| When the lithium battery discharges in the low temperature environment(<0℃),its usable capacity drops sharply and the discharge voltage decreases.And the large current discharge at low temperature will cause a certain degree of damage to the battery.So the battery needs to be pre-heated in the low temperature environment.The difference among the batteries causes the capacity of the battery to decrease after being packed,and a balancing system is required to make full use of the battery pack capacity.This paper studies the low-temperature preheating technology and the active equalization technology of the battery pack,introduces super capacitors,and develops a system that takes into account the low-temperature preheating and active equalization requirements of the battery pack.The following research work has been conducted in this paper around a low-temperature preheating system for a battery coupled with a active equalization function.In view of the ultracapacitor discharge capacity is not affected by the low temperature environment,but the specific energy is small and the cost is high;while the lithium battery has higher specific energy and lower cost,but the discharge capacity at the low temperature is seriously degraded.This paper synthesizes the characteristics of both and proposes the design idea that increasing small-capacity power battery used as a preheating battery,and a super capacitor is used to preheat the battery,and the preheating battery is used as a for heating the battery pack.Based on the purpose to achieve a temperature rise from-25 ℃ to 0 ℃ within 30 minutes of the power battery pack,the capacity and heating power of the preheat battery pack were calculated and determined.The experiment proved the rationality of the selection.Based on the purpose to achieve a temperature rise from-25 ℃ to 0 ℃ within 10 minutes of the preheat battery pack.Combined with the discharge characteristics of the super capacitor,the average heating power and the capacity of the super capacitor are calculated,and the experiment proves the rationality of super capacitor selection.According to the temperature rise curve of the preheat battery,the selection method of the supercapacitor is discussed.According to the quantity characteristics of the selected supercapacitor,a capacitive capacitance equalization topological structure capable of performing energy transfer between any two cells in the battery pack is designed.In order to ensure sufficient equalizing current and improve the efficiency of equalization,a special circuit is designed so that the super capacitor can be connected in series or in parallel according to requirements to achieve the function of a step-up and step-down circuit.The battery equalization was performed with the cell terminal voltage as the equalization variable under standstill conditions.According to the SOC0-OCV curve,the equalization thresholds under different SOC0 were proposed so that the difference between the maximum SOC0 of the power cells after equalization was less than 3%.During the equalization process,the direction of the balanced energy transfer is determined according to the state of charge of the preheat battery,and priority is given to ensuring that the preheat battery is charged,and the effective equalizing current in the PWM duty cycle adjusting circuit is utilized.Combined with the original BMS,the system hardware was designed,and the low-temperature warm-up control software and the active equalization control software were developed,and a systematic verification experiment was conducted.The experimental results show that the system can estimate the temperature required to complete the maximum pulse discharge in the HPPC test experiment based on the SOC0;use two 2.7V/7000F supercapacitors to heat the 48Ah preheat battery pack,The time taken for heating the preheat the battery pack from-25℃ to 0℃ is 7 minutes;the preheat battery pack heats the power battery pack consisting of six batteries with the capacity of 100Ah,and the power battery pack is heated from-25℃ to 0℃ for 27 minutes.In the active equalization process,the transferred energy in the battery is preferably used to charge the preheat battery pack.In the equalization process,the average equalization current is about 8A,and the energy utilization rate is about 80%.The experimental results show the effectiveness of the low-temperature preheating system of the battery coupled with the non-dissipative equalization function developed in this paper. |
