nickel-cadmium batteries (“Abroad”). - Radio, 1996, No. 7, p. 48, 49.
4. Nechaev I. Accelerated battery charging. - Radio, 1995, No. 9, p. 52,53.
5. Alekseev S. Chargers for Ni-Cd batteries and accumulators. - Radio, 1997, No. 1, p. 44-46.
6. Dolgov O. Foreign charger and its analogue using domestic elements. - Radio, 1995, No. 8, p. 42,43.
7. Dorofeev M. Charger option. - Radio, 1993, No. 2, p. 12,13.
8. Tkvchev F. Calculation of a thermosensitive bridge. - Radio, 1995, No. 8, p. 46.
9. Biryukov S. Microcircuit voltage stabilizers for wide application. - Radio, 1999, No. 20 p. 69-71.
10. Self-restoring fuses MULT1FUSE from BOURNS. - Radio, 2000, No. 11, p. 49-51.

Author: M. Evsikov, Moscow

See other articles Section Chargers, batteries, galvanic cells.

Read and write useful comments on this article.

<< Back

Latest news of science and technology, new electronics:

Machine for thinning flowers in gardens 02.05.2024

In modern agriculture, technological progress is developing aimed at increasing the efficiency of plant care processes. The innovative Florix flower thinning machine was presented in Italy, designed to optimize the harvesting stage. This tool is equipped with mobile arms, allowing it to be easily adapted to the needs of the garden. The operator can adjust the speed of the thin wires by controlling them from the tractor cab using a joystick. This approach significantly increases the efficiency of the flower thinning process, providing the possibility of individual adjustment to the specific conditions of the garden, as well as the variety and type of fruit grown in it. After testing the Florix machine for two years on various types of fruit, the results were very encouraging. Farmers such as Filiberto Montanari, who has used a Florix machine for several years, have reported a significant reduction in the time and labor required to thin flowers. ... >>

Advanced Infrared Microscope 02.05.2024

Microscopes play an important role in scientific research, allowing scientists to delve into structures and processes invisible to the eye. However, various microscopy methods have their limitations, and among them was the limitation of resolution when using the infrared range. But the latest achievements of Japanese researchers from the University of Tokyo open up new prospects for studying the microworld. Scientists from the University of Tokyo have unveiled a new microscope that will revolutionize the capabilities of infrared microscopy. This advanced instrument allows you to see the internal structures of living bacteria with amazing clarity on the nanometer scale. Typically, mid-infrared microscopes are limited by low resolution, but the latest development from Japanese researchers overcomes these limitations. According to scientists, the developed microscope allows creating images with a resolution of up to 120 nanometers, which is 30 times higher than the resolution of traditional microscopes. ... >>

Air trap for insects 01.05.2024

Agriculture is one of the key sectors of the economy, and pest control is an integral part of this process. A team of scientists from the Indian Council of Agricultural Research-Central Potato Research Institute (ICAR-CPRI), Shimla, has come up with an innovative solution to this problem - a wind-powered insect air trap. This device addresses the shortcomings of traditional pest control methods by providing real-time insect population data. The trap is powered entirely by wind energy, making it an environmentally friendly solution that requires no power. Its unique design allows monitoring of both harmful and beneficial insects, providing a complete overview of the population in any agricultural area. “By assessing target pests at the right time, we can take necessary measures to control both pests and diseases,” says Kapil ... >>

Random news from the Archive Genes against gravity 31.03.2015 The device of living organisms depends on where they live, and a huge number of very different factors determine the structure of the body, and physiology, and behavior - even in bacteria, even in people. Among these factors, the most universal ones can be distinguished, which are easy to enumerate: for example, the temperature of the environment, or humidity, or the concentration of oxygen in air or water. But there is something that is rarely mentioned. We are talking about the force of gravity, which acts on everyone and always. Could she play a role in shaping the appearance of living beings? Biologists have been asking this question for quite a long time: for example, 100 years ago, D'Arcy Thompson suggested that in animals the shape of the body is largely determined by the force of gravity, and if it were twice as large on Earth, primates would not have developed any upright posture, and in general all tetrapods would be short-legged and move like lizards. Apparently, evolution had to somehow respond to the gravitational factor, but what kind of molecular-cellular mechanisms helped us adapt to gravity, we can only now find out. Makoto Furutani-Seiki (Makoto Furutani-Seiki), together with his colleagues from the University of Bath and with the participation of researchers from Japan, Austria and the United States, were able to find a gene that is responsible for the formation of a "three-dimensional" body in animals. When it was turned off in fish, the development of tissues was disturbed, they were located incorrectly relative to each other, and the whole body was strongly flattened in the direction of the gravitational force. If it did not work in cultured human cells, they ceased to combine into volumetric clusters. In an article in Nature, the authors write that this gene, called YAP, serves as a regulator of the molecular machine that controls the mechanical forces in and between cells - the correct distribution of such forces is necessary to create most organs and body parts. Roughly speaking, thanks to YAP, we can resist gravity and generally have a more or less voluminous, rather than flat, body. How exactly the anti-gravity gene works, how and when it turns on, and what other genes are under its control, we have yet to find out. Further experimentation here will not only reveal why we have come to look the way we do now, but will also help develop reliable methods for creating artificial organs. By controlling the genetic system responsible for the "bulkness" of an organ, we can, for example, grow a liver or kidney of the right size in the laboratory, which will not differ from the real ones - in order to transplant them later to replace the spoiled ones.

Other interesting news:

▪ Gardening is one of the best antidepressants

▪ Cats understand people

▪ Smartphone Gigabyte GSmart GX2

▪ ODROID-GO Advance Retro Game Console

▪ New miniature PWM controller

News feed of science and technology, new electronics

Interesting materials of the Free Technical Library:

▪ section of the website Audiotechnics. Article selection

▪ article There is neither Greek nor Jew. Popular expression

▪ article How much air is on Earth? Detailed answer

▪ Abacus article. Legends, cultivation, methods of application

▪ article Triangle connection. Encyclopedia of radio electronics and electrical engineering

▪ article Application of PIN diodes as damping and switching elements. Encyclopedia of radio electronics and electrical engineering

Leave your comment on this article:

All languages ​​of this page

Arabic	Hebrew	Polish
Bengali	Hindi	Portuguese
Chinese	Italian	Spanish
English	Japanese	Turkish
French	Korean	Ukrainian
German	Malay	Vietnamese

Home page | Library | Articles | Website map | Site Reviews

www.diagram.com.ua
2000-2024