Design of solar power supply based on BQ24200

In order to ensure safe production, many devices need to be monitored online, such as power transmission lines and transmission towers of the power system. However, since these devices are installed in the wild, the monitoring devices cannot be powered by the mains, and the power supply problem becomes a problem. Solar power is the fastest growing green energy. As a clean and renewable energy, solar energy is increasingly valued by people. Using solar energy to power these monitoring devices is an effective way.
The solar power supply designed in this paper consists of photovoltaic cells, lithium batteries, lithium battery charging management units, and supercapacitors. When there is sufficient sunlight, the lithium battery is charged by solar energy and a stable voltage is output to supply power to the power device; when there is insufficient sunlight or rainy weather, the lithium battery is used as a backup power supply to supply power to the power device; at the same time, the use of supercapacitors, taking advantage of their high power density, greatly improves the load adaptability of the power supply (especially high-power pulsating loads).

1 Main features of BQ24200
BQ24200 is a single-cell lithium particle battery charging management chip for current limiting applications. It charges the lithium battery. If the battery voltage is lower than the internal low voltage threshold, it will first be pre-charged, and the battery will be charged by trickle charging, which can repair the deeply discharged battery. After the pre-charge is completed, the battery will be charged with the maximum current below the internal limited current until the battery is charged to the desired voltage value and the charging process is completed.
The basic features and parameters of BQ24200 are as follows:
1) The special design enables it to have a current limiting function;
2) When charging, it requires a very low voltage drop, which is suitable for the design of charging single-cell lithium-ion batteries with coke and graphite electrodes that require a low voltage drop;
3) Internally integrated 500 mA power transistor;
4) The internal voltage adjustment accuracy is 0.5%;
5) Battery insertion and removal detection;
6) With pre-charging function;
7) Automatically enter sleep mode when the input voltage is low;
8) With charging status indication or for host status detection interface indication;
9) With temperature monitoring function.

2 Internal structure and pin function of BQ24200
The internal structure of BQ24200 is shown in Figure 1.

Pin function:
IN: connected to the source of the internal P-channel power transistor.
OUT: connected to the drain of the internal P-channel power transistor.
BAT (Battery voltage sense): battery voltage sensor input, connected to the positive terminal of the battery.
TS (Temperature sense input): external battery temperature monitoring circuit input.
STAT (Charge status output): different charging status indications.
VCC (Supply voltage input): power supply input.

3 Overall design of power supply
This system uses solar cells to convert solar energy into direct current, which is stored in the battery through the charging circuit, and then through the discharge circuit to supply different DC loads according to different power supply requirements.
The overall design of the power supply includes: solar cell power supply circuit, lithium battery, lithium battery charging management unit, and supercapacitor. The circuit is shown in Figure 2.

3.1 Solar panels and lithium batteries
There are two main issues that must be paid attention to when using solar panels: one is the selection of power, and the other is the selection of voltage. The operating voltage of BQ24200 is: the minimum threshold voltage is 214 V, and the maximum operating voltage is 16.5 V. The output voltage of the solar panel must meet the operating voltage range of BQ24200 to avoid damage to it. According to needs, a solar panel with a rated output voltage of 9 V and an open circuit voltage of 10.8 V can be selected for power supply. The output power Wp of the solar cell is the output power of the solar cell under standard sunlight conditions, that is, the 101 standard defined by the European Commission, the radiation
intensity of 1 000 W/m2, the air quality AM1.5, and the battery temperature of 25°C. Due to the different sunshine hours and solar irradiation angles in different regions, it is necessary to select the appropriate capacity according to the application area.
The selection of battery capacity generally follows the following principles: First, on the premise of being able to meet the operation of the device at night, the energy of the solar cell module during the day should be stored as much as possible, and at the same time, it should be able to store the electricity that meets the continuous operation of the electrical device on continuous rainy days. If the battery capacity is too small, it cannot meet the needs of continuous operation of the device; if the battery capacity is too large, on the one hand, the battery is always in a state of power loss, which affects the battery life and causes waste.
3.2 Temperature Limitation
Real-time monitoring of battery temperature is achieved by monitoring the voltage of pin TS to ground. The voltage of TS pin is generated by thermistors RT1, RT2 and Vcc; according to the level of TS pin, high and low temperature protection is performed on the battery charging. When the level on TS pin is between its internal voltage VTS1 and VTS2, BQ24200 charges the battery, otherwise, charging stops. The
selection method of thermistor is as follows:
For thermistor with negative temperature coefficient:

For thermistor with positive temperature coefficient:

Where: RTc is the resistance value of thermistor at low temperature; RTH is the resistance value of thermistor at high temperature. 3.3 Supercapacitor Supercapacitor is a new concept energy storage device between traditional batteries and traditional electrostatic capacitors. Compared with traditional capacitors, it has larger capacity, higher energy, wider operating temperature range and extremely long service life. Compared with traditional batteries, supercapacitors have greater power density (more than 10 times), short charging time, fast energy release speed, long cycle life, and no pollution to the environment. Supercapacitors are added to this power supply circuit design to solve the problem of instantaneous high-power power supply. The circuit is simple and reliable and does not require maintenance. 4 Conclusion This article introduces the main characteristics and internal structure of the lithium battery charging management chip BQ24200 in detail. Using BQ24200 to charge lithium batteries, a solar power supply is designed, which is suitable for active electronic equipment working in the field. By using supercapacitors, the problem of instantaneous high-power power supply of monitoring terminals with GSM/GPRS modules, which are often used in current line monitoring systems, is solved. The designed solar power supply has been successfully used in the cable joint online monitoring system, and the power supply stability is reliable.