Introduction:
The HV system in vehicles with an electrified or fully electric powertrain is equipped with multiple safety features. The system can only be switched on once all safety requirements have been met. The moment a fault is detected, the HV system switches off immediately. This can happen in the following situations:
- A component of the HV system has been removed, and the system is switched on.
- Due to a collision or water damage, electrical components or wiring short-circuit with each other or to ground.
- Components have been damaged due to overload.
The image below shows the components that are part of the safety system. In blue, a section of the HV battery (1) can be seen, with the orange service plug (2) on the left. In the middle are three relays (3 through 5), which are switched on one by one by the ECU (6). Below the HV battery is the ECU (7), which is connected to the loads (8), such as the electric motor, PTC heater, A/C compressor, power steering, and charging unit.
Legend:
- HV battery
- Service plug with fuse
- Relay 1
- Relay 2
- Relay 3
- HV battery ECU
- HV system ECU
- Electrical loads
- Interlock wire
Switching on 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 to the loads.
When the HV system is switched on, 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 circuit on the positive side is switched on through a resistor. In the image below, we see that relay (4) allows current to flow through resistor R1. The resistor limits the current flowing through it, so the inrush current is limited. This allows the capacitors in the inverter to be charged slowly. At this moment, the system can perform a safety check at 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 closes and relay 4 opens, applying full voltage to the inverter and other electrical components.
Interlock:
The interlock system is the safety system that provides protection against electric shock when there are open connections. In every component connected to the HV battery, there is at least one contact that can switch off the HV system when an interruption occurs. These contacts can be integrated into the wiring or included as a switch in a component housing.
In the image at the lower left, we see the active system: relays 3 and 5 are closed, which means the voltage from the HV battery is being switched through to the loads. The interlock circuit is colored blue starting from the vehicle ECU (7). From the ECU, a voltage is applied across resistor R2. The interlock is routed as a series circuit through the electrical loads (8). In the battery pack, the interlock is connected to ground. Between resistor R2 in the ECU (7) and the output to the loads, there is a tap where the voltage on the interlock is measured.
- Interlock OK: voltage after resistor R2 is 0 volts;
- Interlock interrupted: the voltage has not been dropped across resistor R2 and is (depending on supply voltage) 5, 12, or 24 volts.
The voltage after resistor R2 is continuously monitored during switching on, but also while driving.
When the service plug (2) or one of the electrical components (8) is removed, the interlock circuit is also broken. This situation can be seen in the upper right image, where the service plug has been shifted. Both the fuse between the battery modules and the interlock circuit are interrupted. Because the interlock is no longer connected to vehicle ground, the voltage after resistor R2 rises to the value of the supply voltage. The vehicle ECU (7) immediately commands the battery ECU (6), causing relays 3, 4, and 5 to no longer be activated. The HV system is then switched off.
In the image, we see the orange service plug with the large contacts in the center to switch through the positive and negative cables 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 connectors of the HV components.
Short-circuit protection:
The HV system must be protected against excessively high currents, which can occur due to a short circuit in the wiring or in the electrical components. Without protection, this can lead to an arc flash, melting of cables, or even fire. A fuse is designed to protect the system against these hazards. The fuse may be located in the service plug, but also elsewhere in the battery pack. Vehicles may also be equipped with multiple fuses, each designed to protect a specific circuit.
In addition to the fuse protecting the system against excessively high currents, the current sensor in the positive or negative cable of the HV battery passes the current value to the ECU. The ECU makes the decision to switch off the relays when an overload is detected.
Continuous insulation monitoring:
The positive and negative sides of the HV battery do not come into contact with each other, nor with the surrounding environment. Around the positive side (from the + battery to the + of the inverter) there are multiple layers of insulation with a braided shield in between. The negative side is also insulated and does not make contact with the vehicle body or the housing of the components. The vehicle body itself, on the other hand, is connected to the negative terminal of the 12-volt auxiliary battery (12 volts in passenger cars). In the HV section, this is not the case. Causes of a fault may include:
- After a collision, the wiring may have been damaged, causing the copper of the positive and negative wires to come into contact with each other, or touch the vehicle body;
- due to overload—and therefore overheating—the insulation in an electrical component has failed (melted), allowing contact with the surroundings;
- Or there is conductive fluid because the vehicle has been in water, or a coolant leak in the HV battery pack has caused a short circuit between positive and negative. A refrigerant leak in the electric A/C pump can also cause conductivity.
In electrical components, poor insulation can create a connection between the positive or negative cables from the HV battery and the housing. Since the housing is usually mounted to the vehicle body, without the safety systems intervening, poor insulation could allow current to flow. At the moment the positive terminal of the HV battery—due to an insulation fault—is connected to the vehicle body via the housing, several hundred volts of high voltage is present on the vehicle body. However, because there is no way to connect to the negative terminal of the HV battery, nothing will happen, because no current will flow. It only becomes dangerous if there are multiple insulation faults, where both the positive and the negative of the HV battery come into contact with the vehicle body.
In the three images below, we see the HV battery pack (1) with the positive and negative cables, with the vehicle body (2) at the bottom and two electrical loads (3 and 4) in between.
- poor insulation on the positive side of a component: if a load (for example, the electric heater) has poor insulation between the positive cable and the housing, the housing will become energized. Because there is no connection to the negative terminal of the HV battery, no current flows;
- poor insulation on the negative side: now too, a (small) voltage will appear on the vehicle body, but no current will flow;
- poor insulation on both the positive and negative sides: in this situation there is a short circuit between the positive and negative terminals of the HV battery. The vehicle body becomes the connection between positive and negative. The current will rise extremely quickly until the fuse in the service plug and/or the HV battery pack blows to protect the system.
Because with poor insulation on the positive or negative side there is not yet a closed circuit, the fuse in the service plug will not blow. The continuous insulation monitoring in electric vehicles detects such current transfer, warning the driver via a fault message. With an insulation fault, the vehicle may still function, unless the manufacturer shuts it down via software.
Number 5 in the image below indicates the component where continuous insulation monitoring takes place. In reality, this electrical section is of course more complex.
Number 6 indicates the measuring resistor across which the voltage drop is measured in parallel.
The two images below show the situations where there is poor insulation on the positive side (left) and on the negative side (right). Because current starts to flow through the measuring resistor, voltage is dropped in the resistor network. 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 the continuous insulation monitoring detects a deviation, it stores a fault code. Possible descriptions for the P-codes (such as P1AF0 and P1AF4) may be: “battery voltage system isolation lost” or “battery voltage isolation circuit malfunction”. When a vehicle comes into the shop with an insulation fault, after using the scan tool, or manually with a Megohmmeter, the technician can measure the insulation resistances to check whether there is an insulation leak somewhere.
Diagnosis with the Megohmmeter:
In the previous section, the term “insulation resistance” was explained and it was shown how the vehicle uses continuous insulation monitoring to check whether there is leakage from the positive or negative connections from the HV battery to the vehicle body. In this section, we go into this in more depth and describe how you, as a technician, 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 scan tool can, for certain brands, perform an insulation test itself, for example for components that only show an insulation fault after being switched on, such as the electric heater or the electric air conditioning.
In other cases, we can measure the insulation resistance with a Megohmmeter. With a standard multimeter it is not possible to measure insulation resistance, because the internal resistance of the multimeter can be as high as 10 million ohms. The internal resistance is too high to measure high resistance values. A Megohmmeter is suitable for this and outputs a voltage from 50 up to 1000 volts to simulate operating conditions. This high voltage ensures that the output current finds its way even through the smallest damage in the insulation, via 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 leads and setting up the meter correctly, press the orange “insulation test” button. The set voltage (in the image: 1000 volts) is applied to the test leads and therefore to the component, and then we read the ohmic value from the display.
- An insulation resistance greater than 550 MΩ (megaohm, which means 550 million ohms) is OK. This is the maximum measuring range;
- A value lower than 550 MΩ may indicate a leak in the insulation, but it does not necessarily have to be 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. With a nominal HV voltage of 400 volts, the resistance must be (500 Ω * 400 v) = 200,000 Ω.
- Manufacturers often set higher quality and safety standards, resulting in higher minimum insulation resistances. For that reason, factory specifications must always be followed when making a diagnosis.
The manufacturer’s specifications are always leading.
The factory specifications describe the steps, safety procedures, and the minimum insulation resistances.
In the next image, we see a screenshot from a Toyota manual. For the model in question, the minimum insulation resistances of the cables to the electric motor are shown.
The Megohmmeter must be set to 500 volts and the minimum resistance of the wiring (U V and W) to the electric motor with respect to the housing must be 100 MΩ (MegaOhm) or more.
The insulation resistances of, for example, the electric A/C compressor and heating element may be different. For measurements on other components, consult that section of the factory data.
1. Insulation measurement on the negative side (no fault):
With the connector disconnected, we also measure the negative side with respect to vehicle ground. Images 1 and 2 show what this measurement looks like in a wiring diagram and in reality. 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, for example from the inverter, we attach the red test probe to the pin in the removed connector (now on the positive side) and the black test probe to a ground point connected to the vehicle body. In image 1, the diagram from the previous section is shown again, with the HV battery (1), vehicle ground (2), and two of the loads (3 and 4) numbered. The Megohmmeter is connected, and the orange “insulation test” button has been pressed to measure the insulation resistance using the output voltage of 500 volts. This is 133 megaohms. The insulation resistance is lower than in the previous measurement. The manufacturer’s specifications must be consulted. We use the minimum insulation resistance of 100 MΩ specified by the manufacturer. The insulation resistance is OK.
3. Insulation measurement on the positive side (fault):
When measuring at the same connections, we measure an insulation resistance of 65 MΩ. Although the resistance value is higher than the minimum 500 ohms per volt required by the IEC and IEEE (see the previous section), the wiring and/or the component is rejected because the manufacturer specifies a minimum resistance value of 100 MΩ. The wiring and/or connector connections must not be repaired, but must be replaced completely.
4. Insulation measurement on the positive side (fault):
When an insulation value of 0 MΩ is measured, there is a direct connection (i.e., a short circuit) between the HV wire and the housing. The wiring and/or connector connections must not be repaired, but must be replaced completely.
With an insulation fault, the connectors of other loads can be disconnected one by one to measure in the connector, in the way shown in the text and images above.
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