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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Automatic charger for Ni-Cd batteries. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells The article brought to the attention of readers describes an automatic charger, which, according to the author, charges Ni-Cd batteries almost perfectly. In addition, it can also charge Ni-MH batteries. In the author's version, the device is designed to charge a battery with a nominal voltage of 7,5 V and a capacity of 1300 mAh of the Motorola GP1200 radio station. For everyone who wants to repeat this device for charging other batteries, formulas for calculating the main elements are given. It is known [1] that a Ni-Cd battery is considered charged when, when the charger (charger) is connected, the voltage on it is 1,5 V. After the charger is turned off, the voltage quickly decreases to about 1,45. , as this reduces battery life. Normal charging of the battery is possible if it is discharged to a voltage in the range of 1 ... 1,1 V. When discharged to a voltage below the specified level, the battery life is reduced, and at a higher value, a memory effect appears. Therefore, before charging, make sure that the battery is discharged to the voltage indicated above. The approximate charging time is calculated by the formula t=1,4C/I10, where t is the charging time, h; C - battery capacity, mAh; I10 - rated charging current: 110=C/10, mA; 1,4 is a correction factor that takes into account losses, since during charging part of the energy is irreversibly converted into heat. It should be remembered that almost all modern Ni-Cd batteries are created using more advanced technology, so the correction factor for them is in the range from about 1,1 to 1,2. So, how to make sure that after the charging cycle the battery does not recharge and automatically disconnects from the charger. You can, for example, calculate the time required to charge the battery, set the charging current and connect a time relay. However, this decision has its downsides. As mentioned above, the correction factor for a particular battery may vary slightly, which will lead to incorrect timing and, as a result, to its undercharging or overcharging. If the battery has not been completely discharged, a charger that implements this method is very likely to recharge it. If, during the charging process, the voltage in the mains disappears, and then reappears, the time relay will reset its readings and start the cycle again, which will again lead to a guaranteed recharge. Ultimately, battery life will noticeably decrease. Let's consider another option. If you focus on the final voltage value on the battery of 1,5 V, then you can control not the time, but the voltage on it and, in accordance with this, disconnect it from the charger. However, as a rule, there are no identical batteries, and when the battery is charged, some of its cells will be undercharged. If you remove the charging characteristic of the battery, you can find an interesting feature: when recharging, the voltage at the battery terminals decreases. It remains only to check the fact of a decrease in voltage and give a command to turn off the memory. Let's dwell on this in more detail. Let's break the charging process into three stages. The first stage - the voltage on the battery (AB) rises to a level of 1,5 V per cell. The duration of this stage is approximately 80...90% of the total time. The second stage - the voltage on the battery becomes more than 1,5 V per cell. At this stage, the most mysterious process occurs - some batteries are charged, and some experience a slight overcharge. It is almost impossible to predict what the voltage on the battery will be at this moment. It all depends on the identity of the parameters of the batteries. It is noticed that the more the parameters differ, the higher the voltage rises. At the end of this process, the batteries in the battery will be almost equally charged. The duration of this stage is approximately 10...20% of the total time. The third stage - the voltage on the battery decreases and becomes less than 1,5 V per cell. Charging completed. But what if the voltage in the third stage does not become less than 1,5 V per cell. This situation very rarely occurs when charging Ni-Cd, but is typical for Ni-MH batteries. There is a very simple way out. Usually the second stage for all modern batteries lasts no more than two hours (more precisely, 1 ... 2 hours). Therefore, it is sufficient to use a timer that turns off the memory two hours after the start of the second stage. Consider charging the battery from the Motorola GP1200 radio station, which consists of six batteries with a capacity of 1300 mAh. Its nominal voltage, like most batteries for radio stations of this company, is 7,5 V. The presence of a protective diode built into the battery included in the charging circuit should also be taken into account. Typically, the voltage drop across this diode is about 0,28 V. Let's calculate the parameters of the charger for charging this battery. Rated charging current I10=0/10=130 mA. The comparator response voltage is 6-1,5 = 9 V. We add to this value the voltage drop across the protective diode: 9 + 0,28 = 9,28 V. The correction factor for Motorola batteries is approximately 1,2. The maximum battery charging time is t=1,20/I10=1,2-1300/130=12h. The memory circuit is shown in fig. one.
The device consists of the sin of the main nodes: A1 - a rectifier with voltage doubling and a charging current stabilizer; A2 - a comparator that controls a current-setting trigger and a charging timer; A3 is a trigger that determines the battery charging current. The main advantages of the proposed automatic memory:
If the battery (GB1) is connected to the charger, a stable voltage of 1 V appears at the output of the DA5 stabilizer. As a result, the HL3 LED turns on, indicating that the battery is connected to the device. The current-setting trigger, assembled on transistors VT2-VT4, is fed with the same voltage. Due to the presence of capacitor C6, the voltage at the base of transistor VT3 rises more slowly than at the base of transistor VT4. The transistor VT4 opens, the resistor R14 is connected to the current stabilizer DA1 and determines the charging current in the first stage. Therefore, the HL2 LED turns on, signaling the start of charging. When the voltage at the battery reaches 9,28 V, the comparator DA2.1 will work, which will lead to the opening of the transistor VT2. As a result, the voltage at the base of transistor VT4 will decrease sharply and the trigger will switch to another stable state: transistor VT4 is closed, and transistors VT2 and VT3 are open. This leads to the fact that the charging current is now determined by the resistance of the resistors R10 and R11 connected in parallel. It is easy to calculate that the current remains the same. Naturally, as a result, the HL2 LED will go out and HL1 will light up, signaling the second stage. The second stage will end with a voltage drop on the battery, as a result of which the DA2.1 comparator switches again, the HL1 LED goes out and the VT2 transistor closes. Now the charging current is determined only by the resistance of the resistor R11. Charging completed. As practice shows, as a result of multiple and almost ideal charging cycles, the parameters of the batteries in the battery are equalized and the voltage at the end of the second stage tends to 1,5 V per cell, sometimes not exceeding this value. In this case, the comparator will most likely not work. This is where the charging timer, assembled on the op-amp DA2.2, comes into play. Capacitor C5 sets the time (about two hours) after which the timer will switch. After this time, the transistor VT2 will close and, as mentioned above, the charging current, numerically equal to approximately 1/30 of the AB capacity, will be determined by the resistance of the resistor R11. Such a small current only compensates for the self-discharge of the battery. Theoretically, the AB can stay in this mode indefinitely. Trimmer resistor R3 set the threshold of the comparator DA2.1. In fact, the comparator is powered by an asymmetric bipolar voltage, the threshold for its operation is the voltage transition at the inverting input through zero. The comparator is designed so that the lower response threshold is approximately 60 mV less than the upper one [2]. This is done to eliminate the "bounce" at the time of switching transistor VT2. The charger is fed from a transformer, the alternating voltage on the secondary winding of which is 12 V. A rectifier with voltage doubling is assembled on diodes VD1, VD2 and capacitors C1, C2 - its output voltage is about 30 V, which is quite enough to charge a battery of ten batteries. If it is necessary to charge batteries of a different capacity and (or) with a different voltage, the charger parameters can be easily recalculated. This will require three parameters: capacity, the number of batteries in the battery and the presence (or absence) of a protective diode. Knowing the capacitance, calculate the rated charging current. Based on the number of batteries and the presence (or absence) of a protective diode, the comparator switching voltage is calculated. It may be necessary to select a resistor R2 so that the trimming resistor R3 can adjust the response threshold. And it remains to calculate the resistance of resistors R10, R11, R14: R14=5/I10; R11=4R14; R10=R11/3. However, the values obtained are not quite standard, therefore composite, parallel-connected resistors are used in the memory: R14 - four parallel-connected resistors R11; R10 - three resistors R11 connected in parallel. I recommend using compound resistors. Otherwise, if there is a greater spread in ratings, the comparator may not switch. The device is assembled on three printed circuit boards (each node on a separate board), the drawings of which are shown in fig. 2.
Stabilizer DA1 should be placed on a ribbed or pin heat sink with an area of at least 20 cm2. In the device, it is necessary to use capacitors only of the capacity indicated on the diagram. The leakage resistance of the capacitor C5 is at least 2 MΩ. Before adjustment, jumper S1 must be removed. Then voltage is supplied to connector X1 from the mains transformer. Instead of AB, its equivalent is connected. The battery equivalent resistance is calculated by the formula Req=Ucp/I10, where Ucp is the comparator switching voltage (9,28 V). In our case, the battery equivalent from the Motorola GP1200 radio station is a resistor with a resistance of about 75 ohms and a power of at least 2 watts. After setting the equivalent, the HL3 LED should turn on. Further, the comparator switching voltage (3 V) is supplied to the capacitor C9,28 from an external regulated power supply in compliance with the polarity: the negative terminal is connected to the left terminal of the capacitor C3 according to the scheme, and the positive terminal is connected to the right terminal. Trimmer resistor R3 sets the threshold for turning on the LED HL1. Then you should check that with a smooth decrease in voltage from an external regulated power supply from 9,28 to 9,2 V, the HL1 LED is guaranteed to go out. Next, check the performance of the entire memory. To do this, you need to slightly reduce the voltage from the external power supply by at least 1 V. As a result, the HL1 LED will go out, of course, if it was lit. Then turn off the equivalent of AB. LED HL3 should go out. Again we connect the equivalent. LEDs HL2 and HL3 light up. The HL3 LED indicates the presence of a battery in the device, and the HL2 LED indicates the start of charging. Then gradually increase the voltage of the external power supply. At a voltage of 9,28 V, the HL2 LED should turn off and the HL1 LED should turn on, signaling the beginning of the second stage. And finally, it remains to check the charging timer. To do this, a voltmeter is connected between the base and emitter of the transistor VT2. It should show a voltage of about 0,7 V. The HL1 LED is on at this time. After 2 hours ± 20 minutes, the voltmeter reading should decrease. The HL1 LED will continue to light up. But when charging the battery, as soon as the base-emitter voltage of the transistor VT2 decreases, the HL1 LED goes out. Adjustment completed. Disconnect the external regulated power supply, the equivalent of AB and restore jumper S1. The device is ready to work. Literature
Author: Yu.Osipenko, Ufa
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