PPRuNe Forums - In flight battery fires with particular interest in lithium powered devices ?

The primary hazard in battery fires is that they contain the oxidizer for their intended reaction so external smothering that might put out liquid or solid fuels doesn't work too well. The main worries usually expressed about lithium based cells is the word "lithium" which, as a metal, is highly reactive. However there isn't a large amount of free lithium metal in lithium batteries, nor is it easy to introduce water into the cell. What is inside are usually organic (carbon based) electrolytes which themselves are combustible.

The typical failure mode is an internal short circuit which causes the cell to deform from the gases produced and either increase the amount of short circuit or to rupture the cell and force high-temperature electrolyte into contact with the air, leading to an external fire.

Immersing the cells in water absorbs the heat emitted by the electrical conversion from chemical to electrical energy in the short circuit which can slow or prevent the heating from going on to rupturing neighboring cells and potentially leading them to also short circuit and release their chemical energy as heat in a cascade.

Often the process starts during charging. Even though the amount of metallic lithium is small, it can form dendrites - sharp needle-like structures - that can pierce the internal insulation and form a short circuit. Or, in the case of a certain brand of 100% recalled phones, the case of the phone was just slightly too thin for the cell to fit, crushing the cell and increasing the chance for the internal insulation to get breached, particularly when being charged from thermal expansion.

A similar effect can happen when a battery, a group of connected cells, has cells that are not all equally charged. During charging the first cell to be charged continues to have current forced through it - this can either produce electrolysis or the formation of lithium metal dendrites. If electrolysis, the cell can swell or vent, losing further capacity. Likewise, if the cells are discharged, the low one on the capacity will have the rest of the cells forcing current though it in reverse charging, which also damages the cell.

In the case of the car battery recall - cells were made where an internal location was supposed to be insulated but, AFAIK, there was a tiny crease that left a portion uninsulated. Enough thermal/mechanical stress cycles and that allowed the gap to close like a switch - initiating a release of heat due to the short circuit, followed by cooking off and rupturing the cell and then its neighbors.

To cut the chances, don't actively charge or discharge the device when it's in an unobserved location. Certainly don't have an external short circuit, like charged cells loose in a pocket with keys or change. If the item is small, like smaller than a car, submersion to absorb heat to prevent a cascade failure is preferable. For car fires, they now just continue to flood the battery. If not an option (laptops that don't fit in the little sinks on aircraft) then a burn bag seems like the next best thing - just handle the bag like it might fail and put it somewhere a small fire can be contained. Burn bags also are useful in that moving the device while hot and unpredictable with hands is minimized.

The other thread talked about noxious fumes - not much different than any other fire generates - avoid breathing them. I've opened a few cells - the odor is spicey.

My information for this comes from research following the PLGR debacle. This is a GPS/radio unit the US Army uses. Initially designed for coordination if the Russians swept across Europe in a blitzkrieg they were stored with lithium primary (not-rechargeable) cells. Those cells have a 10+ year storage life where they are still quite usable at that time. However, they got used in Desert Storm, which lasted longer than a few days, so they were depleted in use. However, the US Army started supplying li-ion replacements which the PLGR mount could recharge. Bad news - if you put a dead lithium primary cell on a charger it converts it into a fire grenade, typically on the dashboard of the Hummer. So the US Army ended up with a lot of new regulations about handling and shipping lithium batteries, particularly primary (non-rechargeable) cells.

Where I came in - having to write a justification for why a lithium primary cell was certain to be safe. Because it was a coin cell. Usually used in a wristwatch, and in this case in a non-recharged circuit to back the date/time clock in a bit of test equipment. Even shorted, coin cells have such a high surface area to chemical components volume that they won't build up heat enough to rupture.

The bad news - no amount of monitoring can detect when a cell has gotten to the point where it will catastrophically fail. They heat when charged and heat when discharged - so no temperature detection will work. So far it's not economical to put pressure sensors inside cells and the li-ion flat batteries are in basically a plastic bag so they won't see a pressure increase until they are restricted - look for bulged laptops or bulged cell phones for what happens when they fail. Even then, venting or bulging isn't a necessary precursor to fire; they usually just don't work to store electricity anymore, but don't charge them if they do bulge.

I would not store a charged lithium-ion cell anywhere on a plane that would not be suitable to light a charcoal fire unless it's a place that is readily observable and readily accessible. For the more worried - I'd put ionization and smoke detectors in any closed compartment within a few feet of any possible storage. This is the recommendation from the NTSB concerning a sleep-on dive-boat accident that suffered an on-deck fire started by charging li-ion batteries and burning to the waterline with about 20 people killed below deck.

Laptop makers have not helped. Laptops used to have removable battery packs, but even those did not include shutters to cover the battery terminals, so removing them exposed another potential failure mode, not including that some people kept a charged one as a back-up.

So far it seems like the rate of thermal failure is low, though with probably billions of li-ion cells in the world even a low rate can get to the news.