introduction
In view of the coexistence of nickel-cadmium batteries and lithium batteries in the market, the charger designed in this paper can charge these two kinds of batteries, adopt pulse charging mode for nickel-cadmium battery packs, and adopt constant current charging mode for lithium battery packs. Designed with different mechanisms of the battery, it is truly a dual-purpose machine. This is the innovation of the charger, and it is also a difficult design. The widescreen LCD of the charger can simultaneously display the charging status of the four groups of chargers, and can also display the parameters of the battery on a group of chargers separately, so as to achieve real-time monitoring of the battery charging process.
Overall system design
The system design goals are:
1. It can charge and discharge four groups of 8.4V lithium ion batteries or 9.2V nickel cadmium batteries at the same time.
2. Can communicate with the chip in the battery pack to determine the chemistry of the battery.
3. For batteries of different chemical properties, the corresponding charging method will be adopted.
4. It can communicate with the chip in the battery pack to obtain parameters such as voltage, charging current and capacity of the battery pack.
5. The charger has an LCD that displays various data of the battery.
The functional block diagram of the charger is shown in Figure 1.
Figure 1 System overall design structure
System hardware design
Design and implementation of the master control unit
The master control unit is composed of the microcontroller PIC16F873 and the keyboard control chip ZLG7289A. The main task is to communicate with each charging unit and process user input and LCD display information. The keyboard control chip is responsible for the control of 6 buttons and 12 LEDs here.
The ZLG7289A communicates with the microcontroller via the SPI bus in both directions. The main control unit queries each charging unit once a second to obtain information about the current charging unit, such as battery, battery nature, battery voltage, and the like. It is then displayed to the user by the LCD module.
Design and implementation of charging unit
TP8002 lithium ion battery
Charge control chip
The TP8002 is a highly efficient independent switch mode Li-Ion battery charge controller. The controller is available in 4.2V and 8.4V versions.
The TP8002-8.4 has a 500kHz switching frequency and is a high efficiency current mode PWM controller. By driving an external P-channel MOSFET, it can deliver 4A of charge current with up to 90% efficiency. The output voltage is set to 8.4V, the final floating voltage is 1% accurate, and the charging accuracy is 5%. In addition, the device can operate on a variety of wall adapters ranging from 9V to 22V. Compared to hysteretic topology chargers, the TP8002-8.4's fast-running frequency and current mode architecture enables the use of small inductors and capacitors.
Lithium ion / nickel cadmium battery dual purpose
Overall design of the charging unit
From the analysis of the TP8002 from the front, the chip is a charging controller for lithium-ion batteries. To realize the charging of nickel-cadmium batteries, the following problems need to be solved: First, the TP8002 monitors the battery voltage to ensure that the battery voltage does not exceed 8.4V. But for nickel-cadmium batteries, the charge cut-off voltage can reach 9.2V. Secondly, when the charging of the nickel-cadmium battery is coming to an end, the battery needs to be trickle charged with normal current of 30% and 10%. Therefore, the second problem that needs to be solved is how to control the current of the constant current charging. In addition, the charging of nickel-cadmium batteries should use pulse charging. That is, in the cycle of 1 s, 95% of the time is used for charging, 1% of the time is used for discharging, and the rest of the time is not charged or discharged. Finally, how to determine whether a battery is a lithium-ion battery or a nickel-cadmium battery, because if the lithium-ion battery is misidentified as a nickel-cadmium battery, the charging voltage will be higher than 8.4V, which is very dangerous for lithium-ion batteries, and will Nickel-cadmium batteries are misidentified as lithium-ion batteries, which may cause the battery to be undercharged. Therefore, a very low false positive rate must be guaranteed.
According to the working principle of TP8002, this part designs a circuit that can charge the lithium-ion battery with constant current-constant voltage and pulse-charge the nickel-cadmium battery. The overall functional block diagram of the charging unit is shown in Figure 2. Among them, the signal conditioning circuit enables the charger to charge both the 8.4V lithium battery and the 9.2V nickel-cadmium battery, and also controls the charging current.
Figure 2 Overall functional block diagram of the charging unit
The microcontroller is used to control the working state of the TP8002, and the discharge circuit allows the charger to pulse-charge the nickel-cadmium battery.
The microcontroller communicates with the smart battery through a certain communication protocol (HDQ16) to determine key parameters such as its capacity and chemical properties.
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Shenzhen MovingComm Technology Co., Ltd. , https://www.movingcommtech.com