In recent years, new energy vehicles have developed rapidly in China. Due to its high efficiency, energy saving, low noise, and no pollution, it has become a new trend in the development of the automobile industry at home and abroad. Although the power supply, drive and control systems used in new energy vehicles are theoretically not much different from those used in industry, because vehicles have extremely high safety, stability, reliability and other aspects under different working conditions High requirements make electric vehicles more comprehensive and serious in circuit design and selection of components used.As a battery management system for the power part of an electric vehicle, its safety, reliability, stability

In recent years, new energy vehicles have developed rapidly in China. Due to its high efficiency, energy saving, low noise, and no pollution, it has become a new trend in the development of the automobile industry at home and abroad. Although the power supply, drive and control systems used in new energy vehicles are theoretically not much different from those used in industry, because vehicles have extremely high safety, stability, reliability and other aspects under different working conditions High requirements make electric vehicles more comprehensive and serious in circuit design and selection of components used. As the battery management system of the power part of the electric vehicle, its safety, reliability and stability are undoubtedly the key to the performance of the whole vehicle. This article makes the following analysis and discussion on the selection of the precharge resistor in the battery management system.

What is a precharge resistor? Simply put, the power supply needs to charge the capacitor at the initial stage 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 precharge resistor.

The battery management system of new energy vehicles will involve high-voltage pre-charging, because the motor controller (that is, the inverter we often say) has a large bus capacitor. In the case of cold start without pre-charging, if the main relay is directly connected, the high voltage of the battery will be directly loaded on the empty bus capacitor, which is equivalent to an instantaneous short circuit, and a huge instantaneous current will damage the relay. After adding the pre-charging resistor, the bus capacitor is pre-charged through the pre-charging circuit, so that the current when the main circuit is turned on can be controlled within a safe range to ensure the normal operation of the system.

Selection and Analysis of Precharge Resistors in New Energy Vehicles

We can know 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 resistor for this application?

Before resistor selection, we must first clearly understand the operating conditions and parameter requirements, which are summarized as follows:

High voltage battery power output voltage

Rated current of the relay

busbar capacitance

Maximum possible ambient temperature at startup

Temperature Rise Requirements for Resistors

The voltage required to precharge the capacitor

Time required to reach charging voltage

Single pulse or continuous pulse?

If it is a continuous pulse, what is the number of times the resistance can resist continuous pulses and the interval between pulses?

The time the resistor is required to remain in normal operation when the battery is abused

Resistor installation structure and wiring method

Requirements for insulation voltage

After understanding the detailed operating conditions and parameters, it is necessary to do some basic calculations. Usually, pre-charging is required to be completed within 500ms. In such a short period of time, the high heat generated by the current passing through the resistance wire or the resistance body cannot be absorbed by the skeleton of the resistor, and the resistance wire or the resistance body itself will have to bear the great burden. The energy of the partial pulse. So we need to calculate the pulse energy at startup first, and then choose the appropriate resistor scheme.

In the case of a single pulse, the energy is calculated as follows:

Selection and Analysis of Precharge Resistors in New Energy Vehicles

If it is a continuous pulse, when the interval between pulses is very short (for example, less than 1s), the proportion of energy dissipated in practical applications is very small, and we can generally use linear accumulation to calculate the total pulse energy.

Selection and Analysis of Precharge Resistors in New Energy Vehicles

Total energy = single pulse energy x number of consecutive pulses

Then determine the resistance of the precharge resistor:

T = RC * Ln[(Vbat – V0)/(Vbat – Vpre)]

Where: T = precharge time

R = precharge resistance

C = load terminal capacitance

Vbat = battery pack voltage

V0 = the voltage before the load terminal closes the high voltage (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 to be 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 capacitor C is 2000uF, and the maximum precharge time is set to 300ms, when the capacitor voltage reaches 95% of the power supply voltage, the energy of a single pulse is

E = 1/2CV2 = 442J

If it is 5 consecutive pulses in a short time, the total pulse energy is about

Etotal=442*5=2210J

The resistance of the precharge resistor is

R = T / (C * Ln10) = 300 / 4.60 = 65.2Ω

Based on the above calculations, how to choose a suitable resistor? There are several options usually used for battery pack precharging resistors. Let’s compare the characteristics of different types of resistors.

Selection and Analysis of Precharge Resistors in New Energy Vehicles

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

Selection and Analysis of Precharge Resistors in New Energy Vehicles

Selection and Analysis of Precharge Resistors in New Energy Vehicles

According to the above parameters, combined with the requirements of the above example, if it is a single pulse, a 252AEC resistance can be satisfied, and if it is a continuous pulse, two 253AECs need to be used in parallel or in series. In addition, we have also developed other package types of precharge resistors according to market demand, as shown in the figure below.

Selection and Analysis of Precharge Resistors in New Energy Vehicles

HECR series of pre-charged resistors developed based on solid core ceramic resistors

Recently, we often come into contact with some products that use aluminum casing wirewound resistors as pre-charging resistors in battery packs, and there are cases of serious failures or even resistor explosions during their testing and use. Based on this situation, the discussion in this article is hoped to provide readers with useful information on resistor selection in this application area.

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