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How new components are improving high voltage EV safety

21st August 2019

As the specialist EV market continues to grow; bigger, more powerful vehicles are increasingly powered by battery and hybrid solutions. Today, extremely high voltages are running through cables; sometimes as high as 1000V. With this much potential coursing through internal systems, safety for engineers, as well as maintenance professionals and consumers, is paramount.

Due to the presence of these high voltages, there are now hazards attached to every electrical component inside an EV. That’s without considering the risks posed by Li-ion batteries, which are prone to overheating and short-circuiting. Luckily, there are precautions that can be taken which greatly reduce the likelihood of a fault, and safeguard the vehicle in case of an accident or an emergency situation.

Addressing safety concerns early on, and during, the design process is the best way to ensure your vehicle won’t be hazardous; failure to do so can lead to costly re-designs or, worse, having to take your vehicle out of manufacture. If your vehicle is unsafe for operators, the consequences can be devastating and the reputational damages dismal.

Safety should be central to your specialist vehicles design. Whether you’re building a long-haul truck, a dust sweeper, an emergency response unit, or a garbage truck; safeguarding engineers and vehicle operators against sparks, fire, electric shocks and dangerous system failures, should be a top priority.

Thermal Runaway

The battery is a vital part of an EVs makeup. In many ways, it has the most important task: to power the vehicle. A key issue, however, is that an EV battery is highly susceptible to thermal runaway.

Lithium-ion batteries, which contain a flammable liquid electrolyte, are made up of cells. If a cell short-circuits, the electrolyte can combust and the pressure within the cell will rapidly increase until the cell vents the flammable electrolyte.

A ruptured cell can reach temperatures above 1000°C. Thermal runaway describes this rapid and extreme rise in temperature, and due to this rise in temperature, those reactions propagate, causing the same reaction in adjacent cells. In heavier vehicles with far more battery cells, the consequences of this propagation could be devastating.

Thermal runaway propagation can produce smoke, fire and even explosions; an occurrence that has been frequently reported in EVs, including this parked Tesla Model S in Shanghai.

While innovations in battery technologies could do away with our reliance on lithium-ion batteries, potentially enabling better charge rates in EVs; for now, there are protective methods through which engineers are increasing safety in Li-ion batteries.

Cell-to-cell protection is becoming more common in battery design, it works by inserting protective materials – such as phase-change materials – between cells, which absorbs the heat and deflects the flame; thereby protecting adjacent cells. As the temperature of the cell gets too high, heat is dissipated throughout the body of the material, helping to lower the temperature.

With the specialist market making increased voltage demands in EVs – notable in large powertrains such as buses and heavy-duty trucks – battery packs will get bigger, be used in tandem, and become increasingly powerful; beyond even the 800V battery pack sometimes used passenger vehicles. And for those vehicles that have to last longer on a single charge – such as emergency response units – battery packs will need to be denser.

With increasing demands for power, more and more high voltage systems are entering the specialist market, increasing the risks associated with thermal runaway propagation; this makes safety precautions such as cell-to-cell protection all the more crucial.

Arcing

In a high voltage EV, contactors are used to cope with high-intensity motor control demands. A specialist vehicle contactor tends to have additional features; such as increased ruggedness.

When switching high voltage DC at high currents, repeated low-level current overloads occur. This causes electrical arcs which need to be interrupted for an EV to operate reliably, and safely.

Further-to-the-point, a contactor is needed in case of an emergency, such as a crash. This can cause potentially damaging, extremely high short circuit currents – resulting in fire or injury.

In the event of a major incident, contactors must open to avoid extremely high short circuit currents that can occur in high voltage systems. As the contacts of the contactor open, an electrical arc forms between its two contacts.

The arc reaches millions of degrees centigrade and generates a transfer of material from one contact to the other, causing the contactor and consequently the vehicle to fail.

In high voltage machines, specialist contactors are used to deal with switching between electromechanical components. Contactors, built for hybrid and full battery electric vehicles, are adapting to ever-increasing voltage levels.

Innovations in technology have led to contactors that have continuous currents of 250A and a maximum switching capacity of up to 900VDC – allowing for arc suppression even when dealing with incredibly high voltages.

Crimping

In an EV, internal parts are connected using wires. These terminals have to be secured by a quality crimp, a solderless joint that presses together multiple wires by deforming them into one another.

Vital is that wires are crimped using the correct handheld or automated tools; otherwise, they cannot be deemed reliable, or safe. Tools need to be assessed for how suitable they are to a particular component or task, or else the engineer risks a bad quality crimp.

There are several risks associated with poorly crimped terminals. A bad crimp can result in terminals overheating, potentially causing dangerous system failures, or even fires and explosions.

‘An expert quality crimp is gas-tight, to keep oxygen and moisture from corroding the metals, and strong due to it being a single, moulded material with no breaks or joints.’

In a high voltage specialist EV, good quality crimping is even more essential. When dealing with high voltages, exposed wires can have dangerous consequences; with increased risks’ associated with fires and shocks.

The risk is increased further when you consider the requirements of specialist vehicles. Specialist vehicles don’t just have to get passengers from ‘A to B’, they have to be operational in volatile environments; they might have to travel over rough terrain or in icy conditions. Heavy-duty trucks often have to move heavy objects or debris.

Generally speaking, a specialist vehicle will have to cope with more than a passenger car – making a well-executed, good quality crimp, doubly important.

Conclusion

Safeguarding batteries against thermal runaway, reducing the risks associated with electrical arcs, and ensuring your systems are connected safely – via a quality crimp – is absolutely crucial. Also vital, is that manufacturers choose components that are fit for purpose. In the case of high-voltage, specialist vehicles; this means durable, lightweight parts that have a high-voltage rating and are built compact enough to allow room for additional systems. Remember, manufacturing safety into a vehicle is more important than any other design function.

Guide  High Voltage Automotive Components: Preparing for the Future Download Now