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Lithium-ion battery construction: Firefighters should know the differences

From chemical make-up to cell construction, not all lithium-ion batteries are the same

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On the right you can see layers of film found inside a cylindrical battery cell.

Photos/Munro & Associates

Lithium ion (Li-ion) batteries are becoming extremely popular. They can be found in many consumer electronics, e-mobility devices and electric vehicles.

Most firefighters are aware that Li-ion batteries can be hazardous; however, many do not realize that Li-ion is a fairly generic term. There are multiple types of Li-ion chemistries as well as different types of cell constructions. Basic understanding of battery cell chemistry and construction will help firefighters when failures result in fires.

Battery cells and modules

A battery cell is the smallest part inside a battery pack. Inside of the battery cell are layers of film that provide the chemistry needed to supply an electrical charge. The materials inside the cell use different chemicals and are typically only 3-4 volts.

Battery cells are typically packaged into three different types of containers:

1. Cylindrical cells have the layers of film rolled up and positioned inside a hard (typically steel) outer shell. They are very similar to the size and appearance of a AA battery and can be bigger than a D battery. One of the most common sizes is currently an 18650 battery (18mm diameter x 65mm long). Tesla has started using the 4680 battery (46mm diameter x 65mm long).

2. Prismatic cells are square/rectangular containers that have a hard outer shell. The layers of film inside the shell are stacked and roughly the same size as the box that they are inside of. They are typically very durable and heavier than other types of cells.

3. Pouch (polymer) cells are very similar to prismatic cells, but the layers of film are packaged inside of a soft or flexible outer shell. These types of cells will slightly expand as the battery cell is charged and slightly contract when the battery cell discharges. These cells are prone to swelling if they fail.

A battery module is a case that contains multiple battery cells linked together to provide a higher voltage. The number of battery cells and voltage depend purely on the size of the device the module is powering. Smaller products (e.g., cell phones) might only contain a single battery cell. A larger product could contain a battery module.

The number of modules in a device depends on the size of device they are powering. For example, an e-bike may only have one or two modules, while some electric vehicles could have 10 or more.

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Left photo: Cylindrical cells in the middle. AA- and D-size batteries for scale. Right photo: Pouch (top) and prismatic (bottom). AA- and D-size batteries for scale.

Photos/Munro & Associates

Battery cell chemistry

There is a misconception that Li-ion batteries contain pure lithium metal. While there are lithium metal batteries on the market, they are typically not rechargeable. Lithium metal is highly reactive, which is why they are not used for rechargeable products. Further, this is one of the many reasons that a Class D fire extinguisher will not stop a thermal runaway.

Rechargeable batteries use different types of Li-ion battery chemistries with some of the more popular chemistries listed below.

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Battery cell failures modes

A battery cell can fail due to mechanical damage, an electrical short, thermal damage from abnormal operation (i.e., improper charging) or thermal damage from an external source. If the cells heat up for any reason, they can catastrophically fail at a relatively low temperature. The temperature threshold is different for every type of Li-ion battery chemistry.

Signs a battery cell is about to fail include off-gassing, hissing, popping and/or heat buildup. When the cell fails, it gives off a tremendous amount of heat (1,200 degrees F) in tenths of a second. The failure is an exothermic chemical reaction that does not require oxygen from the atmosphere. This is because most chemistries contain oxygen within the chemical makeup.

For firefighters

Having the basic knowledge of battery construction is very similar to studying building construction. It will help significantly when analyzing hazards and deciding the best tactics to move forward with at an incident.

The size and number of cells in a battery will give a rough indication of the voltage inside that battery. An e-scooter, for instance, has a voltage of 24-48 while an electric vehicle is at 300-800. Finding a damaged battery can be hazardous, but a small e-mobility device is far less hazardous when compared to a fully electric vehicle.

If the battery is on fire, your strategy will be based on the size of the battery. A thermal runaway in a smaller battery will finish fairly quick, while a large electric vehicle battery can burn for a much longer time. As firefighters, we must always continue to train and learn as new technology and products become available to the public.

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Ford Mach-E battery module with pouch-style cell on bench.

Photo/Munro & Associates

The future of batteries

Battery technology is advancing at a fast pace. The chemistries used today will likely be different in 5 to 10 years. Ideally these advancements will lead to safer battery cells that are less sensitive to heat and won’t fail catastrophically when mechanically damaged. One thing to remember, even when such advancements are made, firefighters will still face fires related to legacy consumer electronics, e-mobility devices and electric vehicles that pose the risks we are dealing with today.

Battery news and resources

Patrick Durham serves as the captain and training officer at Station 4 within the Troy (Michigan) Fire Department. Durham is a mechanical engineer, presently engaged in cutting-edge automotive industry projects. Notably, he has been involved in designing innovative multi-material battery structures for electric vehicles. Drawing from over 15 years of combined experience as a firefighter and engineer, Durham has developed specialized training courses for firefighters, as well as YouTube content, focusing on various technical aspects, including the specific challenges associated with responding to incidents involving EVs. Durham is also a member of the Technical Panel for Fire Safety of Batteries and Electric Vehicles at UL’s Fire Safety Research Institute, where he contributes his expertise to advance the field of fire safety in the context of emerging battery technologies and electric vehicles. Learn more at StacheD Training or reach Durham via e-mail.
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