I have been using very large Li-poly batteries and am very aware of the potential for a serious melt down/lithium fire.

My chemical knowledge is limited to being a connoisseur of fine solvents! The question is: Is there any chemical or element that would interfere or cancel the reaction of a lithium fire? My thought would be to make a charging box out of an ammo box and line it with more of this material than necessary to snuff the reaction. Sort of like control rods in a reactor. Is this possible????????

From : Don Stackhouse

I'm not sure about lithium, but judging by its proximity to magnesium in the Periodic Table I expect that it would have somewhat similar behavior (although being one row higher and one column further to the left would suggest that it's even more reactive).

In a "previous lifetime" in the full-scale aerospace industry, one of the projects I worked on involved the possible use of magnesium for some weight-critical parts. This required some research into how to safely work with this stuff in our machine shop.

In the case of a magnesium fire (as with any fire), you have to cool the burning material below its ignition temperature, and/or cut off its supply of oxidizer (typically atmospheric oxygen in most cases). However, doing so is quite a bit more difficult than you might expect.

Trying to put out magnesium with water will make the fire WORSE. The heat of the reaction will actually dissociate the water molecules, freeing up the oxygen atoms to react with the magnesium, thereby increasing the supply of oxygen to the magnesium fire!

Halon and other fluorocarbon extinguishants suffer a similar fate, making the fire worse.

Sand (silicon dioxide) also dissociates, but the energy required is so great that the subsequent reaction between the freed oxygen and the magnesium is essentially a break-even deal. Therefore, sand does not accelerate the fire, but it doesn't put it out either.

Carrying the burning material outside (QUICKLY!!) and letting it burn itself out was the common recommendation.

The other recommended practice for folks working with magnesium was to keep buckets of iron filings handy. Iron filings don't add a significant amount of additional oxygen to the reaction (even if they're rusty), they have enough thermal mass to suck much of the heat away from the reaction to help cool the magnesium below its ignition temperature, and a pile of iron filings can smother the fire's source of oxygen in the same manner as sand does in fires of conventional materials.

The other good news was that it's actually fairly difficult to start a magnesium fire. The thermal conductivity is so high that the heat gets conducted away from the spot being heated quickly enough that it's difficult to get that spot up to ignition temperature without heating the entire piece up to ignition temperature. Of course that doesn't help in the case of a battery charging accident, since the charge malfunction that causes the heating DOES heat the entire mass of material.

In my own experience with smaller Li-poly batteries, the hysterical rants in the e-mail forums and press about the dangers seem to have been somewhat exaggerated. I've actually tried to deliberately set a cell on fire, with absolutely no success. Abused cells just puff up, without exploding. The biggest Li-poly batteries I've worked with are 3-cell 3300 mah packs, and so far the dangers certainly can't be discounted, but I have yet to see any real evidence of the sort of behavior we hear about. OTOH, I really would prefer that my first real evidence I see of this NOT be that of my house burning down.

My first recommendation if the size of the pack is really that big would be to simply have the pack outside, away from other structures when you charge it. Put it somewhere that you can just let it burn itself out if it does catch fire. For small packs, particularly new ones that I've never charged before, I like to put them inside a heavy-duty, covered Pyrex casserole dish during their first charges. I'm not sure if that's just for peace of mind or if it's really adding that much safety, but at least in theory the dish should contain the fire and restrict its oxygen supply. For a small pack this is probably significant, but for a large one there is probably enough potential energy being released to require something more, such as the box you're contemplating.

Based on the recommendations I related above for magnesium, it seems to me that a double-walled steel box and lid, with iron filings (or other non-oxygen-containing material) in between the inner and outer walls, might fit your requirements.

Don Stackhouse
DJ Aerotech