Maximising safety in advanced battery systems

In advanced battery systems, the quality of the power electronics helps determine the quality of the final product, its level of functionality, and its reliability. This is emphasised in the latest battery power management and charging systems that use wide-bandgap semiconductors and improved power topologies. In such advanced electronics, if the safety systems do not work quickly and reliably, the entire battery system can suffer a catastrophic failure that could seriously impact the product and its user.

Almost every advanced application space currently under development requires next-generation battery systems. Smart grid-level energy systems need more power storage just as much as advanced electric vehicles (EVs) do and those needs are connected. Each space has its own requirements but the core need of safe, reliable, and economical energy storage is fundamental across every application.

In life-critical application areas, such as automotive and industrial safety, the importance of fail-safe sub-systems is paramount. The battery system in an EV, for example, is similar to the petrol tank in a traditionally combustion engine vehicle, running on petrol, in that the stored energy needs to be safe. There is a considerable amount of energy in a modern battery, and catastrophic failure can lead to thermal runaway – or fire. [Thermal runaway usually occurs during charging. When the temperature rises quickly, the rate of internal heat generation exceeds the rate at which the heat can be expelled. The high heat can propagate to the next cell, causing it to also become unstable.]

Managing short circuits

The higher cell densities in advanced battery systems demand proper safety protocols and devices as the power levels involved present a significant challenge when it comes to managing short circuits. To create a robust and safe system that helps ensure reliability over any given lifetime, manufacturers must integrate high-performance components into their designs.

Fuses, or similar devices, are required circuit-protection components that protect the system in the event of a short circuit by breaking the line under specific conditions. There are many types of fuses, but the primary element in each is a piece of engineered conductor, usually metal, and rated to fail (i.e melt) in a controlled manner under the raised line temperature conditions from a short circuit. This destructive protection method ensures the safety of the circuit by completely severing the power source.

Figure 1: An example of system co-ordination, the blue line represents the triggering threshold

The burn-out response has some drawbacks however, most notably that a power circuit under load will not necessarily have a consistent flow of current. In designs requiring pulses of high power, the wide operating range of the current drives the use of a higher current fuse to avoid nuisance tripping, but this leaves the system more vulnerable to overheating and thermal issues. That, and the non-reversible aspect of fuses, has led to the increasing use of electromechanical safety devices like circuit breakers, which can be reset as they do not rely on destructive elements to function.

Electromechanical protection

A contactor is another type of electromechanical protection device but it differs from a circuit breaker in that it is not intended to interrupt a short. Designed to connect directly to high-current loads, contactors are high-power switching devices operated by an external control. Power contactors are preferred in demanding situations where circuit resetting is needed, and high current levels are present. Though similar in operation to relays, contactors differ in robustness and available features to control and suppress arcs created when switching.

When it comes to battery systems, high voltage contactors provide safe circuit continuity in hybrid and electric vehicles, as well as in charging systems and high-powered industrial applications. These types of contactors can rapidly and securely connect and reconnect the circuit, managing arcing and in-rush situations. For example, in EVs, normally open contactors safely join the battery pack to the system, disconnecting when the vehicle is not in use.

There are a variety of circuit protection solutions that can open a circuit faster than a fuse and can do so closer to normal operating conditions without nuisance tripping, resulting in less potential for damage than if a thermal fuse were used.

Companies are working on hermetically-sealed electromechanical devices with low heat generation, for example, that allow for circuit trips at exact currents and feature a design that significantly reduces resistance, eliminates thermal ageing, and increases system efficiency. Such devices use the magnetic field of the current, the Lorentz force, to trigger the device to open.

Figure 2: Intended applications include charging electric vehicles

The benefit of this new generation of devices being developed is that they generate very little heat in operation. They are immune from the thermal ageing and related nuisance tripping caused by hot/cool cycles in the circuit. Such temperature cycles over time would cause the conductor metal in fuses to become brittle, reducing operational life by compromising the physical integrity of the connection. Devices with fast and consistent clear times, regardless of ambient temperatures, allow the end user to design a safe circuit while at the same time reducing the performance requirements of the contactor.

Performance enhancements

Improving the performance of the contactor in the circuit is a key objective. A contactor that is incorrectly paired with a fuse can actually prevent the fuse from doing its job. As the contactor levitates, it starts to dissipate some of the energy that should be available to trigger the fuse, instead, loading the current on the contactor. An overloaded contactor can prevent the fuse from ‘seeing’ the short, which can result in catastrophic failure.

For example, in a situation with a 500A contactor and 500A fuse, it would take seconds at 1,000A to trip, whereas the GigaFuse, for example, can be set at 1,000A and trip within 3ms. The GigaFuse, by Gigavac and available from Sensata, is believeved to be the only hermetically sealed electromechanical fuse. It is designed for high voltage and high power fuse application requirements.

Figure 1 illustrates how the GigaFuse operates to protect circuits. The blue line represents the triggering threshold. The device can open the circuit faster than a fuse and closer to the desired operating conditions.

With thermal fuses there is a grey zone where current levels may overwhelm the contactor’s ability to interrupt the load without reaching the thermal point for a fuse to trigger. This stretch of time before the thermal fuse can be activated while exceeding the breaking capability of the contactor can now be eliminated.

The GigaFuse can open the circuit closer to the desired operating conditions

Such devices have a wide range of applications in industrial settings.  The contactor-fuse pairing is critical in all high voltage systems. These include battery packs for for hybrid and electric commercial vehicles (trucks, buses, utility vehicles) as well as battery packs for forklifts, scissor lifts and other material handling products.  The contactor-fuse pairing can be especially challenging in high voltage systems above 600V – particularly in energy storage systems (ESS), DC Fast Chargers, and PV/solar inverters. The fast interrupt time of the Gigafuse can ensure system safety in these systems both in daily use and in the event of short circuits and other system anomalies.

The ability of electrical protection devices to address circuit safety in high-power systems is not restricted to advanced batteries. A multitude of applications, from motion control to alternate energy generation, can benefit from such a component’s efficiency, thermal performance, and speed. This type of fast acting, hermetically-sealed electromechanical device is especially beneficial in situations where thermal aging or nuisance tripping are a problem.