Research On Battery Management Module With Cell Equalization Control

The required voltage in an EV/HEV is often as high as200V or more, so singlebattery cells are usually connected in series and parallel to meet the requirements.Due to the limitations of the manufacturing process and the operating circumstance,inconsistency in SOC of each cell will emerge by the use of the battery stack, whichwill decrease the practical available capacity of the battery stack, and will causeover-charge or over-discharge problems to shorten the battery life. Now, researchersat home and abroad proposed many kinds of equalization scheme, but due to the costand complexity, energy transfer equalizers have not generally been applied to thebattery management systems. However, traditional equalizer of energy dissipationtype has shortcomings such as low efficiency, low equalization current, and too muchgenerated heat.Based on thorough analysis of battery equalization technology at home andabroad, this paper proposed a battery management module with cell equalizationfunction. The equalization circuit was based on bidirectional full-bridage converter,and realized bidirectional energy transfer between single cell and battery string. Thisscheme has a higher flexibility. Feedback control make the equalization currentconstantly, so the equalizer can eliminate charge deviation precisely. Synchronousrectification has been adopted to improve efficiency, and protection circuit wasdesigned to cope with problems that may arise. In the formulation of a equalizationstrategy, first research the charge and discharge characteristics and SOC-COVcharacteristics of LFP battery, then proposed a equalization strategy using steadyOCV as equalization criterion. In addition to equalization function, the batterymanagement module also has the function of voltage, temperature, and insulationresistance measurement. Voltage measurement using LTC6803multicell battery stackmonitor IC, temperature measurement using DS18B20digital thermometer, andinsulation resistance was measured in accordance with the method ofGB/T19596-2004.Finally, experiments on the prototype including cell equalization, voltagemeasurement and temperature measurement were implemented through a batterypack of20parallel connection and12series connection. Resistors of various valuesinstead of insulation resistance were used to test the accuracy of insulation resistance measurement. The experiments show that the functions of the battery managementmodule can meet the requirements.