Unveiling the High Voltage Interlock: Enhancing Safety in EVs Lithium Batteries

As the electric vehicle and electric boat (EVs) industry continues to advance, ensuring high-voltage safety has become a top priority for technical personnel and users alike. With the emergence of higher platform voltages (800V and above), the need for robust safety measures is more critical than ever. That’s where the High Voltage Interlock (HVIL) function comes into play. This essential feature has garnered increasing value as it guarantees the high voltage safety of electric vehicles. Notably, the stability and response speed of the HVIL function have witnessed continuous improvements. In this blog post, we delve into the significance of HVIL for EVs lithium battery packs, highlighting its advancements and the benefits it brings to your battery projects.

Overview

1. What is High Voltage Interlock (HVIL)?

2. Why do EVs require High Voltage Interlock (HVIL)?

3. What is the Principle of High Voltage Interlock (HVIL)?

4. Control strategy for high-voltage interlock.

1. What is High Voltage Interlock (HVIL)?

EV High Voltage Inter-lock, Abbreviated as HVIL, also refers to Hazardous Voltage Interlock Loop.

By using low-voltage signals to check the electrical integrity of the high-voltage components, circuit, connectors and covers of the vehicle, if it is identified that the circuit is abnormally disconnected, then the high-voltage power will be disconnected within milliseconds to ensure the safety of users. HVIL acts as a circuit breaker, currently, almost all high-voltage connectors have interlocking terminals.

2. Why do EVs require High Voltage Interlock (HVIL)?

There are three main aspects to consider from the perspective of high-voltage system safety:

1). Using HVIL as a circuit breaker, if the high-voltage connector becomes loose, detached, or damaged during the operation of the high-voltage system, the high-voltage interlocking function can quickly disconnect the entire vehicle’s high-voltage

2). When staff or after-sales maintenance personnel unplug high-voltage connectors while the high voltage hasn’t been cut off, there may be arcing, and multiple live plugging and unplugging may affect the lifespan of the connectors. If the staff accidentally touches the unplugged live connector, there is a risk of electric shock.

3). Use high-voltage interlock to monitor whether the high-voltage connector is inserted properly, so as to avoid abnormal high-voltage drop during vehicle operation caused by not inserting it properly, which will affect driving safety.

3. What is the Principle of High Voltage Interlock (HVIL)?

HVIL uses a continuous low-voltage circuit to monitor all high-voltage connectors and components in electric vehicles (EVs). If the low-voltage HVIL signal is interrupted for any reason, it indicates that there is a problem that needs to be solved in the high-voltage system. When a fault occurs in the HVIL circuit, a diagnostic fault code will be triggered, and an alarm will appear on the vehicle control panel to let the driver know that they should send the vehicle for repair in a timely manner.

In the design of high-voltage interlock, it is necessary to consider how the low-voltage circuit can effectively detect the connection status of each connector and how to transmit signals, that is, the mutual relationship with the high-voltage circuit.

High-voltage connectors are used in pairs. For safety reasons, the male end is generally used for the equipment (product) end, and the female end is used for the cable end. And all high-voltage connectors are equipped with low-voltage signal terminals for connecting interlock signals. When the high-voltage connector is inserted into place, the interlock signal is connected. After connecting all the high-voltage connector signals in series, they are connected to the BMS or VCU for detection. When a certain connector is not inserted properly, the interlock signal is disconnected. Use this to judge whether the connector connection is normal.

Figure 1. Example of High Voltage Connector Interlock Terminals

From the above figure, it can be seen that the high-voltage terminal is longer than the low-voltage interlock terminal. Therefore, we can know that when plugging in the high-voltage connectors, the high-voltage terminals are connected first, and then the interlock terminals are connected. When unplugging the connector, the interlock terminal is first disconnected, and the high-voltage terminal is disconnected. This can ensure the effectiveness of high-voltage interlock signal detection.

Figure 2. Example HVIL (Not represent actual products)

There are various detection schemes for high-voltage interlock, such as duty cycle detection, resistance methods, or voltage detection. Some chip manufacturers such as TI have also launched specialized high-voltage interlock detection chips, and of course, it can also design a high-voltage interlock signal detection circuit using electronic components such as triode.

Figure 3. High voltage interlock monitoring

4. Control strategy for high-voltage interlock.

1) Fault alarm

Regardless of the state of the electric vehicle, when the high-voltage interlock system recognizes an abnormal, the vehicle should give an alarm prompt for the dangerous situation, requiring instruments or indicators to alert the driver in the form of sound or light alarm, so that the driver can pay attention to the abnormal situation of the vehicle in order to handle it in a timely manner and avoid safety accidents.

2). Cut off the high-voltage power

When the electric vehicle is in a stopped state, the high-voltage interlock system should not only give a fault alarm when identifying a seriously dangerous situation but also notify the system controller to disconnect the automatic circuit breaker, completely cutting off the high-voltage source, avoiding potential high-voltage hazards and ensuring property and personal safety.

3) Power reduction operation

During high-speed driving of electric vehicles, the high-voltage interlock system cannot immediately cut off the high-voltage power when identifying a dangerous situation. Instead, the driver should be alerted through an alarm, and then the control system should reduce the operating power of the motor to slow down the vehicle speed so that the entire high-voltage system can operate under low load and minimize the possibility of high-voltage danger, It also allows the driver to park the vehicle in a safe place.

Overall, high voltage interlock plays a vital role in the EV system, it can help the EV system to always be in a safe state and prevent any potential high voltage dangerous situation. Therefore, we need to always pay attention to the operation status of the high voltage interlock to ensure that we need constantly monitor the operation status of high-voltage interlocks to ensure that the electric vehicle system is always in a safe state.

In addition to high-voltage interlock measures, there are also insulation protection, active and passive discharge, PSS, thermal management, etc., which together constitute a system solution for high-voltage safety design.

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