Study On High Speed Battery Balancing Method Based On Switched Capacitor

In today’s world,rechargeable batteries as energy storage carriers are widely used in uninterruptible power supply,artificial satellites,electric vehicles and other fields.Lithiumion batteries are considered to be one of the most attractive rechargeable batteries because of their high energy density,low self-discharge rate and no memory effect.Due to the low voltage and small capacity of a single lithium battery,in most cases,multiple batteries need to be connected in series or parallel to meet the load demand.However,different cells have subtle differences in electrochemistry,capacity,internal resistance,etc.The non-uniformity of battery parameters in the battery pack will lead to the overcharge or overdischarge of the battery cell,which will reduce the capacity and life of the battery pack.In order to extend the service capacity and life of the battery pack,the battery balancing technology is indispensable.At present,there are two main battery balancing methods: passive balancing and active balancing.Passive equalization method has the advantages of small volume,low cost and easy implementation.However,energy dissipation and heating are the main disadvantages of this method.The law of active equalization is to use non-dissipative energy transfer elements to transfer energy from the high voltage battery to the low voltage battery.In these active battery equalization method,based on the switch capacitance balance method has the advantages of small volume,easy to control,easy to implement,thus has broad application prospects,however,with the increase of unbalanced cell number,the traditional switch capacitance equalizer equilibrium speed is often low,cannot satisfy the requirement of battery fast equilibrium.Therefore,it is very important to study a kind of Switched Capacitor Equalizer with fast equalization speed,simple control and stability.Firstly,this paper analyzes a variety of traditional equalization methods based on switched capacitor batteries,and finds out the existing problems of each equalization method.At the same time,the working principle of the equalization circuit based on switched capacitor is studied.Several factors affecting the equalization speed are determined by deducing the formula of charge and discharge current.Aiming at one of the influencing factors,the voltage difference,a fast equalization circuit topology was proposed to improve the equalization speed by increasing the charging and discharging pressure difference.The equalization circuit topology uses a voltage detection device to select the maximum voltage battery to discharge other low-voltage batteries,and its system control is simple,only a pair of complementary PWM wave signals are needed to control the switch on and off.On this basis,by studying the working principle of the multi-cell rapid equalization circuit proposed,the working process of the equalizer in each equalization stage was analyzed,and the current and voltage expressions of the battery and capacitor in each stage were deduced.At the same time,the loss and efficiency formulas of the equalization system were deduced according to the voltage and current formulas obtained.Moreover,in the light of the another variable speed balanced-system switching frequency parameter optimization,through constructing the battery and capacitor system simulating the actual working process of the equalization circuit,charge and discharge system from two aspects of system switching frequency of electric current and efficiency are analyzed,and the influence of the parameters are optimized configuration of switch frequency,The switching frequency which maximizes the system current effective value and system efficiency is selected.The optimal parameters were used to build a simulation model in MATLAB /Simulink platform for verification.At the same time,the equalization effect was compared with the existing four methods based on switched capacitance under the same conditions.The simulation results show that the proposed fast equalization circuit between multiple batteries improves the equalization speed.Finally,an experimental platform of battery pack equalization system composed of four lithium battery monomers was built in this paper.The experimental results show that the proposed multi-battery rapid equalization method can effectively improve the battery equalization speed,which is consistent with the theoretical analysis and proves the effectiveness and feasibility of this method.