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Li-ion battery With small size, light weight, high voltage, power, big, low self-discharge laptop battery and long service life, etc., has gradually become the mainstream of power battery. However, due to lithium-ion batteries have obvious non-linear, inconsistency and time-varying characteristics, and therefore the need for a certain application management. In addition to the charge and discharge lithium batteries demanding, as there have been charging, over-discharge, discharge current is too large, or a short circuit when the lithium battery temperature will rise, seriously undermine the performance of lithium batteries, resulting in shortened battery life. When the lithium batteries used in power equipment in the series, due to the single-cell lithium battery inconsistencies between the internal characteristics will lead to the lithium battery charge and discharge inconsistencies. A deterioration in performance, the entire battery of behavioral characteristics of the battery will be affected by this restriction, reducing the overall battery performance. To enable lithium batteries to maximize its advantages to extend the service life of lithium batteries must be in charge and discharge when real-time monitoring, providing over-voltage, over current, temperature, energy balance between the protection and battery.

Paper designed power management system for lithium batteries installed in the internal lithium battery pack in order to control the microcontroller core of the various sections in the realization of lithium battery energy balance, it is also can be achieved overcharge, over discharge, over current, temperature protection and short circuit protection. LCD displays battery pack through the various states, and can be reserved through the communication port to read the history of Latitude D510 lithium battery performance status.

Overall system design

Power lithium battery charger intelligent management system mainly consists of modules, data acquisition module (including voltage, current, temperature data acquisition), balanced module, power calculation module, data display module and storage communications module. System block diagram shown in Figure 1.

Figure 1 System block diagram

Microcontroller-based controller, the entire system by collecting current information, determine the battery pack is charging, discharging or in idle status, and whether there is over-current phenomenon, and its state accordingly handled. The battery voltage of each section is collected analysis, the system determines whether to activate the equalizer module to conduct the entire battery pack energy balance at the same time to determine whether there is the phenomenon of overcharge or over-discharge. Temperature of the collection is mainly used for system over-temperature protection. The entire system working status, current, voltage of each section, the remaining capacity, and temperature information to go through the liquid crystal display module real-time display. The following methods Latitude D505 of its implementation of each module are introduced.

Microcontroller ATmega8

The system uses micro-controller is the United States ATMEL Corporation introduced a high-performance 8-bit microcontroller ATmega8. The microcontroller with high-end AVR microcontroller series, all the performance and features to support online programming (ISP), only one can be made of the download cable can be carried out SCM systems. ATmega8 microcontroller which has 6-channel A / D converter channels, including 4-channel 10-bit accuracy can be directly used in the design of the battery voltage measurement. ATmega8 of the performance makes it highly adaptable, flexible, low cost and high-performance embedded micro-controller, especially suitable for use in the development phase.

Principle and Implementation of Charge Latitude D500 Control Module

Lithium battery charging method is based on routine pre-charge, constant current, constant voltage in three phases, the timing diagram shown in Figure 2.

Figure 2 Timing diagram of conventional charging method

As the management system is put together with the battery inside the battery and charger is an external, so if communication interface and an external charger to increase the formation of closed-loop control, it will make the management system's general reduced. To achieve high interoperability, so that management systems and external charger working alone, the battery management system using intermittent charging method shown in Figure 3.

Figure 3 Sequence diagram intermittent charging method

Intermittent charging method is in the pre-charge and maintain the stage and through intermittently opened and closed charging circuit, the equivalent to change the size of the charge current, thereby achieving the pre-charge and maintain the state required a small current, in the normal charging large currents performance requirements.

With the management system in the battery pack to recharge, you need to match the constant voltage power supply with external Inspiron 600m battery charger, the value of U, its constant pressure requirements for: U = 4.2V × N + loss of voltage, where N is the battery sections number.

Limit value is the power of conventional lithium battery charging current is 0.3C (C is battery capacity), pre-charging must be carried out during system initialization, and then follow the intermittent charging method automatically recharge the battery pack.

Principle and Realization of voltage sampling module

Lithium-ion battery charging control requirements of its terminal voltage below 4.2V, in order to prevent damage to the battery overcharge, which calls for charging when the real-time detection of single-cell lithium battery voltage. Management system uses the voltage as shown in Figure 4, sample testing program. The working principle is: first, the single-pole double-throw switches K1, K2 hit up voltage measurement files, and through MCU-controlled, multi-channel switch Kn-1, Kn-2 (n = 1,2,3,4,5,6 , 7), simultaneously the capacitance received respectively at both ends of each single cell, so that capacitor charge to the capacitor voltage is equal to the measured single-cell voltage; then disconnect MCU-controlled, multi-channel switch Kn-1, Kn-2, At the same time closing the switch K3 and K4 access to A / D converter measurements. The program can be used directly within the microprocessor 10-bit A / D converter does not need to join another A / D chip, to save design cost. The actual circuit used in the Inspiron 510m analog switch relays.

Equilibrium Theory and Design of the module

Power batteries are a number of single cells in series to form a battery module, due to the internal characteristics of the battery differences between individuals, a number of charging and discharging, the battery pack will be an imbalance, seriously affecting the efficiency of battery power and security. In addition, the battery charging and discharging process of over-charge, over discharge current is too large, high temperature phenomena such as the battery will exacerbate the differences between the properties, which led to between a single lithium battery capacity, voltage imbalance in the performance of the final battery pack as a whole led to a sharp decline, and some characteristics of the acceleration of cell damage. Therefore, lithium batteries must be used in combination to solve the various single cell in the battery pack in the balance.

Sections of the battery pack in the balance of power balance can be used resistors, capacitors balanced, transformer balanced and many other programs. Because this management system is the driving force for large-capacity lithium battery, the use of a resistance balance, balanced fast, but there will be too much energy wasted; capacitor energy balance, while not an extra, but the balance of current generally small, it is competent difficult balance between power Inspiron 500m lithium battery. Therefore, the balance of both efficiency and speed of the module using a transformer balanced program. Used directly in a specific design DC / DC switching power supply module. Since switching power supply modules with power consumption, high efficiency, small size, light weight, etc., be used directly as a balanced module is a good choice. In the specific use, in accordance detected in all single-cell voltage to determine whether the need for batteries for energy balance. If necessary, closing balance master switch K5, switches K1, K2 down to reach a balanced profile, with the overall energy of the battery pack voltage is the lowest section of the balance of the additional battery charge until the cell voltage difference between the value of the scope of the system requirements for inside. Schematic diagram shown in Figure 4.

Figure 4 voltage sampling, balanced charging schematic

Realization of current sampling

Current is the battery capacity estimation of key parameters, so the precision of current sampling, anti-jamming capability and linearity error of the requirements are very high. In this design, use of closed-loop current sensor LEM's LTSR25-NP, as shown in Figure 5,

Figure 5 LTSR25-NP-kind map

The device has excellent accuracy, good linearity and the best response time. Rated current of 25A, maximum 80A current measurement to meet system 312-0305 design requirements. The current sensor may be the charge and discharge current is converted to 0V ~ 5V voltage signal sent to the microcontroller's 10-bit A / D converter to convert measured after charge and discharge current measurement accuracy of 0.2A. Its operator characteristic curve shown in Figure 6.

Figure 6 Current Sensor LTSR25-NP operating characteristic curve

Figure to VREF as a reference point voltage, the default is 2.5V, IP as being measured currents.

The realization of temperature sampling

Battery management system, the temperature detected by DALLAS Semiconductor, Inc. is the U.S. production of digital temperature sensor DS18B20. It is a monolithic structure, no external A / D converter output 9 to 12-bit digital. Communications using a single bus protocol, for DS18B20 the various operations can be done via a data line. Because each DS18B20 contain a unique serial number, so that each bus can simultaneously connect to multiple DS18B20, which makes connections simple DS18B20, system design flexibility, suitable for multi-point temperature measurement system, especially with a single film machine constitutes a combination of temperature 312-0306 detection and control systems.

Realization of display module

LCD displays use DM12864M Chinese Graphic LCD Module. The module can display characters and graphics, built-in 8192 Chinese characters (16 × 16 dot matrix), 128 characters (8 × 16 dots), and 64 × 256 dot-matrix display RAM. The main technical parameters and display characteristics are as follows:
Power Supply VDD: 3.3V ~ 5V (built-in boost circuit, no negative pressure);

Display: 128 × 64 line;

LCD type: STN;

With the MCU interface: 8-bit or 4-bit parallel / 3 serial;

A variety of software functions: cursor display, screen shift, custom characters, sleep mode.

The system uses a serial interface, through the LCD module can display the total voltage of the battery pack, each single cell voltage, charge and discharge current, charge and discharge time, the working temperature, and the remaining capacity and so on.

The realization of data storage communication module

Li-ion battery management system in F5136 lithium battery charging and discharging process of the charge and discharge information, including sections of the battery voltage, charge and discharge current, temperature, battery power and other writing by sampling real-time saved in Flash memory chips SST25VF020. When necessary, through the serial port to communicate with the Host PC, stored in the Flash of the historical data read PC.

SST25VF020 is SST25VF series of Flash memory chips. The chip has the following characteristics: total capacity of 2M; single power supply to read and write operations, the working voltage is 2.7V ~ 3.3V; low-power, operating current 7mA, waiting for current 3μA; clock frequency up to 33MHz; data can be stored 100 years; the package for the SOIC and the small size WSON package. The practical application of the circuit as shown in Figure 7.

Figure 7 FLASH memory circuit schematics

Software Design

The system's software design using C language SCM completed, mainly including single voltage measurement procedure, current testing procedures, temperature testing procedures, energy balance procedure, charge management program, LCD display program and data storage communication program, and several other program modules . Its main flow Inspiron 710m chart shown in Figure 8.

Figure 8 Main Flow

Voltage measurement procedure

Through the microcontroller 10-bit A / D converter module measuring single cell voltage. In order to improve measurement accuracy, the software a "screening value of the average" software filtering method. In the simulation of each cell to measure the amount of time, continuous measurement many times, and then weed out the highest and lowest values, and then the remaining measurements averaged to obtain the best measurement results. Then the calculation of the voltage to obtain the Inspiron 700m battery voltage. In the voltage measurement is complete, run the "bubble sort" process, for all of the battery voltage to sort, marking the lowest, the maximum battery, in order to balance the module service.

Charge Manager

By detecting the cell voltage to determine a single battery charge in which stage and the use of pulse-width modulated output of microcontroller (PWM) to control the MOSFET in order to achieve the pre-charge phase of the charge and maintain a small current charging phase of the pulse charge. When the battery charge is detected after the auto-disconnect the charging circuit. Charging management module by detecting the voltage, current, temperature value to determine if the battery is working in the normal state, as there have been voltage, over-current or over-temperature phenomena, immediately close the charging and discharging through the MOSFET circuit, and the failure prompts lights lit.

Conclusion

This is designed to power lithium battery pack provides the intelligent management system, through which the system battery pack provides a variety of protection and control of energy balance to maximize the overall performance of lithium batteries. The prototype developed by seven 50AH Li-ion battery pack in series Lei days on a trial basis, to achieve the greatest balance of current 2.5A, balance of about 80% efficiency, performance, power lithium battery pack fully meet the requirements of energy balance, with a very good application prospects 5081P and promotion of values.