Talk Nerdy to Me – Lithium Ion Batteries

Power… I need power… in today’s fast-paced world, the equipment you rely on likely needs power, and in many cases, that power comes from batteries. Battery types like lead acid, nickel-cadmium (NiCd), nickel-metal hydride (NiMH), alkaline, carbon zinc, lithium, mercury, silver oxide and zinc air, have a unique fire hazard and documented protection measures, but not the Li-ion battery.

Li-ion technology has advanced and become more prominent over the last ten years. Li-ion batteries power laptops, cell-phones, tablets, vapor cigarettes, power tools, electric cars, and building energy storage systems. Their popularity is due to power efficiency and the reduction of cost over that time period. A typical Li-ion battery can store approximately 4 to 7 times more than the lead acid or NiCd equivalent. For instance, Li-ion 18650 is expected to produce 3.7 V compared to its counterpart the Alkaline AA at 1.5 V. Additionally, Li-ion batteries do not have a memory that requires a full discharge before being charged, and a single battery can handle hundreds of charge/discharge cycles.

A Li-ion battery is typically constructed as either a “jellyroll,” such as the 18650, a round battery about the size of a AA battery; a “prismatic,” which is a flat square or rectangular hard case battery like those found in many cell phones; or a “pouch cell,” a square with soft-pack. All the configurations consist of the same basic parts and include positive electrode, negative electrode, separator, organic electrolyte, and casing. The organic electrolyte is a flammable or combustible liquid depending the mixture. The electrolyte typically consists of a mixture of ethylene carbonate (flashpoint of 293° F) and diethyl carbonate (flashpoint of 77° F).

The primary concern with Li-ion batteries is thermal runaway. Thermal runaway results in temperature increases within the casing, pressure increases within the casing, case venting, and possible ignition of the vented gases followed by battery ejection and possible propagation to other cells. Propagation in other cells is typically a concern in battery packs or arrays such as the Tesla battery pack, which contains over 6800 18650 Li-ion cells.

Manufacturers implement safety features and components to limit the likelihood of thermal runaway, such as temperature sensors, voltage regulators, charge state monitors, and condition monitors. Damage to the battery or the monitoring and control features could circumvent the ability to prevent thermal runaway. Damage may be due to external heating above 177° F, dropping or denting resulting internal structural damage, overcharging, over discharge, short circuiting, or manufacturing defects.

Li-ion batteries have fire protection challenges as well as post response concerns. The fire protection challenge is due to the construction of the battery, volatility of the battery if damaged, and storage configurations. The highest probability of a fire event is during or right after charging. The Li-ion battery is at its most stable condition below 50% of full charge but not in the discharged state.  Therefore, storage of batteries along with the various levels of combustible protective packaging requires a continuous turnover of inventory and monitoring of the charged state of the batteries. LI-ion batteries lose 5% per month without any load applied. Large battery arrays, such as those required for electric cars, pose a significant challenge as a single storage area could require storage of more than 10 million 18650 Li-ion batteries.

FM Global whitepaper “Description of Hazards and Related Protection Schemes Proposed by FM Global for the Storage of Lithium-ion Batteries” provides guidance for fire protection as appropriate for flammable and combustible liquids, rather than gaseous suppression. Burning Lithium ion does not produce hydrogen gas in the same way batteries containing lithium metal does. Currently, NFPA 13 does not have requirements specifically aimed at the protection of Li-ion batteries, nor is there a standard for such protection. Additional research is being conducted by various agencies.

By Corey Stanley
Senior Fire Protection Engineer