Selection and analysis of pre-charging resistors in new energy vehicle systems

In recent years, new energy vehicles have developed rapidly in China. Due to its high efficiency, energy saving, low noise, and pollution-free characteristics, it has become a new trend in the development of the domestic and foreign automobile industries. Although the power supply, drive, and control system used in new energy vehicles are not much different from those used in industry in theory, due to the extremely high requirements of safety, stability, and reliability in different working conditions, electric vehicles need to make more comprehensive and serious considerations in circuit design and the selection of components used . As the battery management system of the power part of electric vehicles, its safety, reliability, and stability are undoubtedly the key to the performance of the entire vehicle. This article makes the following analysis and discussion on the selection of pre-charge resistors in the battery management system .

What is a pre-charge resistor? Simply put, the power supply needs to charge the capacitor at the beginning of power-on . If it is not limited, the charging current will be too large, which will cause a greater impact on the relay , rectifier device and the capacitor to be charged. Therefore, a resistor is used to limit the current. The resistor used here is the pre-charge resistor.

The battery management system of new energy vehicles will involve high-voltage pre-charging. This is because the motor controller (what we often call the inverter ) has a large bus capacitor. In the case of cold start without pre-charging, if the main relay is directly connected, the battery high voltage will be directly loaded onto the empty bus capacitor, which is equivalent to a momentary short circuit . The huge instantaneous current will damage the relay. After adding the pre-charging resistor, the bus capacitor is pre-charged through the pre-charging circuit first, 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.

From the above introduction, we can understand that the pre-charge resistor is an indispensable and critical component in the battery management system. So how should we analyze and select the appropriate resistor for this application?

Before selecting a resistor, we must first clearly understand the operating conditions and parameter requirements, which can be summarized as follows: high-voltage battery power supply output voltage, relay rated current, busbar capacitance value, possible maximum ambient temperature at startup, resistor temperature rise requirements, capacitor pre-charge voltage required. Time required to reach the charging voltage. Single pulse or continuous pulse? If it is a continuous pulse, how many continuous pulses can the resistor withstand and the pulse interval time are respectively? When the battery is abused, the duration of the resistor to maintain normal working state.

After understanding the detailed working conditions and parameters, we need to do some basic calculations. Usually, pre-charging is required to be completed within 500ms. In such a short time, the high heat generated by the current passing through the resistor wire or resistor body cannot be absorbed by the resistor skeleton in time, and the resistor wire or resistor body itself will have to bear most of the pulse energy. Therefore, we must first calculate the pulse energy at startup, and then choose the appropriate resistor solution.

If it is a continuous pulse, when the pulse interval is very short (for example, less than 1s), the proportion of energy dissipation in actual applications is very small, and we can generally use linear accumulation to calculate the total pulse energy. Based on the above calculations, how do we choose the right resistor? There are several options for pre-charging resistors for battery packs. Let's first compare the characteristics of different types of resistors.

By comparison, we can see that solid ceramic resistors are very suitable for pre-charge and discharge applications in terms of performance. Although it is small in size, due to its solid ceramic structure, it has a higher specific heat capacity and mass than the resistance wire of the wire wound resistor, and can absorb extremely high energy in a short time. It has no parasitic inductance and has a very fast response time. The American manufacturer KANTHAL is a leader in the ceramic solid core resistor industry. The following are some specific parameters of this KANTHAL resistor.

According to the above parameters and the requirements of the above examples, if it is a single pulse, a 252AEC resistor can meet the requirements. If it is a continuous pulse, two 253AEC resistors are required in parallel or series. In addition, we have also developed pre-charge resistors of other packaging types according to market demand, as shown in the figure below.

Recently, we often come across some products that use aluminum-shelled wire-wound resistors as pre-charging resistors in battery packs, and there are serious failures or even explosions of resistors during their testing and use. Based on this situation, this article discusses this situation, hoping to provide readers with useful information on resistor selection in this application field.