Cycling UK Forum

Bike lights explode, almost burn down cyclist's house

http://road.cc/content/news/173814-bike-lights-explode-almost-burn-down-cyclists-house#sthash.3qtdApRY.dpuf

Reading that and the original Singletrack article there seem to be a number of lessons he could learn other than not buying cheap Chinese lights.

- if you have an electrical fire, switch off the power to it. I suspect if he had done that one simple step first rather than running around in a panic and then running up and down stairs with bowls of water, he might have prevented the whole fire

- don\t throw water on an electrical fire especially when it's still connected to the power

- have a fire extinguisher in the house - even a fire blanket would probably have helped.

- if you are going to dial 999 or 911 get someone to do it early rather than as an afterthought

Having had a laptop battery go up in smoke right in front of me on a table full of papers I would never leave a charging battery unattended unless it was in a non inflammable area. Also buying cheap junk is just asking for trouble, only okay if you understand the risk.

The problem is not the lights, it's the batteries/chargers. Consumers need a lot more education on this aspect. Air carriers are also worried about transporting lithium-ion batteries - but, hey, eBay bargain hunters always want their lights in a hurry...

Statistically speaking, I bet a lot more of these generic units have been shifted than any of the expensive, big brands. It's hard to tell how relatively safe a good brand is in pure technical terms. However, they definitely have reputations to uphold and have to stand by their products with warranties, etc, which helps for piece of mind. Still, no guarantee that they don't blow up (as evidenced by Sony laptops, etc).

One thing that doesn't happen with "dynamo" lights.

The more "lumens" you have the greater the energy needed to be stored in a battery. Some Li cells eg 18650 don't have built in protection, which is necessary when these cells are used in packs, otherwise they can catch fire.

US safety regulations can be less stringent than European ones (and UK ones if we leave the EEC?), although even we can buy ones that don't meet safety regulations eg via Ebay and sometimes in shops.

For a lithium battery fire, water is the recommended method (after unplugging the charger, in the interests of not getting electrocuted).
The aim is maximum cooling, rather than excluding air.
FAA video about in-flight laptop fires: https://www.youtube.com/watch?v=vS6KA_Si-m8

andrew_s wrote:For a lithium battery fire, water is the recommended method (after unplugging the charger, in the interests of not getting electrocuted).
The aim is maximum cooling, rather than excluding air.
FAA video about in-flight laptop fires: https://www.youtube.com/watch?v=vS6KA_Si-m8

For maximum cooling a CO2 extinguisher surely? Also removes the risk of electrocution - though turning the power off should be the first action in any case so that energy is not being supplied to battery and charger.

the cooling capacity of CO2 isn't that great; the specific heat capacity is only about 700-800 J/kgK (vs ~4200 J/kgK for water). In addition when water boils the phase change absorbs about x10 the energy required to heat liquid water from 20C to 99C.

In a typical (large) CO2 extinguisher you have maybe 2kg of CO2, most of which won't go anywhere near where it is required. If you are exceptionally generous you might suppose that the average temperature of the CO2 gas leaving the seat of the fire is (say) +300C, giving a total heat absorbing capacity (from a 2kg extinguisher) of ~480 kJ.

By contrast just one litre of water (heating from 20C and turning into steam) can absorb over ~3400 kJ, i.e. at least seven times as much heat as 2kg of CO2. I think that water is much more likely to be used efficiently, too, since it can be directed as a small jet to exactly where it is likely to do most good, whereas the CO2 just goes everywhere, and in a small fire most of it is just wasted. Needless to say once a fire is surrounded by steam, only a few materials will carry on combusting with any vigour, and the contents Lithium rechargeables are not amongst them.

So water is remarkable stuff, actually; arguably (provided you turn the power off before you use it in an extinguisher) it would be hard to devise a better fluid to put most fires out with...?

BTW one friend of mine (an electrical engineer who spent his life working in the industry) used to have no truck with so-called 'double insulated' electrical appliances; he would invariably replace the two-core mains cable with three-core and give the fuse a better chance to blow to earth (or indeed an earth leakage trip a better chance to work). If he were still alive I have little doubt that he would advocate using a typical cheapo Lithium battery charger inside a well grounded metal box of some kind, so that any fire would be contained and any (consequent or causal) electrical fault would not cause a hazard either.

cheers

Brucey wrote:the cooling capacity of CO2 isn't that great; the specific heat capacity is only about 700-800 J/kgK (vs ~4200 J/kgK for water). In addition when water boils the phase change absorbs about x10 the energy required to heat liquid water from 20C to 99C.

Yes, CO2 extinguishers are mainly to exclude oxygen from a fire, not to cool things down.

TonyR wrote:Reading that and the original Singletrack article there seem to be a number of lessons he could learn other than not buying cheap Chinese lights.

- if you have an electrical fire, switch off the power to it. I suspect if he had done that one simple step first rather than running around in a panic and then running up and down stairs with bowls of water, he might have prevented the whole fire

- don\t throw water on an electrical fire especially when it's still connected to the power

- have a fire extinguisher in the house - even a fire blanket would probably have helped.

- if you are going to dial 999 or 911 get someone to do it early rather than as an afterthought

After looking at the FAA video posted by Andrew S, I'm not sure if all of that is good advice.
"He said the battery pack was “really, really, really big” and wrapped in plastic, “like a stick of dynamite”. He said the whole thing was bouncing around and exploding with a bright light." This seems to indicate the battery itself (not the charger) had overheated. Therefore pouring water on it to cool it was one of the best options. A fire blanket would have made the problem worse.

Brucey wrote:
BTW one friend of mine (an electrical engineer who spent his life working in the industry) used to have no truck with so-called 'double insulated' electrical appliances; he would invariably replace the two-core mains cable with three-core and give the fuse a better chance to blow to earth (or indeed an earth leakage trip a better chance to work). If he were still alive I have little doubt that he would advocate using a typical cheapo Lithium battery charger inside a well grounded metal box of some kind, so that any fire would be contained and any (consequent or causal) electrical fault would not cause a hazard either.

cheers

You are making the assumption the fire started in the charger. If the fire started in the battery then it would not have been contained. If you included everything in a metal box then you probably would have had an explosion.

Perhaps the safest option would have been to charge the battery outdoors, but in the dry ..... or else don't use re chargeable Li cells for bicycle lights.

MikeF wrote:

Brucey wrote:
BTW one friend of mine (an electrical engineer who spent his life working in the industry) used to have no truck with so-called 'double insulated' electrical appliances; he would invariably replace the two-core mains cable with three-core and give the fuse a better chance to blow to earth (or indeed an earth leakage trip a better chance to work). If he were still alive I have little doubt that he would advocate using a typical cheapo Lithium battery charger inside a well grounded metal box of some kind, so that any fire would be contained and any (consequent or causal) electrical fault would not cause a hazard either.

cheers

You are making the assumption the fire started in the charger.

No not at all (I said 'consequent or causal'); I'm making the assumption that if the power to the thing goes off by itself (fuse blowing or earth leakage trip), that would be a good thing.

If the fire started in the battery then it would not have been contained. If you included everything in a metal box then you probably would have had an explosion.

To get an 'explosion'

a) the box would have to be entirely closed and/or
b) there would have to be some means of liberating a large amount of gas very suddenly.

I did not stipulate the former and the latter seems unlikely. The cells themselves may rupture suddenly (eg if heated by a neigbouring cell). Past that (and unlike non-rechargeable Lithium cells) I don't believe that there is any real risk of 'explosion' with such batteries, don't they just burn? In point of fact if the box was reasonably well sealed (eg with a small vent in addition to a hinged lid), there most likely wouldn't be enough oxygen to sustain any substantial fire inside anyway.

cheers

Brucey wrote:the cooling capacity of CO2 isn't that great; the specific heat capacity is only about 700-800 J/kgK (vs ~4200 J/kgK for water). In addition when water boils the phase change absorbs about x10 the energy required to heat liquid water from 20C to 99C.

cheers

640 calories per gram to turn water into steam is the figure I remember from my physics which is a tenth of the amount required to heat a gram of water by 64 degrees C. The huge amount of energy in steam is why it scalds severely.

Water + Lithium.....? Bang?

Brucey wrote:
To get an 'explosion'

a) the box would have to be entirely closed and/or
b) there would have to be some means of liberating a large amount of gas very suddenly.

I did not stipulate the former and the latter seems unlikely. The cells themselves may rupture suddenly (eg if heated by a neigbouring cell). Past that (and unlike non-rechargeable Lithium cells) I don't believe that there is any real risk of 'explosion' with such batteries, don't they just burn? In point of fact if the box was reasonably well sealed (eg with a small vent in addition to a hinged lid), there most likely wouldn't be enough oxygen to sustain any substantial fire inside anyway.

cheers

I don't know enough about the chemistry of Li cells to know exactly what happens, but I thought it was a thermal runaway process. Thermal runaway itself does not necessarily involve the production of gas. If you contained the battery in a large metal vented box you may well be OK, but any vents of course might allow flames to burst through. Maybe a large unvented box? I would have thought a small unvented metal box might explode though.

well as I understand it you don't get 'a lithium metal fire' per se with lithium rechargeables (which is why such fires can be extinguished with water) although the electrolyte is combustible. To make a reasonably safe enclosure would not be difficult; a hinged lid of some kind that can 'burp open' as necessary plus a small vent hole would probably do it.

cheers