Research On Lithium-ion Batteries Management System For Low Speed Electric Vehicle

Faced with the problem of the dual high pressure of resources and environmental, electric vehicle which is energy-saving and emission-reduction has become special supported and developed industry in country’s long-term development plan. Power battery pack is the energy source of electric vehicle. In order to keep electric vehicle working reliably and safely, a full functional battery management system is the necessary equipment of electric vehicle.Low-speed electric vehicle has been widely spread in market. In order to improve battery reliability, extend battery life and increase battery mileage, a lithium-ion batteries management system of low-speed electric vehicle was designed in this paper. The management system consists of the main control module and the slave control module. The STM23F103VET6 microprocessor is the core of the main control module, which collects battery voltage and temperature through the slave control module. Charge or discharge current is collected through current sensor ACS578 LCB. Battery’s State of Charge(SOC) is estimated using battery’s voltage, current and temperature. According to battery’s voltage and current, protection measure for battery pack is activated when overcharge or overdischarge or overcurrent is detected. The high precise processor LTC6804-2 is used in the slave module to gather battery’s voltage and make battery passive equalization. At the same time, according to the voltage of single battery, the microprocessor controls the independent charging unit to equal battery actively. Active and passive equalization methods are combined to achieve efficient equalization control of the battery pack. The slave module also uses high sensitive NTC thermistors to collect each battery’s temperature. According to battery’s temperature, the microprocessor controls the fan on-off for thermal management.In order to check the status of battery pack, a monitoring software in PC is designed based on the Visual Studio 2010 and C# programmer. The monitoring software communicates with the battery management system through Modbus-RTU protocol and RS232 serial port. The working status of the battery pack is displayed and stored. The protection voltage for over charge and over discharge, protection current for overcurrent and balancing voltage all could be set in PC. In order to get high precise SOC, an advanced Extended Kalman Filter algorithm was presented based on the Extended Kalman Filter algorithm and Ampere Hour Counting algorithm and Open Circuit Voltage algorithm. The algorithm is used to correct and compensate current and temperature for SOC estimation. It makes up of the lack of real-time estimation of shortage of the Open Circuit Voltage algorithm, and improves the estimation accuracy of Ampere Hour Counting algorithm.By charging and discharging tests on a 24 LiFePO4 battery pack, the system’s function and precision is fully tested. The result shows that the error for single battery voltage is less than 5mV, the relative error for battery pack voltage is 0.5%, the error for current is 1%, the error for temperature is less than 1°C, the management system could measure the battery voltage, current and temperature precisely. In battery equalization control, voltage difference among batteries is kept below 15 mV by equalization control. The effect of equalization control is superior. The error for SOC estimation is less than 5%, proving the advanced Extended Kalman Filter algorithm could estimate battery’s SOC accurately.