Design of Battery System in Pure Electric Vehicle–Bonnen Battery (Ⅱ)

Vehicle Parameters
(1)  Determine system voltage range

According to the motor to determine the nominal voltage and voltage application range of the power system.

The power supply voltage level of electric vehicle motor recommended in the national standard is: 120V、144V、168V、192V、216V、240V、264V、288V、312V、336V、360V、384V、408V etc.

The standard requires the motor and controller can safely withstand the maximum current when the power supply voltage is 120% ~ 75% of the rated voltage. Ensure that the DC bus voltage is not less than 80% of the rated voltage of the motor.

Take the vehicle adopts 384V motor as an example. The normal working voltage of the power supply system should be in the range of 300 ~ 460V. LiFePO4 lithium-ion batteries were selected and 120 pc batteries were connected in series.

(2) Maximum output power / current of power system

Power demand of power system
The motor power is 110KW. Assuming that the motor conversion efficiency and controller efficiency are 0.9 and 0.95 respectively, and the power of electronic accessories and air conditioning is 8kw, the minimum power required by the power supply system is 110(0.9×095)+8=137KW

Maximum output current
The nominal voltage of the system is 384V. At high power output, it is calculated as 10% lower than the nominal voltage. dmax=Pmax/=137000/(384 × 09)=396A

Duration
Subject to vehicle requirements. Note SOC requirements (maximum power requirement at 30% SOC)

Design redundancy
It is necessary to ensure 30% design redundancy

(3) Determination of maximum feedback current

The feedback power is related to the vehicle control strategy

The feedback power is less than the maximum power of the motor. The maximum generating power of 110KW motor is about 70KW, and the feedback current is about Icmax=70000/432=162A

Feedback time: according to the requirements of vehicle working conditions (such as 10s)

SOC range: according to battery characteristics, such as 70% SOC.

Vehicle braking feedback strategy: the proportion of mechanical braking and motor braking energy recovery. Motor feedback power is usually small, the actual power is less than the maximum power. According to 50% of the maximum power recovery, the feedback power is 55KW and the current is about 100A.

(4) Determination of SOC application scope of power system
In order to better protect the power supply system, it is not recommended to fully charge the power supply system in general application, and it is not recommended to completely discharge when discharging, otherwise it is easy to damage the battery. It is generally recommended to charge to 95 ~ 100% and discharge to 5 ~ 10% capacity, which can better protect the weak battery in the system. It is suggested that the application range is 10 ~ 90% SOC.

(5) Determination of power system capacity

According to the calculation of vehicle parameters, the average driving speed is 40km / h and the driving range is 200km.
Continuous driving time is required: T = 5h
The driving power demand of the vehicle is: PV1 = 25kW
The energy required is: W=25*5=125KWhC=125*1000/(3840.8)=407Ah
When the capacity of redundancy design is determined, redundancy should be considered. Generally, the design redundancy is 30% (which can be determined according to the actual situation). The design can use 500Ah power supply system.

(6) Cooling design of battery pack

According to the normal operation condition of the vehicle, the heat generated by the vehicle during normal driving and the possible temperature rise are calculated; Calculate the heat production during charging according to charging requirements. Determine the heat dissipation method according to the calculation.

(7) The design of BMS function

The function of BMS is designed according to the requirements of selected battery type and communication protocol requirements for vehicle control requirements. According to the characteristics of the selected battery, the system of the balanced current is determined to use the lithium iron phosphate battery with 500Ah. The self discharge of the battery is about 5% per month (assuming that the capacity difference between the batteries is 5%) and the balance time of charging is about 2H. Then the balance current should reach 5005%/(30*2)=0.417A, so as to eliminate the capacity difference caused by self discharge.

(8) System structure design

The design scheme of multiple battery packs is adopted, and the size of each battery pack are determined according to the spatial position of the vehicle and the allocation on the vehicle. For the convenience of processing and maintenance, the battery pack should be of uniform specification if conditions permit. The weight of each battery pack should not be too large, otherwise it will affect the handling and installation.

(9) Charging system requirements

Pure electric vehicles are usually charged on the ground. At this time, the system, parameters and communication requirements of the system charger are determined according to the selected battery type, the heat dissipation characteristics of the power supply system and the required charging time.

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