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Abstract: The fast charging of battery pack is realized by using PIC16C74 microcontroller, and three fast charging schemes are compared. Keywords: single chip microcomputer, fast charging, battery pack With the increasing demand for portable products, the application of rechargeable batteries is becoming more and more widespread. Many companies have produced fast charging control chips, such as Maxim's Max712, 713, 846, 2003; AD's ADP3810, 3811; Liner's LT1505, etc., but because our products have been finalized, we cannot add new control chips. The portable instrument we developed uses a rechargeable battery pack (8 NiMH batteries in series), which uses 0.1C charging for 16 hours. The charging time is long and inconvenient to use. For convenient use, it is necessary to add a fast charging function. With almost no increase in cost, we used the AD function of the original PIC16C74 chip to develop a fast charger. PIC16C74 is a low-power, high-performance, CMOS, 8-bit microprocessor with an 8-level stack, multiple internal and external interrupt sources, and an advanced RISC architecture. Compared with other similar 8-bit microprocessors, it can achieve 2:1 code compression and 4:1 speed increase. PIC16C74 contains 33 I/O ports, 3 timers/counters, serial communication port, synchronous serial port, 8-channel high-speed 8-bit A/D, 4KX14ROM, 192 bytes RAM, watchdog timer, and the output port has strong drive capability. The schematic diagram of the fast charger is shown in Figure 1. Where: VAD1 is proportional to the battery voltage VAD2 is proportional to the charging current Change terminal controls the charging current DC+, DC- connected to external power supply BAT+, GND connect to battery The CPU collects battery voltage and charging current signals and controls charging through the Charge terminal. The battery pack is composed of 8 1.2Ah NiMH batteries connected in series. There are three specific charging solutions: 1. Limited time charging The charger is a DC13V 500mA power supply. After continuous charging for 3 hours, disconnect the charging power supply. After testing, the discharge capacity of the battery pack is about 1.15Ah (discharge current is about 1A), and the battery life is about 200 times. In mass production, the device should have good consistency. 2. Continuous charging Within 2-5 minutes of the start of charging, the MOS tube is turned on intermittently to pre-charge the battery, and then the MOS tube is always in the on state (a maximum charging time can be set for safety). When the battery voltage is greater than 12V, the MOS tube is turned on intermittently, and charging ends after charging to 12.8V (a maximum charging time is set). After testing, the discharge capacity of the battery pack is about 1.2Ah (discharge current is about 1A), and the battery life is about 300 times. The charger is required to have a certain consistency during mass production. 3. Intermittent charging Within 2-5 minutes of starting charging, the MOS tube is turned on intermittently to pre-charge the battery, and then fast charging begins. During fast charging, the charging current is less than 1.2A by controlling the on and off of the MOS tube. When the battery voltage rises too quickly, the charging current should also be reduced. Once the battery voltage stops rising, fast charging ends, and then trickle charging ends after a period of time. If the battery voltage is still less than 8V within 1 hour, charging stops and an alarm sounds. At this time, check whether the external charging power supply, charging circuit and battery pack are damaged. The entire charging process can be controlled within 1.5 hours to 3 hours as needed. The test results are shown in Table 1.
Table 1 Intermittent charging test results | Charging time (h) | Discharge capacity (Ah) | Battery life (times) | | 1.5 | 1.2 | Approx. 300 | | 3 | 1.22 | More than 300 | The comparison of the three charging schemes is shown in Table 2.
Table 2 Comparison of charging schemes | | Solution 1 | Solution 2 | Solution 3 | | Charging time | 3 | 3 | 1.5-3 | | Discharge capacityAh | 1.15 | 1.2 | 1.2-1.22 | | Battery Life | Shorter | middle | Longer | | CPU time | less | middle | More | | External current requirements | Strict | middle | Loose | | Security | Poor | middle | better | | Complexity | Low | middle | high | Conclusion During the specific design, the charging scheme should be selected according to different focuses.
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