Using battery monitoring chip to realize multifunctional intelligent charger

1 Overview

The energy storage technology (battery) and energy conversion technology (charge and discharge control device) of electric vehicles are two major technical keys. The quality of energy storage and driving effects depends on the advanced level of energy conversion technology, and also determines the practicality, reliability, economy and market competitiveness of electric vehicles. In addition to the principle, design, material and process of the battery itself, another key technology that affects the capacity and life of the battery is the battery charging technology. The battery charging process has the greatest impact on the actual mileage life of the battery, while the protection of the discharge process, current limiting, energy recovery of the controller and the improvement of the efficiency of the driving part account for a smaller proportion. In other words, the vast majority of batteries are scrapped prematurely because of improper charging methods. Therefore, the research and development of high-level, high-performance and high-quality electric vehicle chargers is an important part of promoting the development of electric vehicle technology and industrialization in my country.

2 Design Concept of Multifunctional Intelligent Charger

Smart chargers are fundamentally different from traditional chargers. Most traditional chargers use constant voltage and constant current charging. At present, the better charging technology is only at the level of artificial mathematical simulation, that is, according to the pre-set mathematical model, it is approached in multiple stages. This technology is essentially constant current charging, but it adds a few more stages. This charging mode is better for a specific type of battery with good performance. There are many types of batteries, and the discreteness of the same type of battery is also very large, so it cannot meet the needs of charging multiple batteries. The use of a microprocessor-controlled smart charger can realize the versatility of the battery charger, and can realize the charging of different types of batteries using different charging modes.

We have designed a multifunctional intelligent charger based on a dedicated battery monitoring chip. This intelligent charger uses advanced computer control technology and unique measurement technology to measure the technical status of the battery before and during charging at any time, and uses artificial intelligence technology to conduct comprehensive analysis and judgment and control, so that electrical energy can be quickly converted into chemical energy to achieve the best charging effect. This dedicated battery monitoring chip is the DS2438, which is often used in battery management systems.

3 Application of measurement technology in multifunctional intelligent charger

DS2438 is a microelectronic chip of the BatteryMonitor series produced by Dallas Corporation in the United States in recent years. It is specially used for battery detection. The chip integrates temperature sensor, A/D converter, current integrator and other circuits, and has multiple functions such as measuring battery temperature, voltage, current and remaining power.

3.1 Measurement of battery voltage parameters during charging

DS2438 has a built-in 10-bit voltage A/D converter for measuring the terminal voltage of the battery. When DS2438 receives a voltage conversion command, the on-chip A/D converter will digitally convert the voltage on the VAD pin, and the conversion time is 4ms. The voltage measurement result will be saved in a two-byte voltage register. The measurement range of DS2438ADC is 0~10V, the resolution is 10mV, and the maximum range is 0~10.23V.

Therefore, the voltage parameters of the battery during the charging process can be easily measured through a simple resistor voltage divider circuit.

3.2 Measurement of battery current parameters during charging

DS2438 has a built-in current A/D converter to measure the current of the battery pack. The sampling resistor RSENS is connected to the charge and discharge circuit, and DS2438 measures the current flowing through the battery by measuring the voltage on RSENS. The DS2438 current A/D converter automatically performs a measurement conversion every 27.46ms, and the measurement result is saved in the internal current register. The highest bit S sign bit can be used to determine whether the battery is charging or discharging.

It is worth noting that the DS2438 chip is suitable for weak current loads such as laptop computers. If it is used in the charging and discharging system of electric vehicle batteries, necessary anti-interference measures such as active filtering and digital filtering should be adopted to effectively eliminate the interference of spike current and high-frequency noise signals, so that the DS2438 chip can accurately measure the current parameters of the battery in the charging and discharging circuits.

3.3 Measurement of the remaining battery power during charging and discharging

The DS2438 uses an integrated current accumulator (ICA) to track and measure the remaining battery capacity. ICA is a register that accumulates all currents flowing into and out of the battery after the battery pack is put into use. Therefore, the value of ICA can be expressed as the remaining battery capacity.

As mentioned above, the battery current is obtained by measuring the voltage on the external resistor RSENS every 27.46ms. This value is added or subtracted from the ICA register according to whether it is positive or negative. ICA is an 8-bit binary counter that integrates the voltage on the external resistor RSENS measured each time. The measurement accuracy is 0.488 2mVhr.

Since the DS2438 has the function of detecting the remaining battery power, a charger with better performance can be designed using this function. Most general battery chargers do not measure the battery power during the charging and discharging process. During the charging process, the amount of power in the charged battery can only be determined by measuring the voltage value at both ends of the battery pack. A good charger should have the function of real-time monitoring of the battery power during the charging and discharging process, and can decide to enter the corresponding charging state according to the amount of charge during the charging process. Especially at the beginning of charging, it should be able to control the corresponding charging method according to the discharge depth of the battery (the amount of remaining power). For example, for deeply discharged batteries, a small current repair charge should be used first, and then the normal charging mode should be used. For those who only need to replenish the power lost due to self-discharge, only trickle charging is required.

3.4 Measurement of battery temperature parameters during charging

During the charging process, the temperature of the battery is a parameter that needs to be paid close attention to. For example, the voltage of a lead-acid battery has a negative temperature coefficient of -4mV/℃. For a charger that works ideally at an ambient temperature of 25℃, the battery cannot be fully charged when the ambient temperature drops to 0℃. When the ambient temperature rises to 50℃, the battery will be severely overcharged and its life will be shortened. Therefore, a good charger should have a temperature detection function and be able to adjust the conversion voltage values ​​during the charging process according to different ambient temperatures. At the same time, the charger should have a temperature protection function. During the charging process, when the battery temperature exceeds the specified temperature range, the charger should immediately stop charging or enter trickle charging.

Place the DS2438 chip close to the battery under test. The DS2438 can measure the temperature of the battery at any time through the temperature sensor integrated in the chip. The measurement result is stored in the internal temperature register and transmitted to the microcontroller through the single bus. The temperature range measured by the DS2438 is -55℃~125℃, with a resolution of 0.031 25℃.

4 Intelligent Charger Circuit Design

According to the design concept of the multifunctional intelligent charger mentioned above, a low-cost, high-performance electric vehicle battery charger is designed, which uses a microprocessor as the core control component, a dedicated battery monitoring chip DS2438 as the detection core of the charging and discharging system, and integrates the battery management system with battery charging. Its structural block diagram is shown in Figure 1.

Figure 1 Schematic diagram of charger principle structure From the schematic diagram, we can see that the charger is mainly composed of the main charging circuit, synchronous pulse generating circuit, thyristor trigger driving circuit, DS2438 measuring circuit, single-chip computer system and LCD display. Among them, the DS2438 measuring circuit, single-chip computer system and LCD display are also used for the battery management system when the battery is discharged, and the voltage, current, power and temperature of the battery are measured and displayed in real time. After the discharge is completed, the remaining power of the battery will be saved in the DS2438 ICA register. In the charging system, DS2438 monitors the above-mentioned parameter values ​​in the battery charging process in real time, and uploads them to the single-chip computer through the single bus. The single-chip computer analyzes and judges the detected parameter values ​​in real time, controls the output of the thyristor trigger pulse, controls the conduction angle of the thyristor, and thus controls the charging voltage and charging current to ensure that the charger is neither overcharged nor undercharged according to the selected charging mode, and has overheat protection, and charges safely and reliably.

In order to realize the synchronous triggering control of thyristors, it is necessary to obtain the synchronous pulse signal of the grid voltage. Therefore, the system is designed with a synchronous pulse generating circuit.

This circuit enables the external interrupt INT0 of 8031 ​​to obtain a negative jump valid interrupt request signal near the zero point of each cycle of the grid voltage. Therefore, INT0 interrupts once every cycle. In the interrupt service program of INT0, set the T1 timer Xms,0

In the T1 timing interrupt service program, first control P1.7 to generate a negative pulse, then set T1 timing to 10ms in order to obtain a 100Hz trigger pulse, and then set P1.7 to generate a negative pulse in the second timing interrupt service program. In this way, the 100Hz pulse signal output by P1.7 triggers the thyristor to conduct after passing through the photoelectric isolation and triode amplifier circuit. Thus, a pulsating and adjustable charging voltage can be obtained at both ends of the battery. The waveforms of each point are shown in Figure 2.

Figure 2 Waveform diagram of each point of the charger

5 Features of multifunctional smart charger

The multifunctional intelligent charger is an intelligent charging device controlled by a microprocessor. It uses the microprocessor as the core control component and DS2438 as the detection core of the charging system, realizing digital sampling, digital processing and full digital display of various parameters during the charging and discharging process.

At the beginning of charging, the charger performs a series of tests on the battery under the control of the microprocessor to find out the basic parameters of the battery, confirm the remaining power, voltage level and required charging method of the battery.

During the charging process, under the control of the core control component microprocessor, the charger continuously tracks and tests the charged battery. The microprocessor continuously calculates the test results and adjusts the intensity of the next charging current based on the calculation results.

At the end of charging, the charger determines the saturation of the battery by several factors such as the battery power, voltage, and current changes, and decides whether to end charging based on the set parameters. During the entire charging process, all judgments are automatically completed by the microprocessor.

The multifunctional intelligent charger can display important charging parameters digitally in real time. During the entire charging process, the charger displays information such as charging current, charging voltage, charging time, charged ampere-hours, battery temperature, etc.

Due to the use of microprocessor control, the multifunctional intelligent charger also has strong protection and alarm functions.

The multifunctional intelligent charger uses the more traditional and reliable thyristor voltage and current control plus transformer as the power component. This structure ensures more reliable electrical performance and ensures safe isolation of high voltage.

This paper designs a high-performance multifunctional intelligent charger based on a dedicated battery monitoring chip. The charger uses an advanced and highly integrated intelligent module to monitor and control the battery charging process in an all-round way, ensuring that batteries of different levels can be charged in the proper way to prevent the battery from reducing its service life due to improper charging, greatly improving the performance of the charger.