Introduction:
The HV system in vehicles with an electrified or fully electric drivetrain is equipped with multiple protections. The system can only be activated if all safety requirements are met. When a fault is detected, the HV system immediately shuts down. This can occur in the following scenarios:
- A component of the HV system is dismantled, and the system is activated.
- Due to a collision or water damage, electrical components or wiring short-circuit with each other or with ground.
- Components are damaged due to overload.
The image below shows the components that belong to the protection system. In blue is part of the HV battery (1), with the orange service plug (2) on the left. In the middle, there are three relays (3 to 5), which are activated one by one by the ECU (6). Under the HV battery is the ECU (7), which communicates with the consumers (8) like the electric motor, PTC heater, air conditioning compressor, power steering, and charging system.
Legend:
- HV battery
- Service plug with fuse
- Relay 1
- Relay 2
- Relay 3
- ECU of HV battery
- ECU of HV system
- Electrical consumers
- Interlock wire
Activation of the HV system:
The driver activates the HV system by pressing the start button. When the message “HV ready” appears on the display, the HV system is activated. Before the HV system is active, the relays in the HV battery pack are controlled to connect the battery pack with the consumers.
When the HV system is activated, the ECU (6 in the image below) controls the HV relays in the positive circuit (relay 4) and the ground circuit (relay 5). First, the power circuit on the positive side is activated via a resistor. In the image below, we see that relay (4) allows the current to pass through resistor R1. The resistor limits the current passing through it, so the inrush current is restricted. This allows the capacitors in the inverter to be charged slowly. At this point, the system can perform a safety check with a lower voltage. After the voltage across the capacitors in the inverter is approximately equal to the voltage of the HV battery pack, relay 3 is closed, and relay 4 opens, allowing full voltage on the inverter and other electrical components.
Interlock:
The interlock system is the protection mechanism that prevents electrical contact when there are open connections. Each component connected to the HV battery contains at least one contact that can disable the HV system when a disconnection occurs. These contacts can be integrated into the wiring or included as a switch in the housing of a component.
In the bottom left image, we see the active system: relays 3 and 5 are closed, which means that the voltage from the HV battery is passed on to the consumers. The interlock circuit is shown in blue from the vehicle ECU (7). From the ECU, a voltage is placed on resistor R2. The interlock is fed through the electrical consumers (8) as a series circuit. In the battery pack, the interlock is grounded. Between resistor R2 in the ECU (7) and the output to the consumers, there is a tap where the voltage on the interlock is measured.
- Interlock in order: voltage after resistor R2 is 0 volts;
- Interlock interrupted: the voltage is not consumed in resistor R2 and is (depending on the supply voltage) 5, 12, or 24 volts.
The voltage after resistor R2 is constantly monitored during activation and also during driving.
When the service plug (2) or one of the electrical components (8) is dismantled, the interlock circuit is also interrupted. This situation is shown in the upper right image, where the service plug is shifted. Both the fuse between the battery modules and the interlock circuit are interrupted. Because the interlock is no longer connected to the vehicle’s ground, the voltage after resistor R2 rises to the supply voltage value. The vehicle ECU (7) immediately sends a signal to the battery ECU (6), preventing relays 3, 4, and 5 from being activated. The HV system is then deactivated.
The image shows the orange service plug with large central contacts to connect the positive and negative lines of the HV battery, and on the left, a smaller connector with two pins. These are the two pins of the interlock. We also find these connections on the connectors of HV components.
Short Circuit Protection:
The HV system must be protected against excessive currents that can arise from a short circuit in the wiring or electrical components. Without protection, this can lead to an arc, melting of wires, or even fire. A fuse is designed to protect the system from these hazards. The fuse can be located in the service plug or elsewhere in the battery pack. Vehicles may also be equipped with multiple fuses, each designed to protect a specific circuit.
In addition to protecting the system against excessive currents, the current sensor in the positive or negative lead of the HV battery relays the current to the ECU. The ECU decides to deactivate the relays when there is an overload.
Permanent Isolation Monitoring:
The positive and negative sides of the HV battery do not come into contact with each other or the environment. The positive side (from the + battery to the + inverter) is surrounded by multiple layers of insulation with a braided shield in between. The negative side is also insulated and does not contact the chassis or housing of the components. However, the vehicle’s chassis is connected to the negative of the onboard battery (12 volts in passenger cars). In the HV section, this is not the case. Causes of a malfunction can include:
- After a collision, damage may have occurred to the wiring, causing the copper of the positive and negative wires to come into contact with each other, or touching the vehicle’s chassis;
- due to overload – and thus overheating – insulation may have become defective (melted) in an electrical component, allowing contact with the environment;
- Or there is conductive fluid because the vehicle has been immersed in water, coolant leakage in the HV battery pack causing a short circuit between positive and negative. Also, coolant leakage in the electric aircon pump can cause conduction.
In the electrical components, poor insulation can create a connection between the positive or negative leads from the HV battery and the housing. Since the housing is usually mounted on the vehicle’s chassis, without interventions from the protections, a current could flow with poor insulation. When the positive of the HV battery – due to an insulation fault – is connected to the vehicle’s chassis through the housing, there will be high voltage of hundreds of volts on the chassis. However, as there is no way to connect with the negative of the HV battery, nothing will happen because no current will flow. It will only fail when there are multiple insulation faults, with both the positive and negative of the HV battery coming into contact with the chassis.
In the three images below, we see the HV battery pack (1) with the positive and negative leads, with at the bottom the vehicle chassis (2) and in between two electrical consumers (3 and 4).
- poor insulation positive side component: when there is poor insulation between the positive and the housing in a consumer (e.g., the electric heater), the housing will become live. Since there is no connection with the negative of the HV battery, no current flows;
- poor insulation negative: again, a (small) voltage will appear on the chassis, but no current will flow;
- poor insulation in both positive and negative: in this situation, there is a short circuit between the positive and negative of the HV battery. The chassis becomes the connection between positive and negative. The current will increase rapidly until the fuse in the service plug and/or the HV battery blows to protect the system.
Since with poor insulation in the positive or negative there is no closed circuit, the fuse in the service plug will not blow. The permanent isolation monitoring in electric vehicles detects such current transfer, warning the driver with a fault message. A vehicle can still operate with an insulation fault, unless the manufacturer software disables it.
Number 5 in the image below indicates the component where permanent isolation monitoring occurs. In reality, this electrical part is more complex.
Number 6 indicates the measuring resistor where the voltage drop is measured in parallel.
In the two images below, the situations are shown where there is poor insulation in the positive (left) and in the negative (right). The current flow through the measuring resistor causes voltage consumption in the resistor circuit. The voltage drop across the measuring resistor is a measure of the amount of current flowing through the resistors.
As soon as the ECU with permanent isolation monitoring detects an anomaly, it records a fault code. Possible descriptions of the P-codes (such as P1AF0 and P1AF4) may include: “battery voltage system isolation lost” or “battery voltage isolation circuit malfunction”. When a vehicle comes into the workshop with an isolation fault, the technician can use diagnostic equipment or manually with a Megohmmeter measure the insulation resistances to check for any insulation leak.
Diagnosis with the Megohmmeter:
In the previous paragraph, the concept of “insulation resistance” was explained and shown how the vehicle uses permanent isolation monitoring to check for a leak from the positive or negative connections from the HV battery to the vehicle chassis. In this paragraph, we delve deeper and describe how, as a technician, you use a Megohmmeter to locate the fault. Of course, as a technician, you must be certified to work on HV systems. The software in a diagnostic tester can perform an isolation test on certain brands, for example, on components that only show an isolation fault after activation, such as the electric heater or the electric air conditioner.
In other cases, we can use a Megohmmeter to measure the insulation resistance. A normal multimeter cannot measure the insulation resistance as its internal resistance can reach up to 10 million ohms. The internal resistance is too high to measure high resistance values. A Megohmmeter is suitable for this and applies a voltage of 50 to 1000 volts to simulate the operating condition. This high voltage ensures that the emitted current even finds its way through the smallest damage in the insulation, through the copper core, to the insulation. To measure with the Megohmmeter, set the meter to the same voltage as the HV battery or one step higher. After connecting the test cables and correctly setting the meter, click the orange button “insulation test”. The set voltage (in the image: 1000 volts) is applied to the test cables and therefore to the component, and we then read the ohmic value from the display.
- An insulation resistance greater than 550 MΩ (Megaohm, which means 550 million ohms) is acceptable. This is the maximum measurement range;
- A value lower than 550 MΩ may indicate a leak in the insulation, but this is not necessarily the case;
- According to the International Electrotechnical Commission (IEC) and the Institute of Electrical and Electronics Engineers (IEEE), the insulation resistance of an EV must be at least 500 Ω per volt. At a nominal HV voltage of 400 volts, the resistance (500 Ω * 400 v) = 200,000 Ω.
- Manufacturers often set higher quality and safety standards, setting higher minimum insulation resistances. For this reason, factory specifications should always be followed during diagnostics.
Factory specifications are always paramount.
The factory specifications describe the steps, safety regulations, and the minimum insulation resistances.
In the next image, we see a screenshot from a Toyota manual. The minimum insulation resistances of the cables to the electric motor are shown for the relevant model.
The megohmmeter must be set to the value of 500 volts and the minimum resistances of the wiring (U V and W) to the electric motor relative to the housing should be 100 MΩ (MegaOhm) or more.
The insulation resistances of the electric air conditioning compressor and heating element, for example, may vary. Consult the factory data for measurements on other components.
1. Insulation measurement on the negative side (no fault):
With the disconnected connector, we also measure the negative side relative to the vehicle’s ground. Images 1 and 2 show a schematic and actual view of this measurement. The measurement results in an insulation resistance of >550 MΩ, indicating that the insulation is in good condition.
2. Insulation measurement on the positive side (no fault):
After disconnecting the connector, e.g., from the inverter, we attach the red probe to the pin in the disassembled connector (now on the positive side) and the black probe to a grounding point connected to the vehicle’s chassis. Image 1 re-shows the schematic from the previous paragraph, with the HV battery (1), vehicle ground (2), and two of the consumers (3 and 4) numbered. The Megohmmeter is connected, and the orange button “insulation test” is pressed to measure the insulation resistance with the sent voltage of 500 volts. This measures 133 Megaohms. The insulation resistance is lower than in the previous measurement. The manufacturer’s minimum insulation resistance of 100 MΩ should be adhered to. The insulation resistance is acceptable.
3. Insulation measurement on the positive side (fault):
During the measurement on the same terminals, we measure an insulation resistance of 65 MΩ. Although the resistance value is higher than the minimum 500 ohms per volt stipulated by IEC and IEEE (see the previous paragraph), the wiring and/or component are rejected as the manufacturer has prescribed the minimum resistance value of 100 MΩ. The wiring and/or connector connections should not be repaired, but completely replaced.
4. Insulation measurement on the positive side (fault):
When an insulation value of 0 MΩ is measured, there is a direct connection (i.e., short circuit) between the HV wire and the housing. The wiring and/or connector connections should not be repaired, but completely replaced.
In the event of an insulation fault, connectors of other consumers can be disconnected one by one to measure in the connector, as shown in the text and images above.
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