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Heat lock: Passive fail-safe technology for resilient battery systems

Rechargeable lithium-ion (Li-ion) batteries are an indispensable decentralized source of energy. In light of the Paris Agreement, the European Green Deal, and the pricing of greenhouse gas emissions, the use of electrochemical energy storage options is a strategic imperative in a wide range of applications. That spans from supplying decentralized units such as in the military sector to being used in uninterruptible power supply (UPS) systems like for hospitals and data centers, and from storing energy for personal use generated with an in-house photovoltaic system to enabling the operation of battery electric machines such as battery electric vehicles (BEVs), e-bikes, e-scooters and power tools.

Accounting for the lion’s share of battery-powered applications is the accumulator segment, usually designed as a rechargeable battery pack. This kind of pack usually consists of multiple Li-ion cells. Thanks to the continuous development of this technology, its use is also becoming increasingly attractive from an economic point of view, as demonstrated in a 2020 study by Horváth & Partners.

Figure 1 Here is how the global price trends for Li-ion batteries look like. Source: Energie & Management Powernews

It’s mainly due to two reasons.

  1. Economies of scale and cost digression in production and manufacturing processes.
  2. Miniaturization of the individual cell along with a simultaneous increase in energy density.

Despite the increasing presence on the market, it’s important to bear in mind that rechargeable battery packs continue to carry a certain residual risk of latent hazards, in particular due to point number two, resulting in rechargeable Li-ion batteries having disadvantages in terms of safety.

Domino effect due to excessive thermal load

An integral component of the Li-ion cell is the electrolyte. It often comprises a mixture of flammable organic solvents like ester compounds and a conducting salt (lithium salts), which improves the electrical conductivity. This mixture is highly flammable and, when combined with an excessive thermal load, can lead to the formation of explosive mixtures. Due to constant efforts to further increase the energy density of Li-ion cells, this constitutes a potential hazard for the end user.

Continuous and unwanted thermal input can lead to irreversible damage to the rechargeable battery pack or, and in a worst-case scenario, to thermal runaway, which is an unintentional and extremely dangerous sudden release of the stored energy.

The key parameter here is the temperature, as electric battery cells have a narrow operating range of +15°C to ±45°C. When this range is exceeded, the high temperature poses a threat to the functional safety of the overall system.

The highest statistical probability of a cell defect occurs when the battery is overcharged. This may lead to destruction of the cell structure, which is usually associated with heat generation and in some cases even an explosion.

Naturally, the manufacturers of rechargeable battery packs are aware of this risk factor, which is why there is a battery management system (BMS) as well as a primary and a secondary protection circuit embedded in the electronic safety architecture. Among other things, it ensures that the battery remains within its specified operating range in terms of charging and discharging cycles. However, it should be noted that algorithms and the hardware they control are not immune to failure. The same holds true for a potential collapse of semiconductors used in the primary protection circuit. In a worst-case scenario, both can fail, and an undetected, excessively high load may cause the battery system to ignite and explode.

Heat lock: An autonomous, passive fail-safe element

To counter the above-mentioned concerns, RUAG Ammotec GmbH has developed the heat lock technology that is capable of protecting the battery pack in the event of an excessive thermal load and placing it in a safe state while decoupled from the electronic safety architecture of the conventional protection circuit. The heat lock technology is based on a passive, thermosensitive agent. A sample illustration of the heat lock element is shown in Figure 2.

Figure 2 The heat lock element is shown here without battery peripherals. Source: RUAG Ammotec

In the context of electric battery systems, heat lock technology is to be understood as an entirely independent pyrotechnical switch-off device. The underlying idea is that current flows from a battery into a load, heating up the battery in the process. However, this heating up remains unnoticed by the primary protection circuit even after it exceeds the permissible level.

The basis of the application is a physicochemical sensor that continuously monitors its environment, is triggered by heat input (heat), and as a result, permanently blocks a flow of electrons (lock).

When a critical temperature is reached, a process is set in motion in the heat lock element, which, by means of an increase in internal pressure, causes an insulating piston to shear off a current conductor passing through it, thus electrically insulating the remaining ends of the conductor from one another permanently.

The heat lock element permanently interrupts the conductor, thereby preventing further current flow and the resulting dangerous heating up of the battery. A single-use technology of this kind prevents the damaged system from being switched back on in an uncontrolled fashion.

The focus here is on protecting against overheating in battery-electric applications with the aim of protecting the overall system and ultimately the user against the resulting damages described above.

The rotationally symmetrical design of the heat lock shown in Figure 3 allows rechargeable battery pack manufacturers to easily add an additional layer of safety to the primary system during the development cycle. As the unit is hermetically encapsulated on the inside, it can easily be handled in an automated battery assembly line, thereby ensuring a consistent workflow in manufacturing.

Figure 3 The heat lock element is embedded in a battery system. Source: RUAG Ammotec

Initial impressions from the pilot environment on the integration side point out the watchdog character of the application, which keeps an eye on the environment alongside the hardware/software architecture. It’s important to understand that the heat lock technology functions entirely autonomously, and therefore, it doesn’t require a separate power supply. However, additional control via an electrical pulse or the aforementioned BMS can also be integrated as an option.

Heat lock vs. thermal fuse

Heat lock technology must be clearly distinguished from conventional thermal fuses. While such components serve important roles in a wide variety of applications, heat lock is unique in a sense that it’s not triggered by either current or voltage. Instead, the condition of the environment being monitored is what is used to ensure that the primary system is protected in the event of a failure of the electronic safety architecture. Limiting factors such as the low rated current of commercially available fuses and the comparatively high cost of semiconductor devices have had a significant influence on the design of heat lock. The goal is to make the development of electric battery systems safe and economically worthwhile.

Furthermore, the threshold value, which is an important consideration in the low temperature range, merits a special emphasis. It can be configured precisely to ± 2 K from a temperature starting at approximately 60°C, depending on the specific application scenario on the integration side. The basic version already allows rated currents of up to 40 A. In addition, the application is scalable in terms of the dimensions of size, temperature range and current, and it’s harmonized with the integrator following a requirements analysis.

Figure 4 provides insight into the characteristic triggering behavior of heat lock technology. Both the configuration of the threshold temperature as well as the specifics of the thermal bridge design are case-dependent parameters that rely on the heating rate and are performed in synchronization with the underlying application.

Figure 4 A heat lock element’s tripping behavior is shown at a threshold temperature of 93°C. Source: RUAG Ammotec

The temperature curve shown in black traces the gradual increase of the surface temperature on the battery cell that is to be protected, caused by the current flow shown in red. When a temperature value of 98°C—in this case, as an example—is reached, a change in the slope of the temperature curve can be observed, which is due to the increase in pressure inside the heat lock element, accompanied by an increase in temperature. After a duration of 8.3 seconds—in this case, as an example—a sudden drop in the flow of current (red rectangular signal) can be seen, which means that the conductor has disconnected, preventing further overheating of the rechargeable cell.

The deviation from the nominal triggering temperature (93°C) can be explained by the fact that the heating rate shown here was intentionally chosen to be higher for experimental purposes than is the case in reality. Another decisive factor is the design of the thermal bridge, which has a significant effect on temperature-related behavior. It can also be seen that, after separation has occurred, the temperature value continues to rise sharply for a short period—cooling time of the heat lock element—before reaching the safe system state discussed at the beginning.

Going extra mile toward sustainable energy

The benefits of electrification can only be fully exploited if the design at the component level is already fine-tuned to ensure the longest possible service life. Proactive integration of safety levels such as heat lock technology make it possible to design the energy storage system and its life cycle as sustainably as possible and to set the course early on for the success of a sustainable energy industry.

Tom Balogh is project manager for new business development at RUAG Ammotec GmbH.

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