Key takeaways
- A DC arc does not self-extinguish, so the disconnect needs a longer gap, a blowout or both.
- Precharge limits the inrush into the inverter capacitance and protects the main contacts from welding at make.
- Contact material must resist erosion and welding at high DC current, not just carry the load.
- Weld detection and isolation monitoring are safety functions, not optional diagnostics.
What changes in an EV battery circuit
A battery disconnect sits between a high voltage battery and the rest of the vehicle. It has to carry the drive current, isolate the pack for service and break the circuit under fault. The source is a low impedance battery, so the available fault current is high and does not fall away like a typical mains supply.
The load is not a simple resistor. The inverter has a large DC link capacitance, which looks like a short circuit at the moment of connection. The motor is inductive. A fault may be a short circuit, a ground fault or a crash event. Each case produces a different current waveform and a different arc duty.
The contact system also has to work in a sealed environment, with limited service access and a long life target. That combination makes the contact design a safety function, not just a component choice.
Precharge and the closing sequence
If the main contact closes directly onto an uncharged DC link capacitor, the inrush current can be very high. The contacts may weld at make, and the capacitor and wiring see a stress they were not designed for. Precharge avoids this by connecting the pack through a resistor first.
The sequence is usually: close the precharge contact through a resistor, wait for the link voltage to rise, close the main contact, then open the precharge contact. The main contact then closes with little or no voltage across it, so the make current is small. The precharge contact and resistor carry the inrush for a short time.
The timing and the resistor value set the precharge current and the time to reach an acceptable voltage. Too fast and the resistor and precharge contact are overloaded. Too slow and the vehicle start is delayed. The control system should verify the link voltage before the main contact closes, not rely on a fixed delay alone.
Infographic
Precharge and main contact sequence
How an EV disconnect avoids a high inrush at make
Check
Confirm the link voltage and the isolation state before closing.
Precharge
Close the precharge path through the resistor.
Rise
Wait for the DC link to reach the accepted voltage.
Main close
Close the main contact with little voltage across it.
Precharge open
Open the precharge path once the main contact carries.
Break
Open the main contact under load with the arc driven away.
The DC arc and the blowout
A DC arc has no current zero. It continues until the voltage across the gap is no longer enough to sustain it, or until the arc is stretched and cooled below the arc voltage. In a high voltage pack, the arc can persist for a long time and deposit a lot of energy in the contact and the chamber.
A magnetic blowout uses a permanent magnet or a coil to push the arc away from the contact face. The arc moves onto runners, is stretched and is cooled by splitter plates. The blowout has to be strong enough at the lowest current it must break and controlled enough not to trap the arc or re-strike.
The gap and the chamber are part of the rating. A larger gap helps withstand the recovery voltage after the arc extinguishes. A sealed gas volume can improve the dielectric strength and control contamination. The contact material, the gap and the blowout are a system, and a change to one can change the breaking capacity.
Breaking capacity is a system rating
A contact material data sheet does not give a DC breaking capacity. The rating belongs to the contact, the chamber, the blowout and the circuit together.
Contact material and the welding risk
EV disconnects usually use silver tin oxide or silver tungsten based materials. Silver tin oxide with a fine structure resists welding at make and gives good erosion resistance at moderate DC currents. Silver tungsten and tungsten carbide resist erosion and welding at higher energy, but they need more force and are harder to join.
The contact face is often large and may be divided into several contact points. Multiple points share the current and reduce the current per spot, which lowers the welding risk. They also give redundancy if one point is contaminated. The geometry has to keep the points loaded evenly, which is a challenge with a tilting armature.
The joint between the contact and the carrier is part of the current path. A void or a poor braze creates a hot spot under continuous load and can grow under thermal cycling. The joint should be inspected by a method that can find voids, not only by a visual check.
| Requirement | Design response | Verification |
|---|---|---|
| High inrush at make | Precharge resistor and sequenced contacts | Link voltage check before main close |
| DC arc at break | Magnetic blowout, runners and a longer gap | Breaking test at rated voltage and current |
| Welding at fault | Weld resistant material and multiple contact points | Fault current make and break test |
| Continuous carry | Low resistance joint and a good thermal path | Temperature rise at rated current |
| Isolation for service | Adequate gap and isolation monitoring | Dielectric and insulation resistance test |
| Sealed environment | Controlled gas, clean surfaces and a leak check | Leak test and contamination inspection |
| Long life | Erosion allowance and a stable face | Electrical life at the load profile |
A design prompt. The actual values come from the vehicle specification and the applicable standards.
Force, holding and shock
The contact force has to hold the contacts closed against the electrodynamic force at the fault current. At very high current, the current path can produce a force that pushes the contacts apart. A blow-on geometry can use the fault current to help hold them closed, but it has to be designed for the direction of current in both charge and discharge.
The force also has to survive shock and vibration. A vehicle event can produce acceleration that the armature and the contact have to resist without opening or chattering. A contact that opens momentarily under shock draws an arc and can weld. The mechanism and the contact spring should be checked as an assembly under the specified shock profile.
The holding force is not the same as the closing force. The magnetic circuit may hold the contact closed with a force that is lower than the peak closing force. The design should state both, and the contact should be stable at the lower value.
Sealing, gas and contamination
A sealed contact chamber keeps contamination out and can control the gas composition. That matters because the dielectric strength and the arc behaviour depend on the gas. Moisture, dust and organic vapour can all change the surface and the arc.
The seal also has to survive thermal cycling and pressure changes. A leak can let moisture in, which raises the risk of corrosion and tracking. A leak test at the end of assembly is a basic check, and a sample from each lot should be checked after thermal cycling.
Contact materials can outgas or produce debris during arcing. The chamber design should provide a place for the debris to settle away from the contact gap and the insulation surfaces. A small amount of debris in the wrong place can reduce the creepage distance and cause a failure.
Weld detection and isolation
A welded contact is a safety hazard because the pack cannot be isolated. The control system should be able to detect a weld, usually by checking the voltage across the contact after an open command or by monitoring the current path. The detection has to work at the full pack voltage and in the presence of noise.
Isolation monitoring checks the insulation resistance between the high voltage system and the chassis. It is a separate function from the contact, but it depends on the contact being open. A contact that leaks or tracks can defeat the measurement.
The diagnostic coverage should be defined with the safety case. A contact that is assumed to open but does not is worse than a contact that reports a fault. The design should fail safe where possible and give a clear indication where it cannot.
Testing the disconnect
Test the contact at the rated current for temperature rise, at the fault current for make and break, and at the full voltage for isolation. Add a precharge test at the worst case link capacitance and a shock and vibration test with the contact energised.
For the break test, use the real circuit inductance and the real blowout. A resistive test at a short time constant is much less severe than the vehicle fault. Measure the arc duration, the peak current and the contact face damage.
Inspect the contact after the test and check for welding, deep pitting, material transfer and loose debris. A contact that cleared the current but welded is a fail. Keep the test conditions and the material lot with the results.
Frequently asked questions
Why does an EV disconnect need a precharge circuit?
The inverter DC link capacitor looks like a short circuit at make. Without precharge, the inrush current can weld the main contacts and stress the capacitor. Precharge limits the current through a resistor before the main contact closes.
Why is DC arcing harder to break than AC?
An AC arc passes through zero twice per cycle and can extinguish there. A DC arc has no natural zero, so it continues until the circuit or a blowout drives the voltage below the arc voltage. That means a longer arc and more contact erosion.
How is a welded EV contact detected?
The control system can check the voltage across the contact after an open command, or monitor the current path. The method has to work at full pack voltage and in the presence of electrical noise. The detection strategy belongs in the safety case.
Which contact material is used in EV relays?
Silver tin oxide and silver tungsten based materials are common. Fine structured tin oxide resists welding at make. Tungsten grades resist erosion at higher energy. The choice depends on the current, the voltage, the blowout and the life target.
Sources and further reading
- IEC 60947-2, Low-voltage switchgear and controlgear, circuit breakers
- IEC 60664-1, Insulation coordination for equipment within low-voltage supply systems
- Qumek EV disconnect contact and precharge test notes
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