Research On AC Heating Of Lithium Ion Batteries At Low Temperature Based On Switched Capacitors

The intensification of energy crisis and environmental pollution has promoted the establishment of green transportation and the development of electric vehicles.As the core of electric vehicle,automotive power battery is the key part of electric vehicle,and its performance directly determines the safety,power and driving mileage of electric vehicle.Lithium-ion batteries are widely used in electric vehicles because of their excellent performance.However,the current performance of lithium-ion batteries needs to be further improved.Especially in low temperature environment,the performance of lithium-ion batteries becomes worse,which is manifested in the limitation of battery charge and discharge,and their discharge capacity is weakened.With the decrease of temperature,the internal resistance of batteries increases.The discharge voltage of batteries will soon reach the cut-off voltage,the discharge capacity of batteries will be less,and the range of electric vehicles will be shorter.Charging batteries in low temperature environment will shorten the constant-current stage,lengthen the constant-voltage stage and increase the charging time,but the amount of electricity charged is lower.At the same time,there is lithium plating phenomenon in low-temperature charging,which will cause irreversible damage to lithium-ion batteries and increase the risk of safety accidents.In order to improve the utilization effciency of lithium-ion batteries in low temperature environment,batteries need to be preheated.The existing battery preheating methods have some problems,such as large volume,external power supply,limited range of use,and further improvement of heating efficiency.In this paper,a vehicle-mounted switched capacitor-based battery heating topology is proposed.It uses switched capacitors as energy storage components,and uses the alternating current generated between capacitors and batteries to heat the batteries internally.The heating circuit has the advantages of simple structure,small size and low cost.And it can realize the rapid and uniform heating of lithium ion batteries.Firstly,the performance of batteries in low temperature environment is analyzed,and some current heating methods are introduced.Then,the heating topology based on switched capacitor is proposed.At the same time,the proposed circuit is simulated in MATLAB to verify the voltage and current wavefonns.Next,the hardware and software of the circuit are realized,and the experimental platform of the battery heating system is designed.At the same time,the current formula in the circuit is deduced.And the RMS of the current in the heating circuit is affected by the switching frequency of the circuit.The circuit switching frequency frequency corresponding to the maximum heating current in the heating circuit is obtained by particle swarm optimization.Finally,the experimental results show the effectiveness of the proposed heating circuit.and the circuit can get the fastest heating speed by heating the battery at the heating frequency obtained by particle swarm optimization.Experiments show that the heating circuit has a faster heating rate and can achieve 122S heating up from-20 ? to 0 ? at the fastest time.The loss of battery is small and the loss of battery power is 5.47%/5.38%.The heating effect is good.It will not cause inconsistency of battery power and thermal inconsistency,and will not cause damage to battery life.At the same time,the heating circuit can realize the heating of batteries within the normal range of power consumption,that is,the batteries can be heated regardless of the high or low residual power of the batteries.