Dangers Of A Battery Thermal Runaway?

[Recuperator]

I had a red "Battery Temp" warning light illuminate today.

My fellow aviator did not seem to be to concerned or grasp the dangers of this condition or the dangers of a thermal runaway on helicopter batteries (NiCad's).

Any PPruners have any practical experiences with this phenomena and don't mind sharing them?

An explanation of what happens internally in the battery will also be appreciated.

Will switching off the battery always guarantee the prevention of the thermal runaway?

I have heard some horror stories about batteries exploding and helicopters crashing as result of it.

Please enlighten me...

[Lama Bear]

The Red light is serious. Get the ship on the ground and the battery out of the ship if possible. I have had batteries vent after being removed.

Turning the battery switch off will always stop the runaway.

I too have heard the story of explosions but they were always 3rd or 4th hand. I think they are folklore.

My bad

\"Turning the battery switch off will always stop the runaway.\"

There should have been a big NOT in that sentence. Opps!!

[blade771]

Manual states (if in 206) to turn off the battery, test the warning lights if the Batt Hot light is unservicable then treat as though it were a Batt Hot situation - land and evacuate the aircraft. If the Batt Hot light is servicable continue the flight. Remembering you only have 30 ish minutes of battery life.

If the Bat Temp caption extinguishes, and after the battery is reselected, the caption re-illuminates, then switch the battery off and leave it off. With the Battery off failure of the generator will result in a total electrical failure.

Thermal runaway is very serious and can result in the battery exploding - always treat these cautions as serious.

recuperator - I have PM'd you.

[Eurochopper]

My non-expert take on this, to get the ball rolling, is as follows:

Normally as a battery charges up, its internal voltage increases. If the charging voltage is set to the correct value this means that the charging current falls to near-zero when its fully charged. Conversely the charging current will be high when the battery is in a low state of charge. In other words the charging current is a function of the difference between the internal voltage and the charging voltage.

When the battery is charging, a chemical reaction occurs which stores the energy being put into the battery by the charging current - the battery only heats up a bit due to resistive losses within it. Once the battery is fully charged, any remaining charging current just goes to making heat in the battery (and decomposing - aka "boiling" the electrolyte.)

However a feature of NiCads is that the internal voltage also changes with temperature. Unfortunately it decreases with increasing temperature so that a hot battery will take more current than a cold one for a given state of charge and charging voltage.

This means that a thermal runaway state can occur when, for some reason, the charging current is too high with the battery fully charged - the excess current causes temperature rise which decreases the internal voltage of battery, which increases the charging current which decreases the internal voltage etc - you have thermal runaway.

I think this would be likely to occur either because the charging voltage is incorrect, or because some fault in the battery, such as a shorted cell, has reduced the fully-charged internal voltage.

My take is that as soon as you remove the charging current the process stops. The only exception to that would be if you have left it so long that the battery has overheated to the point of distortion and possible short circuit on the cells.

But it will take a long time for the heat to escape and the temperature to return to normal, so you can expect the light to remain on for a long time (in my one experience it was 45 minutes). The other hazard is that the decomposing electrolyte will produce explosive gases (oxygen and hydrogen like a lead acid battery I believe?) so if they are not properly vented, or if a spark occurs it could go boom anyway, even though you have removed the charging current.

When I did a groundschool course at another operator, we were told that slowly discharging the battery helped to reverse the process and actually helped to cool the battery faster than isolating it completely, and this was the reason why only the battery positive contactor was switched off (there being some services that were powered direct from the battery positive terminal). However I am not sure that I believe this and it might have been purely so that the direct battery services remained powered. (Super Puma family)

Well that's my offering, who is going to start the argument?

EC

[toolguy]

One possible cause of thermal runaway is a build up of deposits on the plates. The ni-cad battery uses nickel hydroxide as its cathode, a cadmium compound as its anode, and potassium hydroxide solution as its electrolyte. The chemical reaction during charging and discharging leaves crystalline deposts on the plates. Periodic deep cycling of the battery can help remove these deposits.

When the deposits form, the internal resistance increases and creates heat when the battery is charging. The problem comes when the battery gets too hot and leads to melting, catching fire, exploding, etc.

Either way, follow your RFM and follow your emergency procedures. Disconnecting the battery does not mean the battery will cool down by itself. Once the temp gets too high, disconnecting the charge does not guarantee the battery will cool down.

During the late 1960s and early '70s, occurrences of thermal runaway or battery overheat in ni-cads caused by internal resistance, prompted the FAA to issue Advisory Circular 00-33 to alert maintainers of why periodic maintenance is so important on a nicad. Keep the electrolyte levels in the green, connectors tight and treat corrosion to prevent high internal resistance another cause of thermal runaway.

Improper voltage can lead to thermal runaway, disconnecting the battery may help, but if the temp is too high, flipping the switch may not make the problem go away. Follow your RFM.

[Arm out the window]

I have had a battery get into the runaway situation doing night circuits out in the training area to a large-ish pad; we smelt it and noticed the load meter reading v. high about the same time as another aircraft which was also in the pad called that there was 'steam' coming from the battery vents.
Turned off the battery and shut down, got out and stayed well away from it for half an hour or so and then had a look;
By the way it was steaming and carrying on before that, I'm sure it could have exploded if left to keep going.

We used to be told basically this:

Symptoms - high ammeter / loadmeter readings, venting, smell, bat hot lights if fitted.

Action - isolate battery by switch, land ASAP and leave it at least 30 min, preferably overnight.
There was also some talk of physically disconnecting it (leads) but maybe not so smart because you'd be going near it while hot.

So definitely not something to be taken lightly. I also remember seeing a photo of a Huey with the nose hatch area blown apart by a thermal runaway, that's probably on the net somewhere.

[Thomas coupling]

Had a thermal runaway in a Seaking around 1980 ish. It ripped a hole about 2 foot diameter out of the front end of the nose cone!
Damage came back as far as the tail rotor pedals.

Happened very quickly and in hindsight with only the whiff of a battery acid acrid smell.

My view now is to get the a/c down immediately, switch battery off, evacuate the a/c:eek:

[Max Contingency]

Happened to me in the small hours of the morning. Like a lot of emergencies it started with someone saying 'can anyone else smell burning?'. This was followed up with a quick check that everything looked OK inside the aircraft. When I shone my torch down towards my feet, I couldn't see them for the acrid smoke that was now pouring into the footwell (through some holes in the back of the battery compartment that shouldn't have been there!). As Wittering had only just passed down the left hand side of the aircraft, it was windows open, a quick pan call and an approach with my head half out the window. Shut the aircraft down on the roll, ran away bravely and, as Wittering was closed at the time, I called the fire brigade from a mobile phone at the side of the runway. It was only from outside the aircraft that we first realised the battery was the source of the smoke. We opened the battery bay door to help it cool but It carried on boiling/smoking away for a good 10 minutes afterwards.
Cause of the incident - Thermal runaway.
Cause of the thermal runaway - not determined.
Certainly not an emergency you would want operating out to sea!

[John Eacott]

IIRC, the Sea King didn't have a battery hot light: if so, it didn't bl**dy well work! November 1973, Bay of Biscay, dark and dingy night off RFA Olna on CASEX when we smelt "something electrical". As always, life wasn't meant to be easy, and Olna had a radio failure, so our pleas to come home fell on deaf ears. We went through a full ditching brief, UC at the hatch to the avionics bay with fire extinguisher, all non essential electrics turned off, and we got back to Olna it was to find the other SK on the flight deck, preventing a landing :rolleyes:

Anyway, when we landed the battery (just forward of the two pilots, for those not familiar with the Sea King) was so hot, the black hand gang couldn't disconnect it. So they promptly cut the battery cable with bolt croppers, rendering the cab U/S for longer than it took to replace the battery :eek: :p

At the time, we had no idea that it was a thermal runaway, and we were obviously very lucky not to have it blow up on us.

Although NiCads have their attributes, I am more inclined these days to use lead acid batteries, with many of the new generation coming quite close to NiCad for performance. Cost wise (as an owner) they are much cheaper to operate than NiCads, and have less down time considerations: plus, no propensity to thermal runaways as with the Nicads :cool:

[Arm out the window]

A couple of other thoughts that spring to mind -

1. Pressing on with a start with a low voltage battery can not only lead to hot starts, but if you do get it going, a depleted battery will suck lots of current which may then increase the chance of a thermal runaway. Best to forget about the start and change the battery if you can.

2. If memory serves me correctly, a very depleted nicad can have internal reversal of polarity within individual cells, which can set up a situation where the cells are feeding eachother - high current causing high temp, and with a generator to keep pumping current in, it will quickly lead to the hot kaboomish situation described above.

Recuperator, hopefully you can convince your colleague that a hot battery can turn pretty bad pretty quick!

[KikoLobo]

But it surely affects LiIons and LiPolys which i think are banned for good reason from aircrafts (at least i think)..

If individual cells get unbalanced in voltage charge, one cell will be charging the other, until the other reaches more voltage and then the process reverses (not the polarity in this case), this causes the battery to get very very hot even when its not connected to anything on the aircraft.

I don't know of a case where this causes a critical temperature, but it surely is a factor for a batt temp light, and i don't know how much or how critical.

Another thing that might happen is your regulator goes bad and starts charging the battery to much, in this case turning off the batt switch will fix it, provided that the batt relay works and disconects from the charging bus, if this does not work... YOU HAVE to turn of your generator, and if the gen relay is not working.. Well.. It was not your day to fly :)..

From here on, i only write my opinion and what i think is the best to do.
Any how, in the b206, you have two lights, one yellow and one red (battery temp, battery hot). If you get a battery temp, you can turn off the batt switch, look for a nice good safe place to land and while you are going there wait to see if the light stinguishes, if it does not then continue to make your pattern and land to investigate (even though the manual does not require this, i think its smart, its like a midium point of land as soon as possible and land as soon as practicable, like looking for a good spot, safe and close to a place to call for help, although it might not be an airport). if i remember well the manual states, turn of bat switch and land as soon as practicable, and if the light stinguishes you are allowed to turn on the batt switch again (not 100% sure now).

If while you are flying along with the yellow light on, and suddenlty get a red light on, (battery hot), then i will look for the closest place to land safely (not the sweetest place but if it fits i am landing there), shut the generator switch off while landing or as soon as i get the light, and after touchdown. Shut the heli down and..... run forest run!!!.... After an hour of so of no explosion, THEN go back and remove the batt from the heli and try to investigate... I believe that the first thing is your life and carrying an object that might explode its like carring a bomb that might go BOOM at any minute.

Any way this is only my opinion :)

[outhouse]

Back around 1968-9 BAC had a machine called the Strike Master a development of the Jet Provost. The early version had a NiCad battery fitted in the nose, behind the landing light and the clear plastic window for it to shine through. Some of these aircraft operated in Saudi Arabia and experienced extremely high temperatures, certainly in the nose with the sun shining through the nice window mentioned. The dangers of battery overheat and a subsequent possible explosive event had been seemingly forgotten during the design stage as the crew oxygen bottles had been mounted on a bracket above the battery. Sods law was working well one hot summer’s day in Riyadh!!! Battery overheat followed by a very, very violent explosion as a large part of the nose section of the aircraft parted company. After landing safely and a change of underpants by the pilot it was found that both battery and oxygen cylinder had exploded. Battery overheats should be taken seriously and if the battery continues to stay hot after switching OFF even more so. Designers of aircraft also need to be mindful when placing possible explosive equipments in close proximity to one another. In the case above modification was carried out to change battery type but its location remained the same, some small comfort to future aviators flying the machine when battery overheats occurred.
outhouse

[N Arslow]

Was in a Seaking heading away from Culdrose one night when a seriously strong electrical smell was detected. FRC actions and went home pronto. Shocked later to see that the corner of the battery had melted (about the size of a baby's fist).
Now very happy to have a BATT TEMP caption - not illuminated obviously.