The optimum discharge termination voltage of the nickel-cadmium battery is about 1V, at which time the chemical crystallization on the internal electrode plate of the nickel-cadmium battery disappears or remains small. If the nickel-cadmium battery has not been reduced to this value, it will cause a "memory effect". This effect will reduce the battery capacity, which is not conducive to the rational use of nickel-cadmium batteries. The fast charger introduced in this paper adds a discharge circuit before charging to the problem, and sound and light alarm when discharging to 1V, prompting people to put the function switch to the charging position, so that the nickel-cadmium battery charging is more reasonable.
The charger features fast charging, fast discharge, safety and high efficiency.
It can charge 2 or 4 x 5 cadmium batteries; fast charging current is 900mA, charging time is 45 minutes (450 ~ 500mAh battery) and 66 minutes (700 ~ 800mAh battery). The discharge battery has 2 sections and the discharge current is 0.5 to 0.9A.
working principle
The charger circuit is shown in the figure, which consists of a charging circuit and a discharging circuit.
The charging circuit charging circuit is composed of IC1 dual time base circuit 7556, IC2 fast charging integrated circuit MAX713 and a small number of peripheral components (for the working principle of MAX713, please refer to the third issue of Programmable Fast Charge Integrated Circuit MAX712/713 in 1994). One article). One time base circuit in IC1 constitutes the iookHz oscillator, and the other time base circuit in IC1 cooperates with 1C2 to form a pulse-width-modulated switching power supply circuit. Q3 is controlled by transistors Q1 and Q2. When Q3 is turned on, current is supplied to the load, that is, the battery is charged; at the same time, energy is accumulated in the coil L2. When Q3 is turned off, the energy accumulated in the coil L2 continues to supply a charging current to the battery (the current forms a loop through DZ).
When the battery is charging quickly. IC8's 8 pin is low level, LED3 is bright; when charging, the voltage increases gradually, and its voltage change rate △ is positive (ie △V/Δt is positive), before the end of fast charge, its voltage change rate △ is negative ( That is, ΔV/Δt is negative). When IC2 detects a △, the fast charge is stopped and changed to trickle charge. By the set time, the entire charging process ends.
The fast charge current depends on Rs. When Rs is 0.Z7Ω, the fast charge current is about 900mA.
During the entire charging process, LED1 is lit, indicating that it is in a charging state.
When the charging and discharging switch K1 is in the downward position, the charging circuit loses power, so the charging circuit does not work. The discharge circuit is turned on and the LED 2 is lit, indicating that the circuit is in a discharge operating state.
After the power supply is regulated by the 5v regulator IC4, it is divided by a resistor of 15kΩ and 10kΩ. The voltage at point A is 2V. An operational amplifier A1 of the operational amplifier LM324 (IC3) is connected to a voltage comparator, the inverting input terminal is connected to point A, and the non-inverting input terminal is connected to the positive electrode of the battery to be discharged. When the voltage of the two discharge batteries is greater than 2V, the output of the voltage comparator A1 is high, Q4, os are turned on successively, and the battery is discharged to the ground through the Q5, 1~2.7Ω resistor. The discharge current of the battery is related to the parameters of Q4.Q5 and related resistors. When discharging to the termination voltage of 2V, the voltage comparator turns over, the Al output is low, Q4, Q5 are turned off, and the battery stops discharging.
The other operational amplifier A2 in IC3 is also connected to a voltage comparator circuit as an end of discharge indicator. Once the battery is discharged to 2V, the output of A2 is high level from low level, LED4 is on, and the buzzer sounds, indicating that the discharge operation is over.
The component selection transformer uses a 10W transformer with a 10V secondary. The rectifying part can be made of 2A plastic full bridge or 4 1N5100. The power supply filter capacitor can be a 3300μF electrolytic capacitor with a withstand voltage of 16V.
Q1 can be 2N3904 or 2SC2710, Q2 can be 2N3906 or 2SA1452. Q3 can use 2SJ123-class power FET, requiring PD=30W, ID=10A. The power FET will generate heat during operation, so it must be equipped with a 15 × 15 × 25mm heat sink. Ll uses l-shaped MXD-2000 magnetic core and is wound with ψO.1 enameled wire (if the requirement is not high, Ll can also be omitted, replaced by a short wire). L2 is made of ring-shaped ferrite MXD-2000 (outer diameter ψ18~ψ22) with ψ0.6 enameled wire. D2, D3 must use a Schottky diode with a small forward voltage drop and a small recovery time (cannot be replaced by 1N4001), such as 31DQ6 (UR=O.6V}1o=1.4A) or 1N5822.
Q4 "Q6 uses 9013 (β value is in the range of 80--150), os can use NPN power triode with ICM greater than 2A (also needs a heat sink on os).
The contact current of the charging and discharging switch K1 and K2 should be greater than 1A, and the double-pole double-throw switch with reliable contact and slightly larger size is selected.
Production and debugging The reader can design and make a printed circuit board according to the circuit diagram shown in the drawing. Install the selected components on the printed circuit board, and adjust the charging current, voltage at point A, and discharge current after soldering.
The charging current is adjusted by the charging current depending on the resistance Rs, if R. Use a series of values ​​o. 27Ω (1/2W) resistor, no adjustment is required. If Rs is self-made, especially when a resistor composed of nickel-chromium wire is used, since the cold state and the hot state resistance change greatly, when adjusting the fast charging current, it is necessary to connect an ammeter in the charging circuit to adjust the resistance value. Make it current at 900mA.
The voltage at point A is adjusted to 2V (discharge termination voltage). Its value is related to the resistance of 15kΩ and lOKΩ. If the voltage at point A is greater than or less than 2V, one of the resistance values ​​can be adjusted so that its voltage is 1.9~2. Between OV. These two resistors should use metal film resistors as well.
The discharge current is adjusted by the discharge current of the battery and the amplification factor of Q4 and Qs and the base and emitter resistance. The emitter resistance can be selected from the range of 1 to 2.7 Ω (2 W) to adjust the base resistance so that the discharge current is in the range of 500 mA to 900 mA.
Use the method to discharge two 5th batteries into the battery holder (E, F end in the figure), put the switch in the "discharge" position, and switch the switch K2 in the "4" position (complete with the charging circuit) Cut off). Connected to the power supply, LED2 is on, indicating that it is in a discharged state. After the discharge is completed, LED4 is on, and there is a buzzing sound, indicating that the discharge is completed. If the power LED4 is connected and a beep sounds, it indicates that the battery voltage is lower than 2V (refer to two batteries).
Charging If the rechargeable battery is 450--500mAh, then K3 will be in the 45-minute position. If the rechargeable battery is 700-800mAh, then K3 will be in the 66-minute position. If you charge 4 batteries, K2 will be in the "4" position, and the two will be in the "2" position. Kl is placed in the "Charging" position. When the power is turned on, LED1 is on to indicate that it is in the charging state; LED3 is on, indicating that it is in a fast state, and LED3 is off when the rapid charging is completed. Charging ends when the set time is reached.
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