Insulation failure of new energy vehicles

What does insulation failure look like?

If an insulation fault occurs in the entire vehicle, the instrument will definitely light up the fault light. For example, the insulation fault light of the EQC is battery + exclamation mark.

Different new energy vehicle manufacturers may have different warning words for insulation faults, but the fault lights are basically the same, because this sign is specified in GB/T 4094.2.

GB/T 4094.2-2017 Marking of electric vehicle controls, indicators and signal devices

Didn’t we talk about the insulation failure of the drive motor before? Why did the battery fault light come on?

Because the general practice in the industry is that the battery BMS detects the insulation resistance of the entire vehicle. If the insulation resistance is less than the fault threshold, the BMS will report the fault. However, the BMS detects the entire high-voltage circuit of the vehicle. If a fault occurs, it cannot confirm which high-voltage part in the circuit has a problem, and can only turn on the battery fault light.

If an insulation failure occurs, after-sales service will need to use an insulation meter to insulate all high-voltage parts in turn to confirm which component has the problem.

There are generally two situations when a vehicle has an insulation failure: if the insulation resistance is ≤100Ω/V, the vehicle will be under high voltage and cannot be driven; if the insulation resistance is 100Ω/V＜500Ω/V, the vehicle will be limited in power and can only be driven at a low speed. In actual use of the vehicle, the insulation resistance is generally very large or very small, and power limitation is relatively rare.

How is insulation fault detected?

First of all, we need to know that resistance is the value between two points of an object. The same is true for the BMS to detect the insulation resistance of the vehicle. GB 18384-2020 mentioned that, in simple terms, the insulation resistance is the smaller value of the resistance between the main positive circuit/main negative circuit of the power battery and the vehicle ground. After the resistance value is detected, it is divided by the battery voltage, and the calculated value is compared with the fault threshold to see if it is normal.

When it comes to this, some people may think that there is a relay between the main positive/negative of the battery and the whole vehicle. Indeed, before the high voltage of the power battery is powered on, there is no high voltage connection between the battery and the whole vehicle.

Therefore, before the main relay of the power battery is closed, the BMS only detects the insulation resistance between the battery and the vehicle ground; after the main relay is closed, the BMS detects the insulation resistance between the main positive circuit/main negative circuit and the vehicle ground. Therefore, when the vehicle is in ACC or ON gear, the BMS works but there is no high voltage, and the instrument lights up the insulation fault light, then it is basically a battery problem.

GB 18384-2020 also gives the test steps and calculation methods for insulation resistance.

GB 18384-2020 Safety requirements for electric vehicles

As for BMS in practice, there are many methods to measure insulation resistance, such as bridge measurement method (a passive measurement method, which seems to be the most commonly used method now) and DC power supply measurement method.