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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Automatic prefix to the charger. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells The magazine has always paid a lot of attention to the issues of competent maintenance of automotive batteries. So, for example, the previous article on this topic was published last year (I. Herzen. "Automatic prefix to the charger" in "Radio", 1997, No. 7, pp. 45, 46). The work below is another step in that direction. In the process of long - several months - storage of car batteries, they self-discharge, and therefore it is recommended to recharge the battery at least once a month. However, conventional recharging is not able to prevent sulphation of the plates, which gradually leads to a decrease in battery capacity and a decrease in its service life [1]. Therefore, the battery is subjected from time to time to discharge with a current, in amperes, numerically equal to 1/20 of the rated capacity, expressed in ampere-hours, to a voltage of 10,5 V, followed by charging to a voltage of 14,2 ... 14,5 V. Such the charge-discharge cycle should be repeated several times if the battery is heavily sulfated or has been in a semi-discharged state for a long time. The attachment described below is designed to work in conjunction with chargers that provide the necessary charging current and have a pulsating charging voltage at the output. Suitable, for example, are industrially produced devices UZ-A-6/12 (Vyborg), UZR-P-12-6,3 (Yuriev-Polsky), as well as amateur devices described in [2, 3]. The prefix allows you to discharge the battery to a voltage of 10,5 V and, after discharging, automatically start charging with a current with a discharge component (with a ratio of the charge and discharge components of 10:1). The device stops charging when the voltage at the battery terminals reaches 14,2 ... 14,5 V, which corresponds to its 100% charge. It controls the voltage when there is no charging current. When the mains voltage fails, the device stops discharging the battery. Discharge-charge cycles can be single or multiple. Schematic diagram of the prefix-machine is shown in fig. one.
The set-top box is powered - combined - from the mains, from the charger and from the rechargeable battery GB1 at a time when the optocoupler dinistor U3 is closed. Timer comparators DA14,2 [14,5] with voltage dividers R10,5R1 and R4R7 are used as a threshold element that generates a signal at two battery voltage values - 10 ... 8 V during charging and 11 V during discharge. At its inputs R and S, the voltage on the battery being charged or discharged is compared with the above threshold values determined by the timer supply voltage, the resistance of the resistors of the timer's internal voltage divider, and the voltage at its input UR (it is taken from the zener diode VD2). The lower and upper thresholds of the comparator operation can be changed with trimmers R10 and R11. The timer is powered by a parametric stabilizer VD3R9. The voltage of a twelve-volt battery that is not too discharged is usually 12 ... 12,6 V. When the device is connected to the network with the battery connected, the timer will be set to a state corresponding to a high level voltage at its output, transistor VT1 will be open. The dinistor of the optocoupler U3 will open, and the battery will start charging, which will be indicated by the HL1 LED turning on. However, as a rule, the degree of charge of the connected battery is unknown, therefore, before starting charging, it is advisable to discharge it to a voltage of 10,5 V. To turn on the discharge mode, after connecting the battery, briefly press the SB1 "Start" button. Through the contacts SB1.1, the input R of the timer will receive voltage from the battery connected to the output and switch it to the opposite state (low level at the output), the transistor VT1 will close and turn off the HL1 LED. At the same time, through the closed contacts SB1.2, a low level comes to the upper input of the RS-flip-flop assembled on the elements DD1.1, DD2.2. The trigger is set to a state where a high voltage appears at the output of the DD1.1 element. With the position of the contacts of the switch SA1 shown in the diagram, the output of the elements DD1.3, DD1.4, included by the inverters, has a low-level voltage. Since the phototransistor of the optocoupler U2 is open (and it is open all the time while the mains voltage is applied to the prefix), a current flows through the base of the transistor VT4, resistor R23, the phototransistor of the optocoupler and the output of the logic elements DD1.3 and DD1.4, sufficient to saturate this transistor. The discharge current of the battery flows through the EL1 incandescent lamp - about 2,5 A, which corresponds to the 20-hour discharge mode of the 6ST55 battery. When servicing a battery of a different capacity, a lamp of the appropriate power should be used. The mains voltage through the quenching resistor R1 is supplied to the diode bridge VD1 and, after rectification, feeds the series-connected LEDs of the optocouplers U1 and U2. Capacitor C1 and resistor R2 form a smoothing filter for the LED of optocoupler U2. When the mains voltage fails, the phototransistor of this optocoupler closes, which leads to the closing of the VT4 transistor and the termination of the battery discharge. As the battery discharges, the voltage across its terminals decreases. When it reaches 10,5 V, the timer will switch, transistors VT1 and VT2 will open. Opening the transistor VT1 will cause the device to switch to charging mode, switching the RS-trigger and closing the transistor VT4, as well as opening the transistor VT3. The charging current is set using a charger in accordance with the operating instructions for the battery, i.e. equal to 1/10 or 1/20 of the battery capacity. If charging is carried out without the operator's control, it should be ensured that the fluctuations in the charging current are limited due to fluctuations in the mains voltage. The easiest way to stabilize the current is to turn on a circuit of two or three parallel-connected car lamps with a power of 40 ... 50 W in the gap of one of the output wires of the charger [5]. The same effect is obtained by turning on a lamp with a voltage of 220 V and a power of 200 ... 300 W in one of the input (network) wires of the charger. The charging current contains a dosed discharge component, which has a beneficial effect on the course of electrochemical processes in the battery [1]. The current of the discharge component determines the resistor R19 (approximately 0,5 A). During charging, the voltage at the battery terminals gradually increases. It is known that the voltage of a fully charged battery is 14,2...14,5 V [1]. This voltage is measured in the absence of charging current, since charging pulses, depending on the degree of battery discharge, increase the instantaneous voltage value at its terminals by 1...3 V. To ensure this measurement mode, the device uses elements U1, R4, VT2. In charging mode, transistor VT2 is open. On fig. 2 shows voltage and current diagrams explaining the operation of optocouplers U1 and U2. The mains voltage is rectified by a diode bridge (Fig. 1) and supplied to the LEDs of the optocouplers U1 and U2.
The phototransistor of the optocoupler U1 opens when the current through the LED of the optocoupler U1 (Fig. 2) exceeds the current of the phototransistor opening. In this case, the resistor R4 shunts the tuning resistor R11 and the upper threshold for the operation of the timer DA1 increases. At the moments when the mains voltage passes through zero, the phototransistor closes and the timer threshold decreases to a value of 14,2 ... 14,5 V. It is at this time that the charging current does not flow through the battery. The measurement takes place in each half-cycle of the network, i.e. 100 times per second. Measurement duration - 1...3 ms. The current through the LED of the optocoupler U2 flows all the time while the mains voltage is applied to the prefix, due to which the phototransistor of the optocoupler U2 is open. As soon as the voltage on the battery reaches 14,2 ... 14,5 V in the absence of charging current, the timer DA1 will switch (the output will appear low) and charging will stop. Since the output of the RS flip-flop is still high, the device can remain in this state for a long time, up to several days. The current consumed from the battery is small (20...30 mA) and cannot cause its significant discharge. If repeated training of the battery with discharge-charge cycles is necessary, the contacts of the switch SA1 are transferred to the lower position according to the scheme. In this case, the RS flip-flop is disabled and charging and discharging will alternate as long as there is mains voltage and a rechargeable battery is connected. Capacitors C2, C3 increase the noise immunity of the timer. Resistors R19, R22 provide reliable retention of transistors VT3, VT4 closed in the absence of base current. Instead of KT608B, any transistors from the KT603, KT608, KT3117, KT815 series can be used in the device; KT503B - KT315, KT501, KT503, KT3117; KT814B - KT814, KT816, KT818, KT837 and instead of KT825G - any of this series. Optocoupler dinistor TO125-10 can be replaced with T0125-12.5, TO2-10, TO2-40, TSO-10. We will replace the KTs407A diode bridge with KTs402, KTs405 with letter indices A, B, C. It is advisable to use the VD3 zener diode with a small TKN stabilization, any zener diodes of the D818 series are suitable. Oxide capacitor C1 - K50-16, K50-35 or K50-29; C2, C3 - KM-66, K10-23, K73-17, etc. Trimmer resistors R10, R11 - any multi-turn, for example SP5-2. Resistor R20 - PEV with a power of 10 or 15 W (in extreme cases, 7,5 W); the rest - MLT, OMLT, C2-23. Button SB1 and switch SA1 - any, for example, KM2-1 and MT1, respectively. Most of the elements of the device are mounted on a printed circuit board made of foil fiberglass 2 mm thick (Fig. 3).
Optocoupler dinistor U3 and transistor VT4 are installed on heat sinks with a cooling surface of 100 ... 150 cm2. The board is reinforced in any case of suitable dimensions (in the author's version - 260X100X70 mm). Connections carrying charging and discharging current must be made with a wire with a cross section of at least 2 mm2. The wires connecting the device to the battery should preferably be flexible. To establish the device, you will need a laboratory DC source with a voltage regulated in the range from 9 to 15 V at a load current of at least 0,6 A, and a voltmeter. First, the charger and the EL1 lamp are temporarily turned off, and the battery being charged is replaced with a laboratory current source. By setting the source voltage to 10,5 V using the voltmeter, the trimmer resistor R10 sets the lower threshold for the comparator to turn on the HL1 LED, and then, by setting the voltage to 14,2 ... 14,5 V, the upper threshold for turning on the HL11 LED is set with the trimmer R2. The appearance of the assembled console is shown in Fig. four.
To ensure the electrical safety of the entire charging installation as a whole, it is necessary that the load (battery) be galvanically isolated (separated) from the supply network. The role of decoupling elements in the attachment is played by optocouplers (U1 and U2. Unfortunately, the optocouplers of the AOT110 series chosen by the author are not able to eliminate the danger of electric shock, since their rated insulation voltage does not exceed 100 V. Only those optocouplers whose insulation voltage is not less than 500 V, the phototransistor is composite (especially for the U2 optocoupler), for example, from the AOT127 series. Literature
Author: A. Evseev, Tula
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