In recent years, new energy vehicles have developed rapidly in China. Because of its high efficiency, energy saving, low noise, no pollution and other characteristics, it has become a new trend in the development of the automotive industry at home and abroad. Although the power supply, drive and control systems used in new energy vehicles are not much different from those used in industry in theory, due to the high requirements for safety, stability and reliability under different working conditions, electric vehicles should be considered more comprehensively and carefully in circuit design and component selection. As the power part of electric vehicle, the safety, reliability and stability of battery management system is undoubtedly the key to the performance of the whole vehicle. This paper analyzes and discusses the selection of precharge resistor in battery management system.
New energy vehicles have developed rapidly in China. Because of its high efficiency, energy saving, low noise, no pollution and other characteristics, it has become a new trend in the development of the automotive industry at home and abroad. In the era of Internet of vehicles, electronic products are more and more widely used in automotive products, with more and more functions, such as on-board navigation, automatic start and stop, oil-electric hybrid engine, etc. these products need high reliability electronic components as support, such as vehicle gauge capacitance, vehicle gauge resistance, vehicle gauge inductance, vehicle gauge fuse, etc. for example, vehicle gauge capacitance needs high temperature resistance, anti fracture, anti failure and other requirements, The resistance of the car gauge needs the requirements of anti surge, low temperature drift, corrosion resistance, vulcanization, etc.; the inductance of the car gauge needs high current, high temperature resistance, high stability, etc.; the fuse of the car gauge needs high reliability, high sensitivity, high life, etc. Shenzhen Shunhai technology focuses on the new energy vehicle market, such as new energy battery protection panel, new energy charging pile, new energy engine control panel, on-board navigation, electric windows, etc. the price of vehicle components is excellent, the delivery time is fast, and most of the spot transactions.
What is a precharge resistor? Simply put, at the initial stage of power on, the power supply should charge the capacitor. If it is not limited, the charging current is too large, which will cause a great impact on the relay, rectifier devices and capacitors to be charged, so the resistance used here is the pre charging resistance.
The battery management system of new energy vehicles will involve the high-voltage precharge link, because the motor controller (that is, we often say the inverter) has a large bus capacitance. In the case of cold start without precharge, if the main relay is directly connected, the high voltage of the battery will be directly loaded onto the empty bus capacitor, which is equivalent to an instantaneous short circuit, and a great instantaneous current will damage the relay. After adding the precharge resistance, the bus capacitor is precharged through the precharge circuit, so that the current when the main circuit is connected can be controlled within a safe range to ensure the normal operation of the system.
We can learn from the above introduction that the precharge resistor is an indispensable and critical device in the battery management system. So how should we analyze and choose the appropriate resistance for this application?
Before selecting the type of resistor, we should first clearly understand the operating conditions and parameter requirements, which are summarized as follows:
Output voltage of high voltage battery power supply
Rated current of relay
Bus capacitance
Possible maximum ambient temperature at startup
Temperature rise requirements of resistance
Voltage required for capacitor precharge
Time required to reach charging voltage
Single pulse or continuous pulse?
If it is a continuous pulse, what are the times that the resistance can resist continuous pulses and the interval time between pulses?
When the battery is abused, the resistance is required to maintain the normal working state for a long time
Installation structure and wiring mode of resistance
Requirements for insulation voltage
After knowing the detailed working conditions and parameters, we need to do some basic calculations. Usually, the pre charging is required to be completed within 500ms. In such a short time, the high heat generated by the current passing through the resistance wire or body is not absorbed by the resistance skeleton, and the resistance wire or body itself will have to bear most of the pulse energy. Therefore, we should first calculate the pulse energy at startup, and then select the appropriate resistance scheme.
In the case of a single pulse, the energy is calculated as follows:
In the case of continuous pulses, when the interval between pulses is very short (such as less than 1s), the proportion of dissipated energy in practical applications is very small. We can generally use linear accumulation to calculate the total pulse energy.
Total energy = single pulse energy X number of continuous pulses
Then determine the resistance value of the precharge resistance:
T = RC * Ln[(Vbat - V0)/( Vbat - Vpre)]
Where: t= precharge time
R= precharge resistance
C= load end capacitance
Vbat= battery pack voltage
V0= voltage before closing the high voltage at the load end (which can be expressed as 0)
Vpre= load terminal voltage at the end of precharge
Generally speaking, vpre is selected as 90% or 95% of the total voltage Vbat, which is considered as 90% here, so the formula can be expressed as follows:
T = RC * Ln10
Then r = t / (c * ln10)
Example analysis: if the total voltage is 700V, the load capacitance C is 2000uf, and the maximum pre charging time is set to 300ms, when the capacitance voltage reaches 95% of the power supply voltage, the energy of a single pulse is
E = 1/2CV2 = 442J
If there are five consecutive pulses in a short time, the total pulse energy is about
E total = 442*5=
