By Felix Frisch, VP Sales and Marketing, Epsilor Electric Fuel
The military vehicle industry was one of the first ones to adopt lithium ion battery technology during the last decade.
With quadrupole energy density, high power rates and superior life cycles, Li-ion batteries are favored by most military vehicle developers and advanced armored corps.
At the same time, safety hazards arising from the use of lithium batteries, and their anticipated behavior in case of thermal runaway, are causing many military organizations to cautiously wait for a significant improvement in this field prior to allowing wide integration of this promising technology into their main vehicle fleets.
A possible breakthrough in lithium battery fire safety has been recently announced by Epsilor Electric Fuel, an Israeli based defense battery manufacturer, following a rigorous series of bullet penetration tests it has performed. The positive results of these tests together with a new NATO 6T battery design based on these tests may support a positive deployment decision to be made by such doubtful vehicle operators.
Despite their wide usage in military applications for almost two decades, and their wide penetration to the emerging electric vehicle sector, lithium batteries are still considered by armies as a safety hazard. The caution is due to the high-profile reports of battery thermal runaway in cell phones, tablet computers and sometimes also in cars. Moreover, in military applications, batteries are subject to combat related risk such as bullet penetration where the vehicle must ensure a safe recovery of its crew from the threat zone.
The series of bullet penetration tests held by Epsilor during last January have demonstrated how two different types of lithium batteries - Li-Ion NCA battery and Lithium-Ion LFP battery - can contain thermal runaway in the severe case of bullet penetration. The tests have proven two main operational assumptions:
- A lithium-Ion LFP (Lithium Iron Phosphate) battery installed inside a crew compartment and even near soldiers, can "catch a bullet" and still prevent a severe thermal runaway that may directly endanger the crew
- A fireproof casing of batteries can ensure that Lithium-Ion batteries installed in external battery compartments will continue to operate and enable vehicle to evacuate to safety even if one of the batteries caught fire after being penetrated by a bullet.
Two Sets of Standards
The tests have been formulated based on two sets of requirements regarding the usage of Li-ion batteries onboard military vehicles:
- The US army Ground Vehicle Systems Center (GVSC) requirements, named MIL-PRF-32565B: this standard defines that thermal runaway must be restrained both when the battery is penetrated by a projectile (bullet or nail) or is exposed to extremely high temperatures in excess of 500 degrees C. The latter requirement appears to be particularly relevant to naval situations where the crew cannot evacuate from a vessel. However, the prevention of battery disintegration or a release of particles and hot plasma in case of bullet penetration, are widely adopted by military users around the world.
- The Israel Defense Force (IDF) requirements: armored vehicles of the IDF are equipped with external battery compartments. Their main requirement is that in case a battery compartment experiences a direct hit by enemy fire, the vehicle will be able to continue its mission at least until a safe evacuation of the crew from enemy fire threat. According to this requirement, it is less important how the penetrated battery will end-up, but it is most important that fire will not propagate to neighbouring batteries, that have to continue operating normally.
To summarize the difference between the two requirements, in one case the penetrated battery has to stay "in one piece" in order not to endanger the crew, while in the other case, the battery compartment has to protect the other batteries form the penetrated battery. The second, more tolerant case opens the possibility of using more energetic batteries, that do not need to be located near personnel.
In a series of tests that took place in December 2019 and January 2020, Epsilor exposed two of its products to 7.62 mm armor piercing incendiary bullets shot from an AK-47 assault rifle. Although thermal runaway developed in both batteries, causing them to burn, in both cases batteries did not explode and presented positive results.
- Lithium-ion LFP 6T batteries (Type ELP-02426; 2.74kWh) are designed to be installed within crew compartments. Epsilor’s 6T LFP battery is covered by an integral steel case incorporating a directional pressure relief. When installed on armored vehicles, such pressure relief is directed towards external parts of the vehicle or towards a manifold, enabling safe release of pressure and gas to the open air outside the vehicle. After being penetrated by a bullet the battery ignited slowly and burned moderately, releasing flames and steam from the directional pressure relief. The battery burned for some 20 minutes, however, no solid particles were released from the battery and it did not disintegrate.
- Lithium-ion NCA batteries (Type ELI-52526; 4.2kWh) have superior energy capacity of approximately four Lead-Acid batteries of the same size. These batteries are designed for installation outside crew compartments. This battery was installed in a fireproof composite case aiming to prevent battery disintegration as well as fire propagation to neighboring batteries. This battery burned for approximately 45 minutes, next to another similar battery, contained in a separate fireproof case. During the test a neighboring battery was installed in a similar composite case. During the test this battery did not catch fire, and it continued to operate and to generate power after the end of the test.
The promising results demonstrate that 6T lithium batteries can be designed to address the most demanding requirements of two pioneering developers and users of defense vehicles. A sufficient implementation of the new battery and compartment designs, will enable other defense vehicle users to widely deploy lithium batteries in their fielded vehicle fleets.
There is, however, another most important aspect of lithium battery safety that is not discussed in this article. The most common cause for thermal runaway in lithium batteries and particularly in military lithium batteries is short-circuit. This "daily life" event is not as popular among defense organizations as the infamous AK-47 bullet, but the vast majority of reported thermal runaway cases has been a result of batteries the following: crushed lithium cells resulted by bad battery craftsmanship, short circuit resulted from water penetration to poorly sealed batteries and uncoated circuitry, short circuit resulted from poorly soldered wires, etc. As always, safety is mostly a result of professional design and high-quality manufacturing.