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SCHEME OF DEVICE FOR ACCELERATED CHARGING OF ACCUMULATOR BATTERIES
Ni-Cd and Ni-MH

The author of the article: M. Evsikov

The device described in the article is intended for accelerated charging of Ni-Cd and Ni-MH batteries by an exponentially decreasing current. Its advantages include the ability to choose the charging time in the range from 45 minutes to 3 hours , the ease of manufacture and adjustment, the lack of heating batteries at the end of charging, the ability to visually monitor the charging process, automatic recovery of the process when the power is turned off and then turned on, convenience of use. The device can be used as a stand for removing the charge-discharge characteristics of batteries.

When charging a large unchanged current ( 0.5E or more, where E is the capacity of the battery), the battery begins to heat after 75 ... 80% charge, and Ni-MH batteries are heated 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 terminates with the ignition or explosion of the battery. The recommended stopping temperature is + 45 ° C [2] . However, this criterion is only suitable as an emergency: the combination of recharging with overheating reduces the capacity of the battery and, therefore, shortens its service life.

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

When charging unchanged 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 is reduced and at the end of its service life is approximately 80% of the nominal value. Therefore, limiting the charge to a nominal capacity does not guarantee that there is no recharge and overheating of the batteries and, therefore, can not be the only criterion for completing the charge.

The most difficult criterion for the end of the process is the moment when the voltage on the accumulator reaches a maximum, and then begins to decrease. The maximum voltage on the battery corresponds to the full charge, but in [2] it is shown that it is a consequence of heating the battery during the recovery of the charge. The magnitude of the maximum is very small, especially for Ni-MH batteries (about 10 mV ), so ADCs or voltage converters are used to detect it [2] . When charging the battery, the maximum voltage of its various elements is reached at different times, so it is advisable to monitor each of them separately. In addition, there are batteries with anomalous charging characteristic, on which this maximum is absent. In other words, only voltage control is insufficient, it is necessary to control both the temperature and the amount of charge passed through the battery.

Thus, when charging the battery with a large unchanged current, it is necessary to monitor each of its elements by several criteria, which complicates the charger. Only charging with a low current (no more than 0.2E ) does not cause an emergency overheating of the batteries, even with a large recharge. In this case, the state of each element is not necessary to control, the charger is very simple, but its drawback is obvious - a long charging time.

There are chargers in which the initially large charge current decreases with time [4-6] . In this case, you do not need to monitor the status of each battery cell. But in these devices there is no control of the magnitude of the charge, and as a criterion for full charging a certain voltage is used, which, as mentioned above, is not satisfactory.

[7] describes a charger in which a battery is charged as a capacitor from a source of constant voltage through a resistor. In this case, the charging current should theoretically decrease with time exponentially with a time constant equal to the product of the equivalent battery capacity by the resistance of this resistor. In practice, however, the dependence of the charging current on time differs from the exponential, since the equivalent capacitance and output impedance of the source change during charging. But even if we neglect this difference, the most important parameter - the charging time constant - is unknown, and therefore the control of the charge passed through the battery is not possible. Therefore, charging ends 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 realize by means of a simple RC circuit . It ends naturally, as a result of which there is no need for a timer that turns off the batteries after a predetermined time, 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 the current generator, therefore its value and shape do not depend on the voltage on the batteries or on the nonlinearity of their charging characteristics.

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

I = I ₀ exp (-t / T ₀ ), (1)

Where: t - time; I ₀ - initial charging current; T ₀ is the charging time constant. In this case, each battery receives a charge q , which is estimated by the expression

Q = I ₀ T ₀ [1 - exp (-t / T ₀ )] = (I ₀ - I) T ₀ . (2)

Graphs of dependences I and q on time t are presented in Fig. 1.

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

It is seen that during the time 3T _{0 the} charge reaches the value 0.95I ₀ T ₀ and then approaches the value I ₀ T ₀ . It is recommended to select the values I ₀ and T ₀ by formulas

I ₀ = nE, T ₀ = 1 h / n, where n = 1, 2, 3, 4. (3)

The most convenient value is n = 1 . The initial charge current in this case is equal to the electrical capacity E , charging time is 3 hours . (You can practically leave the batteries in the charger for the night, and by the 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 h with the initial charging current 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 3E . Finally, at n = 4 , the charging time is shortened to 45 minutes , and the initial charging current is increased to 4E . Values ​​of n , equal to 3 and 4 , are acceptable for Ni-Cd batteries, since their internal resistance is slightly (less than 0.1 Ohm ). As for Ni-MH batteries, their internal resistance is several times larger, so a large current can warm them up at the beginning of charging, which is unacceptable. Values ​​of n greater than 4 are not recommended. You can choose I ₀ to be 5% larger than the one defined by formula (3) . Then the exact charging time will be 3 h / n , and a further 5% charge transfer is unimportant.

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

Fig.2. Principle of operation of the device

The capacitor C1 , pre-charged to voltage U ₀ , is discharged through the current amplifier A1 with the input resistance Rin and the current gain Ki . The current in the input circuit of the amplifier Iin is given by

Iin = U ₀ exp (-t / RinC1) / Rin. (4)

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

I = KiU ₀ exp (-t / RinC1) / Rin = SU ₀ exp (-t / RinC1), (5)

Where: S = Ki / Rin - the slope of amplification of the amplifier, if it is considered as a voltage-to-current converter. Comparing (2) and (5) , we have

T ₀ = RinC1, I ₀ = KiU ₀ / Rin = SU ₀ . (6)

It is convenient to choose U ₀ = 1 V , C1 = 1000 μF , then it follows from (3) that Rin = 3.6 MOhm / n ,

S = nЕ, Кi = SRin = 3600000E. (7)

For example, for E = 1 Ah and n = 1, the following parameters should be: Rin = 3.6 MΩ , S = 1 A / B , Ki = 3600000 = 131 dB .

Schematic diagram of the device is shown in Fig. 3. The current amplifier is assembled on the DA2.1 op-amp and the transistors VT2 and VT3 . The supply voltage of the op-amp is stabilized by the chip DA1 . A node on the 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, LED HL1 lights, signaling the normal operation of the device. By switch SA1 , the charging mode is selected: 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 motor of the variable resistor R3 determines the charging current. In the "Constant" mode, this voltage through the resistor R1 and the closed contacts of relay K1.1 goes 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 for the nominal values ​​shown in the diagram is approximately 10 ⁷ , and the steepness of the voltage to current conversion is S = 1 / R11 = 3 A / V.

Fig.3. Schematic diagram of the device

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

When the power is cut off in charging mode, the transistor VT1 closes and the relay opens contacts K1.1 , preventing further discharge of capacitor C2 . The LED HL1 goes out, signaling a power failure. With power recovery, the transistor VT1 opens, relay K1 closes contacts K1.1 and the battery charging automatically continues from the current value at which it was interrupted. The LED HL1 lights up again, signaling that the charging has been resumed. By pressing the SB1 button, you can briefly stop charging when the charging characteristics are removed. 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 is housed in a housing measuring 310x130x180 mm . AA batteries are placed in the gutter on the top cover of the casing. Contact sockets are made in the form of pieces of tape from tinned sheet metal, which are pressed against the batteries by a spring from the standard compartment for an element of size AA . The current does not flow through the spring. It should be noted that the commercially available plastic compartments are suitable only for currents not exceeding 500 mA . The fact is that the current flowing through the contact springs, warms them, while the accumulators are heated. Even 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 it impossible to continue using it.

Transistor VT3 is installed on a ribbed heat sink with a surface area of 600 cm ² , diode VD2 - on a plate heat sink of 50 cm ² . Resistor R11 is made up of three connected in parallel resistors MLT-1 resistance of 1 ohm . All high-current connections are made by pieces of copper wire of 3 mm ² cross section, which are soldered directly to the terminals of the corresponding parts.

OU K1446UD4A (DA2) can be replaced with the chip K1446UD1A or another of these series, but from the two op-amp you need to choose one with a bias voltage less. The second op-amp can be used as part of a temperature-sensitive bridge [8] for emergency disconnection of the batteries when they are overheated during DC charging (when charging with a decreasing current, the battery was not overheated). If other types of op amps are used, it should be borne in mind that in this design the power supply is unipolar, so it must be operative at zero voltage on both inputs.

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

Transistor VT1 - any of the series KP501 , VT2 should have a static current transfer coefficient of base h _21E not less than 100 . The transistor KT853B (VT3) differs in that its h _21E exceeds 1000 . As VT2 , VT3 , other types of transistors can be used, but the total current gain should exceed 100,000 .

The capacitor C2 , which sets the charging time constant T ₀ , must have a stable capacitance, not necessarily equal to the nominal value in the circuit, since the required value of T _{0 is} set when the resistor R5 is selected by adjusting. The author used an oxide condenser from Jamicon with a large margin for voltage ( 25 times ).

Relay К1 - reed switch EDR2H1A0500 of ECE company with voltage and current of 5 V and 10 mA respectively. A possible replacement is a domestic relay KUTS -1 (passport RA4 .362.900 ).

The ammeter PA1 should be designed for the maximum charging current (in the author's version the device M4200 is applied to a current of 3 A ). Fuse FU1 - self - healing MF-R300 company BOURNS [10] .

Adjustment of the device is reduced to setting the necessary value of the charging time constant T ₀ , chosen according to the formula (3) . Resistance of the resistor R5 is chosen equal to Rin according to the formula (7) , assuming that the capacity 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 engine of the variable resistor R3 is transferred to the lower (in the circuit) position and the contacts of the switch SA1 are closed (this is necessary for discharging the capacitor C2 ). Then power is turned on and, by moving the resistor R3, the initial current I _{0 is set to} about 1 A. Then SA1 is moved to the "Decreasing" position. After a time T ₁ (approximately equal to T ₀ ), the current I ₁ is measured. The corrected resistance value of the resistor R5 * is calculated by the formula R5 * = R5 [ln (I ₀ / I ₁ )] . Finally, set the resistor R5 to the resistance equal to this adjusted value.

Batteries must be discharged to a voltage of 1 ... 1.1 V before charging to avoid overcharging and the appearance of a memory effect [2] . If the batteries become hot during discharge, they should be cooled to ambient temperature ( 0 ... + 30 ° C [2] ) before charging. Before connecting the batteries to the charger, you need to make sure that it is de-energized, the resistor R3 is in the lower (in the circuit) position, and SA1 is in the "Permanent" position. Next, observing the polarity, install the batteries, turn on the power and use the variable resistor R3 to set the initial current I ₀ according to the formula (3) . After this, SA1 is transferred to the "Decreasing" position, and after 3T _{0 the} batteries are ready for use.

To power the device requires a voltage source from 8 to 24 V , it can be unstabilized. At the same time, you can charge from one to ten items. The minimum supply voltage taking into account ripple should be 2 V per element plus 4 V (but within the specified limits).

The device can be used as a stand for removing not only the charging, but also the discharge characteristics of the batteries. In the latter case, the battery under test should be connected to the device in reverse polarity. The voltage at its electrodes must be constantly monitored by a voltmeter. Do not change the polarity of the battery so that it does not cause an accidental damage to the battery. For this reason, it is not recommended to discharge the battery from several series-connected elements in this way, since the moment of failure of the element with the smallest capacity can be missed.

Sources

1. New types of batteries ("Abroad"). - Radio, 1998, No. 1, p. 48, 49.

2. Http://www.battery-index.com/

3. A little bit about charging nickel-cadmium batteries ("Abroad"). - Radio, 1996, No. 7, p. 48.49.

4. Nechaev I. Accelerated charging of batteries. - 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 the temperature-sensitive bridge. - Radio, 1995, No. 8, p. 46.

9. Biryukov S. Microcircuit voltage stabilizers of wide application. - Radio, 1999, No. 2, p. 69-71.

10. Self-restoring fuses MULTIFUSE firm BOURNS. - Radio, 2000, No. 11, c. 49-51.

print version
Author: M. Evsikov, Moscow
PS The material is protected.

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