Fast charger for Ni-Cd and Ni-MH batteries. Encyclopedia of radio electronics and electrical engineering

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

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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 the charging time in the range from 45 minutes to 3 hours, ease of manufacture and adjustment, no heating of the batteries at the end of charging, the ability to visually control the charging process, automatic recovery of the process when the power is turned off and then turned on, 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 starts to heat up after 75 ... 80% charge, and Ni-MH batteries heat up more than Ni-Cd [1]. After the battery is fully charged, the temperature rises rapidly [1], and if this process is not stopped in time, it ends with the battery igniting or exploding. The recommended charging termination temperature is +45 °С [2]. However, this criterion is only suitable as an emergency: the combination of overcharging with overheating reduces the capacity of the battery and, consequently, 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 voltage on the battery 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 the heating of the battery in the process of charge recovery. 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 cells is reached at different times, therefore

it is desirable to control each of them separately. In addition, there are batteries with an abnormal charging characteristic, on 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 impedance of the source change during the charging process. But even if we neglect the indicated difference, then 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 = I₀exp(-t/T₀), (one)
where t is time; I₀ - 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 = I₀Т₀[1 - exp(-t/T₀)] = (I₀ -I)T₀. (2)

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

Rice. 1. Dependences of I and q on time t

It can be seen that during 3T₀ charge reaches 0,95I₀T₀ and then approaches the value I₀Т₀.

 It is recommended to choose values ​​I₀ and t₀ formulas

 I₀ = nE, T₀ = 1 h/n, where n = 1, 2, 3, 4. (3)

The most convenient value is n \u1d 3. The initial charging current in this case is equal to the electrical capacity E, the charging time is 2 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. With n = 1,5, it will be 2 hours with an initial charging current of 3E. This mode is suitable for Ni-Cd and Ni-MH batteries. Increasing n to 1 reduces the charging time to 3 hour, but the initial charging current increases to 4E. Finally, at n = 45, the charging time is reduced to 4 min, and the initial charging current is increased to 3E. Values ​​of n equal to 4 and 0,1 are acceptable for Ni-Cd batteries, since their internal resistance is low (less than 4 ohm). As for Ni-MH batteries, their internal resistance is several times greater, so a large current can heat them up at the beginning of charging, which is unacceptable. Values ​​greater than XNUMX are not recommended. I can choose₀ 5% more than determined by formula (3). Then the exact charging time will be 3 h/n, and a further 5% recharge is not significant.

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

Fig. 2.

Capacitor with capacity C1, pre-charged to voltage U₀, is discharged through current amplifier A1 with input resistance Rin and current gain Ki. The current in the input circuit of the amplifier Iin is determined by the expression

Iin = U₀exp(-t/RinC1)/Rin. (four)

The current in the output circuit of the amplifier I \u1d KiIin charges the battery GBXNUMX:

 I = KiU₀exp(-t/RinC1)/Rin = SU₀ exp(-t/RinС1), (5)
where S = Ki/Rin is the gain slope of the amplifier when viewed as a voltage-to-current converter. Comparing (2) and (5), we have
 T₀ = RinC1, I₀ = KiU₀/Rin=SU₀. (6)

Convenient to choose U₀ \u1d 1 V, C1000 \u3d 3,6 μF, then from (XNUMX) it follows that Rin \uXNUMXd XNUMX MΩ / n

 S = nE, Ki = SRin = 3600000E. (7)

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

The schematic diagram of the device is shown in fig. 3. The current amplifier is assembled on the op-amp DA2.1 and transistors VT2 and VT3. The supply voltage of the op amp is stabilized by the DA1 chip. The node on the transistor VT1 controls the value of this voltage. When it is normal, this transistor is open, current flows through the coil of relay K1, the contacts of relay K1.1 are closed, the HL1 LED lights up, signaling the normal operation of the device. The SA1 switch 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 on the engine of the variable resistor R3 determines the charging current. In the "Constant" mode, this voltage is fed through the resistor R1 and the closed contacts of the relay K1.1 to the non-inverting input of the op-amp. Its output current is amplified by transistors VT2, VT3 and is set so that the voltages across resistors R11 and R5 become the same. The current gain Ki = R5/R11 and with the ratings indicated in the diagram is approximately equal to 10⁷, and the voltage-to-current conversion slope S = 1/R11 = 3 A/V.

Rice. 3. Schematic diagram of the device

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 the charging mode with a decreasing current, the transistor VT1 closes and the relay opens the contacts K1.1, preventing further discharge of the capacitor C2. The HL1 LED goes out, signaling a power outage. With the restoration of power, transistor VT1 opens, relay K1 closes contacts K 1.1 and battery charging automatically continues from the current value at which it was interrupted. The HL1 LED lights up again, signaling the resumption of charging. By pressing the SB1 button, you can briefly stop charging when removing the charging characteristics. In this case, the capacitor C4 prevents the penetration of network interference to the input of the op-amp.

The device is assembled on a universal printed circuit board and housed in a housing with dimensions of 310x130x180 mm. AA batteries are placed in a groove on the top cover of the case. The contact sockets are made in the form of pieces of tinned sheet tape, which are pressed against the batteries by a spring from a standard compartment for an AA cell. 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, while 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 case of the compartment, making its further use impossible.

Transistor VT3 is mounted on a ribbed heat sink with a surface area of ​​600 cm², diode VD2 - 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 conclusions 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 series, VT2 must have a static base current transfer coefficient h_21Э not less than 100. The transistor KT853B (VT3) is different in that its h_21Э exceeds 1000. Other types of transistors can be used as VT2, VT3, but the total current gain must exceed 100000.

Capacitor C2, which sets the charging time constant T₀, must have a stable capacity, not necessarily equal to the nominal value indicated on the diagram, since the required value of T₀ set when adjusting the selection of resistor R5. The author used a Jamicon oxide capacitor with a large voltage margin (25 times).

Relay K1 - reed relay EDR2H1A0500 from ECE with a voltage and current of operation, respectively, 5 V and 10 mA. A possible replacement is a domestic-made 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 a current of 3 A was used). Fuse FU1 is a self-resetting MF-R300 from BOURNS [10].

Establishing the device is reduced to setting the required value of the charging time constant T₀selected by formula (3). The resistance of the resistor R5 is chosen equal to Rin according to the formula (7), assuming that the capacitance of the capacitor C2 is exactly 1000 μF. Instead of batteries, a digital ammeter is included. Before turning on the power, both when charging the 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 the SA1 switch are closed (this is necessary to discharge the capacitor C2). Then turn on the power and, moving the slider of the resistor R3, set the initial current I₀ about 1 A. Next, SA1 is transferred to the "Decreasing" position. After time T₁ (approximately equal to T₀) measure the current I₁. The corrected resistance value of the resistor R5* is calculated by the formula R5* = R5[ln(I₀/I₁)]. Finally, a resistor R5 is installed with a resistance equal to this corrected value.

Batteries before charging must be discharged to a voltage of 1...1.1 V to prevent their overcharging and the manifestation of the memory effect [2]. If the batteries become hot during discharging, they should be cooled down to ambient temperature (0...+30 °С [2]) before charging. Before connecting the batteries to the charger, you must make sure that it is de-energized, the slider of the resistor R3 is in the lower (according to the diagram) position, and SA1 is in the "Constant" position. Further, observing the polarity, install the batteries, turn on the power and use the variable resistor R3 to set the initial current I₀ by formula (3). After that, SA1 is transferred to the "Decreasing" position, and after a time of 3T₀ batteries are ready to 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.
2. battery-index.com
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 batteries. - Radio, 1997, No. 1, p. 44-46.
6. Dolgov O. Foreign charger and its analogue on domestic elements. - Radio, 1995, No. 8, p. 42, 43.
7. Dorofeev M. Charger option. - Radio, 1993, No. 2, p. 12, 13.
8. Tkachev F. Calculation of a thermosensitive bridge. - Radio, 1995, No. 8, p. 46.
9. Biryukov S. Microcircuit voltage stabilizers of wide application. - Radio, 1999, No. 2, p. 69-71.
10. BOURNS MULTIFUSE resettable fuses. - Radio, 2000, No. 11, p. 49-51.

Author: M. Evsikov, Moscow; Publication: cxem.net

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