Judging by the fact that, despite my objections, members of my team regularly touch the positive and negative battery terminals at the same time, I’d say not much
I’m looking forward to reading the full report.
In the meantime, based solely on the picture of the battery, I see no evidence that the carbon fiber conductivity could have caused this. There is no visual indication that the positive terminal had any serious heating. Only the negative terminal appears to be involved.
To me this indicates that the description of the negative terminal being jammed into the battery seems more likely the cause.
Either way, finding ways to prevent this from happening again is the best thing the FIRST community can do.
To be honest, I think the field crew did a pretty good job of extinguishing the fire, but everybody at the competition should know where the extinguishers are.
It wouldn’t be that hard to add a short explanation of where they are at the opening ceremonies.
XD. I’d be worried if the quickness of this field staff was quicker than the last fire.
They were about 45 seconds into march before they even e-stopped, which is where the clip that was posted in this thread, started.
There were not many fire incidents at FIRST events, that’s great and hope there won’t be any. Unfortunately there was at least one last year and already one this year. At every event they must keep couple of first extinguishers of different type, close to scoring table or a better place and all volunteers near the field should know the location. Same goes for pits. Also volunteers near field and those at pit admin must be trained/educated on which type of extinguisher is best for a situation. Since volunteers fron UL are always present at events, they could be the best to train, or anyone with fire depr can train. Fire incidents don’t happen, but when it happens there will be few hands to help.
So I have read explanations and I have seen photos that are not in this thread. I think everyone agrees that the battery was inserted up side down in the robot. So what likely occurred?
Our batteries are available in a about seven variations involving terminals. As such all variations start with a basic battery that has no terminal and a depression where the red and black epoxy filling is located. During manufacture, each terminal type is pressed down and soldered in place in this void and then the completed connection is filled with epoxy. When the battery was inserted up side down, extreme stress likely broke the internal connection on the negative terminal causing a high resistance connection. High resistance coupled with high current cause high temperature. So I believe, the terminal failure resulted in temperatures high enough to set the tape insulation on fire. From the picture of the battery above, the tape may have a paper base. Once ignition took place, other materials in the battery case also began to burn which resulted in the first picture. Notice the fire damage to the area surrounding the missing terminal. One of the pictures I saw still has the negative lead with the battery terminal still attached. Note that there is no swelling of the battery case (common with an internal short). It is possible that the battery was also shorting to the frame which produced the excessive current. Since there was no reported leakage and no swelling, I think the damage was primarily confined to the outside of the battery. It is also possible that wire terminal was not secure to the battery terminal which also results in a high resistance connection.
So a few things to take away from this incident…
Tight connections are essential in the high current paths of our robot electrical system. That means all crimps, all hardware must be tight. Tug on the wires, mover them around. If anything moves, correct it. We (Team 111) solder all crimps in this area and use #10 lock washers between the battery terminals and wire terminals on the battery. By adding washers between the terminals prevents them from sliding and loosening the retaining hardware. Do not lift the battery by the wires as this damages the internal battery connections made during manufacture. Train everyone on your pit crew on the proper method of handling your batteries (two hands please) and exactly how they should be installed in your robot. Check your batteries often for signs of having been dropped. A flat spot on a corner is a obvious sign or someone telling you that they dropped it is another. Recycle dropped batteries, they will fail under Al’s Einstein corollary to Murphy’s Law. Any robot item that can fail will fail on Einstein.
Watching the robot burn live, it looked like the bumper was also on fire.
According to this link it looks like a dry powder extinguisher was the way to go as it will extinguish a wood/plastic + electrical fire.
We were fortunate that the carpet and floor did not catch fire.
That wasn’t an FTA, that was an inspector.
Electrically harming a person is a function of the current passed through the person and the path the current takes. Perhaps the most critical path is one crossing the body; either arm to arm or arm to leg.
While the resistance of skin, in good conditions is 100,000 kOhms or so, once the skin is penetrated the resistance drops significantly - Values ranging from 1000 Ohms to 500 Ohms.
Ventricular Fibrillation is probably the first effect to be concerned about. I see numbers as low as 30 mAmps, but those are associated with AC. That’s the magnitude level I have typically been instructed in. I see information indicating higher and lower levels of current at DC, but it probably depends upon the path!!
Electrically, the rules require the system to be isolated from chassis. The thing to realize is that in failure conditions, such as this, the chassis of the vehicle may no longer be isolated.
Even a fully charged battery should not be capable of doing harm, but under the precisely wrong conditions the current could become closer than you want to be!
They did a great job, but the response wasn’t quite as quick as it would appear from the video, which started a bit after the flames were noticed. The fire extinguisher was not at the scoring table – someone ran behind the bleachers to fetch it. (A fire extinguisher was prominently visible at fieldside near the head referee’s position for the rest of the event.)
The cause of the fire was due to the battery. The terminals came in contact with our carbon fiber baseplate, which is conductive. This caused it to create a circut and short causing the firebyou saw here. Thiscwas maimly because we attached our battery upside down which we encourage no one to mount their battery upsidemdown or against a conductive surface. Though, luckily nothing was damaged
Correction: The fire was due to the battery being installed upside down which caused the negative terminal to be forced into the battery.
You mean I shouldn’t be able to undo all the battery hardware by hand? :ahh:
The most recent addition to my hall of electrical shame… Insulation was starting to get crispy too.
I always find a lot of sketchy battery connections helping teams out at events.
After the batter was removed. The damage was minimal with only a burned spot on the carbon fiber base plate and the failure of one encoder. A decorative piece of poster board right next to the battery did not even get scorched. Not sure who ran for the extinguisher but he saved the bot. Here is a link to the battery long after the bot was up an running again.
That was our FTA Lorenzo who came in with the extingusher. We checked things out before handling the robot. The fire was relatively small when you were up close. The fire was mostly the insulation and the negative post area on the battery. FTA John was trying to not use the extinguisher if we could get away with it. They make a mess.
Had not had a fire in the PNW since 2010. The extinguisher usually lives in road case 8, which is normally right behind he FTA table. This was a double sided event, so case 8 was moved off field and the extinguisher was not placed well.
We have changed that for all future events. The extinguisher will live at the FTA table rather than case 8.
We use an ABC extinguisher on the recommendation of our friends at UL because the breadth of possible fire types is very wide.
Your photo has a number of items I would like to address…
If your team is using this type of terminal, it should be turned towards the inside of the battery. Please note the snag hazard of hanging that retention screw over the side of the battery. Even covered in tape allows it to snag a chassis part and break the terminal off the battery.
The dicoloration of the copper is a good indicator of high temperature.
Note that the split washer appears to be mounted under the screw head not under the nut where it belongs.
The one good think is the wire sticking through the retention feature. This will help with keeping the wire in the connector. Frequently teams trim too little insulation and when tightening the screw will actually force the wire out of the connector.
For minimal cost, you can buy crimp on Burndy terminal from AndyMark and the crimper as well. A more reliable solution to this connector.
As to electrocution hazards of a 12 volt battery. Under most circumstances, this will not be an issue. Please remember that the GFCI protectors are specifically designed to sense and open under currents that would trigger a cardiac event. Those are tested to open at 4-6 ma currents.
While the body has skin resistance, the skin acts as an insulator to the tissue inside. 12 volts is not likely to pass into the tissue and the lower resistance path so it can be contacted by the fingers and not produce any noticeable effects. However, everyone knows that holding a 9 volt battery to your tongue will produce some nasty reactions if fully charged. Your tongue is wet, very conductive and does not have the insulating properties your fingers do. However, once the voltage reaches a point where the insulation breaks down, you will feel the effects of the current to some degree as it passes into the tissue below. Contacting the 48 volts on analog phone lines will give me a tingle. To compare, the 110 volts AC at the outlet in your pit is actually 280 volts peak to peak. That will easily overcome any skin insulation and pass into the underlying tissue with ease.
As pointed out in this thread, the battery still has a lot of energy stored inside. Do not work around the battery with dangling jewelry or large tools. The available current is capable of welding metals. It will also break bones if dropped on your foot or your finger.
No matter the source, battery or wall, treat all electricity with respect. If you don’t it will slap you when you least expect it.
*Consider a sine wave with a peak-to-peak
amplitude of 1 unit. It has the equation:
y = (1/2)*sin(x)
The integral of the square of y
over one full cycle is given by:
integrate(y^2,x,0,2*pi) = pi/4
The average value of y^2 over the cycle is:
(pi/4)/(2*pi) = 1/8
The RMS value is given by:
y_rms = sqrt(1/8) = 0.35355339059327
Therefore, a sine wave with an rms amplitude
of 110 units has a peak-to-peak amplitude of:
110/y_rms = 311.12698372208
Battery post driven into cell shorting it bypasses fuse link safety of multi cell batt resulting in >1000A current, melting lead which could spurt Once on skin a serious burn results like solder only worse due to mass of molten lead does not wipe off skin/living tissue instead buries itself deeper into tissue… If in eye… sight could be lost as Molten Lead will melt thru plastic lens like butter… consider using better “Glass” safety lens. Inhaled fumes from burning electrolyte mixing with chemicals from extinguisher mixture could be quite hazardous. Explosion is possible before and after extinguisher is used so carefully consider proximity when examining fire especially in battery (FTA and field crew responders)
The sidewalls of our batteries are very thin… a consideration in mount as any piercing produce equal danger as it bypasses all current limiting (limited only by ‘R’ of Low ‘R’ conductors, lead / aluminium, and cautionary reminder: P=I^R so R is worst for heating when non zero!!! say 1000A^2 * .1 ohm even 100A^2*.1 =10kW! for some short period of time)
Apply lesson learned… Post photo of vaporized melted Negative Battery Terminal – Pic is worth thousand words!
The likely explanation here is that CO2 fire extinguishers are vastly more expensive than a standard ABC extinguisher. The average ABC extinguisher will set you back $20-$30, but even the cheapest CO2 extinguisher runs over $150 (and many are closer to the $300 range).
Obviously this is a drop in the bucket compared to the cost of an FRC robot, but I suspect many teams and/or events either don’t know the difference between extinguishers, see the price and go into sticker shock, or some combination of both.
There is also the question of priorities; If the priority of the field staff is to protect the field/venue/people from damage/injury, then the onus of preventing a robot from being damaged by a fire extinguisher falls on the team, who should have built the robot in such a way as it wouldn’t catch on fire in the first place.