Battery fumes

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In the good old days of lead acid batteries pilots were aware of that batteries could boil and battery compartments were vented to release the gas (hydrogen if memory serves) to atmosphere.
Now we are using NiCad batteries is there any risk from fumes if it gets hot? I know there is a theoretical 'thermal runaway' condition but what about explosive gases?

[heedm]

Now it's toxic gases.

http://www.safetycenter.navy.mil/media/mech/issues/summer03/thermalrunaway.htm

[[email protected]]

Matthew have you got another link - that one didn't work for me?

[airborne_artist]

http://www.safetycenter.navy.mil/media/mech/issues/summer03/thermalrunaway.htm

Thermal Runaway

By Cranston Dickson

Thermal runaway is a condition in which the current for a fully charged nickel-cadmium battery rises out of proportion to the impressed-voltage level. This condition is caused when heat from oxygen recombination-an inherent property of most rechargeable batteries-causes the battery's voltage to drop as it gets hot.

During thermal runaway, the battery can become dangerously hot, emit excessive amounts of toxic gas, and spew electrolyte. This condition can occur in either the battery workshop or the aircraft.

Low electrolyte levels, electrolyte contamination, no gas barrier, or a deteriorated gas barrier can cause thermal runaway.

When oxygen recombination occurs, heat is generated, causing the battery temperature to rise and the battery voltage to drop. This process causes the battery to draw a higher charge current. As the temperature of the battery increases, the battery voltage continues to decrease, and the current progressively becomes greater. As this process continues, electrolyte eventually reaches the boiling point. If allowed to continue, the electrolyte level may fall below the top of the plates, causing them to dry out. The cadmium plates may then ignite and burn like steel wool, melting the separator, causing the cell to short circuit, and igniting the hydrogen in the cell. A battery's stored energy can feed a short circuit, allowing it to burn through the entire battery.

In event of a thermal runaway, electrical power should be isolated, and no attempt should be made to handle or move the battery for at least 30 minutes. After this period or when the battery is cool enough, carefully remove the vent caps of the affected cells. Make sure you wear a facemask, gloves and protective apron when doing this work.

The Royal Australian Navy uses nickel-cadmium batteries in Sea King and Squirrel helicopters. Over the last ten years, two incidents of thermal runaway have occurred in Sea Kings; RAN Squirrels have had none. In both H-3 incidents, the aircraft were not damaged significantly, but both suffered major damage to its batteries.

Thermal runaway is preventable, but it requires maintainers rigidly to follow servicing procedures at appropriately equipped battery-charging facilities. It is imperative only trained people service and maintain nickel-cadmium batteries. These preventive steps will work in most cases, but a situation occasionally is beyond the control of the battery maintainer or squadron personnel. This usually happens when aircraft are required to operate in extremely high temperatures, which is less than ideal for premium battery operation.

Mr. Dickson works with Hunter Aerospace Corporation PTY Ltd., Australia.

The U.S. Navy hasn't had too many cases of thermal runaway, but this story and the one in the January 2003 issue of Approach, "What Could Go Wrong Now?" show the danger is real and can be catastrophic. This story is reprinted courtesy of Touchdown-the Australian Navy Aviation Safety and Information Magazine.--Ed

[leemind]

"In the good old days of lead acid batteries pilots <cut>
Now we are using NiCad batteries <cut>"

All of the helicopters I fly (a couple of B206's and H500's) were factory fitted with NiCads out of the factory and a plethora of Battery over temp warning lights, but have all since been retrofitted with good old lead-acid, presumably for this reason. Does anybody fit NiCads as standard these days? Given their discharge characteristics and memory effects I think they make a poor choice (witness the automotive industry: Still with good old lead acid...)

[[email protected]]

Thanks Airborne artist

[Shawn Coyle]

It's my understanding that thermal runaways on Nicads are mostly a thing of the past, thanks to changes to the internal structure of the batteries.
This is from discussion with a battery manufacturer (but could also be thought of as 'they would say that, wouldn't they').
Lead acid batteries are often fitted in warm climates as they seem to be slightly more reliable, and cheaper(?).

[419]

Shawn,
Thermal runaways can, and do still occur with Ni-cad's. The biggest cause is poor maintenance in the battery workshop. Something as simple as having a few mis-matched cells can start it happening.
As you say, lead-acid batteries are more reliable and cheaper(considerably). One of their main advantages is in maintenance time. A lead-acid can be removed, cap-checked and refitted the same day. If a Ni-cad buggers about during maintenance, they can take days to test. (and lots of money in replacement cells)

With the new lead acid batteries, there is not usually any requirement for these to be vented, as they are totally sealed, and any gasses produced are recombined into the cells.

419

[offshoreigor]

leemind,

I agree, most operators have gone back to the sealed lead acid, specifically the Concorde. Works great.

Cheers,

:eek: OffshoreIgor :eek:

[SASless]

I haven't flown with a ni-cad in ages.....lead acid is the only way to fly....much less prone to problems....also a few less questions to answer on the checkride and writtens.....fewer procedures to pull out of the recesses of one's alcohol ravaged brain.:E

[[email protected]]

Thanks for all the info chaps, why do I not find it surprising that we are way behind the times in the UK Military, we probably bought all the NiCads nobody else wanted and paid 3 times the going rate for them!

[Arm out the window]

I find it unusual that machines don't have vented battery compartments even with nicads.
The only thermal runaway I experienced, in a nicad-batteried Bell 204, was noticed first by another machine in the circuit with us at night (both in the pad at the same time) by the 'steam' coming from the battery vent down near the tailboom/cabin junction.
Aren't nicad installations set up with vents, in some cases through telltale jar arrangements with treated material in them that changes colour in the presence of battery fumes?
Maybe not on all types.