Vehicle power on and off management

The whole vehicle power on and off has become a routine operation in daily car use. The editor used to understand this function from movies, but after becoming an automotive electronics engineer, he found that in reality, factors such as vehicle anti-theft authentication limit the possibility of car theft. The whole vehicle power on and off seems simple, but factors such as anti-theft authentication, robustness, and emergency power off need to be considered in system design.

The editor drew a simple state machine to assist understanding, but the actual design state machine will be much more complicated than this. The design of the whole vehicle power on and off needs to take into account many factors. For example, for anti-theft authentication, it is necessary to add anti-theft measures such as password verification to the vehicle system to ensure that only the owner or the authorized person of the vehicle can power on and off the vehicle. In addition, the robustness of the vehicle system needs to be considered, so that the normal operation of the whole vehicle power on and off can be guaranteed even in abnormal situations.

At the same time, in order to ensure safety, emergency power off needs to be considered. For example, when the vehicle fails and needs to be powered off quickly, the vehicle system needs to be able to power off quickly and safely. These factors need to be fully considered in the design of the whole vehicle power on and off to ensure the safety, reliability and stability of the vehicle system. Therefore, in actual design, the complexity of the state machine will be much more complicated than the simple state machine drawn by the author. It is necessary to carefully consider various abnormal situations and take corresponding preventive measures.

Vehicle power on

When the user steps on the brake or presses the one-button start switch, if there is a legitimate key in the car, we will immediately see the instrument or large screen start to light up, which means that the vehicle has been powered on successfully. This is an almost imperceptible process, and the whole process generally takes about 200ms, mainly used to confirm whether the key exists and is legitimate.

There is a slight difference between the power-on status of fuel vehicles and electric vehicles. Generally speaking, the power-on of a fuel vehicle only involves the energization of the KL15 relay. If it is an electric vehicle, the power-on process actually includes two steps. One is what we often call the energization of the KL15 relay, and the other is the energization of the high-voltage relay.

It should be noted that the capacitance in the circuit of the high-voltage relay is zero before power is applied, so care should be taken before directly closing the high-voltage relay. Directly closing the high-voltage relay will cause a sharp increase in current, thus causing a huge impact on the entire power circuit, so the current flow needs to be limited to avoid damage to related components.

Therefore, in order to protect the power system of electric vehicles, a pre-charging circuit must be added. Before the high-voltage relay is energized, the pre-charging relay must be turned on to ensure that the capacitor voltage in the circuit is consistent with the voltage of the power battery. In this way, the impact current during power-on can be effectively reduced, and key components such as the motor controller, battery, and main relay can be protected from damage. This not only improves the safety and reliability of electric vehicles, but also extends their service life.

Power certification

Powering up a vehicle does not mean that the power system is ready. The power system includes the engine of a fuel vehicle or the motor of an electric vehicle. Before the power system is ready, anti-theft certification, namely IMMO certification, is required. This anti-theft certification usually uses the AES128 or 256 algorithm, but most people find it difficult to crack this password. In fact, anti-theft certification is usually authenticated by the controller that controls the power on and off and the engine ECU or VCU.


During the manufacturing process, the vehicle will use a special key generation device to generate a key and write it to the controller that needs to be authenticated so that the vehicle can pass the IMMO certification. For electric vehicles, anti-theft certification will be automatically performed when the vehicle is powered on. When the certification is passed, the user only needs to switch the vehicle to the forward gear to travel.

In fuel vehicles, the one-button start of the start switch will start the engine and IMMO certification at the same time, but if interference causes an authentication error, the authentication will be performed at least three times to avoid a single failure causing authentication failure.

Vehicle power off

When the user stops the car and shifts the gear to P, press the one-button start switch, and the engine will be turned off. The purpose of this is to prevent the vehicle from accidentally touching the one-button start switch while driving, causing the engine to turn off.

The regulations also stipulate that there needs to be a switch for users to power off in an emergency. If an emergency occurs during driving, causing the vehicle's brakes to fail and the gears to fail to switch, the vehicle cannot stop. At this time, the vehicle can be powered off through the emergency switch. This switch will also be slightly different according to the strategy of each OEM. Generally, the more common ones are long-press one-button start switches or P gear switches.

The author once saw a complex state machine for the power on and off of the entire vehicle, which covered dozens or even hundreds of jump paths. When encountering abnormal power on and off of the vehicle, it takes a lot of time and effort to use this diagram to find the problem. It takes a talented person to draw such a complex state machine. However, one day the author figured out that this is not something that can be completed in a day or two, but the result of years of accumulation and repair. Every time a problem occurs, it needs to be patched on the original basis, just like three years of sewing and patching. On the contrary, it would be wiser to redesign a simple power on and off process.

In the process of using our car, often no one pays attention to the function of powering on and off the whole vehicle, because it is too inconspicuous. No one will praise the car for the smooth powering on and off of the whole vehicle, but an occasional failure to power on and off normally will cause great complaints.