Rocketman

I would like to present a hypothesis for the rocket-mode failures of PVs with an eGo type form factor.
Usually thought to be less threatening than an in-the-face explosion of a sealed, or inadequately vented metal tube PV, the 'rocket-mode' failure can still cause injury and damage to property. These adverse events are disturbing as evidenced by highly excited recounts by those unlucky enough to have experienced one.

The form factor I am referring to is a tube PV with an internal Li-ion cell, usually with a capacity between 2 and 4 watt hours that is wrapped in a Mylar film 'bag' inside a thin metal tube. The ends of the tube are a metal cap at the bottom end and a circuit/switch/connector housing at the top end, both pressed into the metal tube case. This device is not intended to be disassembled by the customer, in contrast to removable cell PVs.

I have found that the force to manually remove the end cap and the front housing is significant. With a cross sectional area (internal) of about 0.22 square inches, 100 pounds of axial force equates to an internal pressure of about 450 psi to blow one end. The front end is thought to be harder to 'blow' than the bottom end cap.

The typical failure of a Li-ion cell is chemical runaway that generates an elevated temperature and a release of gas. The internal bagged cell will easily swell to totally close off the internal cylindrical cavity of the tube, preventing gas exchange between the top and bottom of the PV. The continued gas generation will pop the Mylar bag filling any cavity inside the PV tube with gas. A small amount of the gas pressure may escape past the switch button but the flanged construction of the button most likely functions as a seal when pressurized.

With no place to vent the pressure builds to the point that the press fitted switch housing and the PV tube separate. The rapid release of this pressure creates a thrust that moves both parts of the separated PV.

Most reported examples of this have been while the PV has been on charge. The path taken by the housing end is restrained by the charger, but the tube section, containing the still venting and sparking Li-ion cell is free to move.
The initial impulse applied to move the tube is the release of about 500 PSI of hot gas, further aided by additional rapid but lower pressure gas from the cell. This is often described as "it made a loud bang, and shot across the room, LIKE A ROCKET".

Once the tube has been opened, the hot flaming sparking gases and internal cell parts create a visual effect much like a Fourth of July sparkler following the tube that was propelled by the 'pop' of the internal pressure blowing the two PV sections apart. The sparks and flames can even continue after the tube has come to rest.

I propose that the motion imparted to the 'exploding' PV is more like an air gun or paint ball gun, with a flaming projectile, instead of a rocket propelled device. It may look like a rocket but the majority of the acceleration was a result of the initial pressure pulse of tube/housing separation.

Thank you for not referring to this as a 'Rocket Mode Failure'

Rocket

Steampunk

Well, THAT just gave me the warm fuzzies...

Rocketman

Sorry, didn't mean to scare everyone but this 'Rocket mode' stuff is a little over the top.
Aside from the one eGo report (that I can't find now) that "flew from one room, bounced off a door, and knocked a refrigerator over" most don't go very far. One "flew off the table and rocketed over by the couch" a few feet away and sat there and burned a spot in the carpet.

To put the amount of force required to Rocket an eGo into perspective: Take a rubber band, just a plain old rubber band like you used in school to shoot paper clips, and see how far you can shoot an eGo. Takes a little practice to get it balanced, but you can send an eGo anywhere in the room you want (maybe go outside) with the energy in a stretched rubber band.

You could probably even wop someone upside the head with it.
Might hurt a little, but that's about it. You could even call it your eGo Rocket Launcher

Flying across the room like a 'Rocket'? OR flying across the room like a sparkler shot from a rubber band gun?

This even distorts how some think about other types of tube PVs.
Obviously, a vent hole the size of an eGo ain't big enough, right? Not if an eGo can go into 'Rocket mode'. So vent holes for mods must be even bigger.

Make an assumption, and then multiply it.

tek

Seems more like a Roman Candle with one shot effect.. Due to the inertial velocity and mass of the object such as a ego and the aerodynamics of it, I would have to say more similar to a Single Shot Roman Candle (ssrc) then a rocket.

However a compressed canister of gas at pressures over 500 psi, typical of say, 50 lb co2 carbonic liquid would be more like a rocket with the push and maintained push of such a object

Rocketman

The roman candle works because the mass of the powder ball is small and the charge that propels it is loaded serially in the tube. Pop, fizz, pop, fizz. Even a bottle rocket doesn't have much force (about a pound of thrust for about half a second). It's the Bang at the end of flight that's impressive. I like the paint ball analogy. Charge the chamber, then vent it, whoosh.

What if it took 200 pounds of axial force to pull the nose off an eGo?
Close to 1000 PSI?
But once the tube popped open, pressure gone.
It's also likely that the nose will creep out and finally give way.
It just doesn't take much force to propel an eGo 20 to 30 feet.
Toss one underhanded to someone. Did that hurt?

If it's going fast, it goes further. Gravity works like that.
And if you were to ask Sir Newton, the nose piece would go flying around the room also.
Sorry, he's dead,but they usually just sit there with the charger.
If it was going 500 fps, it would go through a wall.
It ain't. They don't.
Would I want to have one in my hand when it popped? Nope.
But it does seem that most eGo style failures are less intense than heavy duty sealed PVs.
(the American made ones)



(now tek is going to try and make flaming paint balls)

Steampunk

Sheesh! Give him ideas, eh?

Rocketman

Paint ball aerial recon flares

Steampunk

WHAT?!?!? OH! AERIAL! Phew! I gotta leave my glasses on more often...

Rocketman

You know, it just occurred to me.

You have specs on your specs.

berger

I guess its just that the image of a PV flying across the room with a trail of flames it conjures a more appropriate literary portrait.... even if its not an accurate scientific definition ...

Rocketman

Comet?

Rocketman

Ask someone that just had an adverse PV experience, before they have had a chance to calm down.
(I think that's how the fridge got knocked over )

Steampunk

Now THERE'S a name for a mod!

Steampunk

And the screen I view the pics on...

Problem is, the ones on the specs or the screen don't MOVE when you increase or decrease the size of the pic!

Rocketman

Not near as impressive as SuperNova
Hey man, my SuperNova just had a meltdown.

or

What in the H*ll was that?

My UFO PV.

Look at all the names for PVs,
and then tell me BM is weird.

Steampunk

<SNERK!>

Rocketman

You jus a slob
didint your momma told you to always keep your 'sensor' clean?


A clean sensor is a happy sensor.

Steampunk

If I clean it any more I'll prolly wipe the coating off it!

fsors

RM, you must have heard about the crap storm GV smoktek pv debacle?

Rocketman

I stuck my nose in a few times over on ECF.
But if you were talking about VF, they won't let me in.

No big loss for them, I wouldn't have contributed much.

Steampunk

Um...WHAT?!?!?

Rocketman

After I've done a little more thinkin about this, I'm going to see if it is logical that the slot/linear hole array concept will actually be any better than holes around the bottom and/or a large bottom blow out plug.
Comparing PV explosive failures to a firearm may not be applicable to the pressure profile of a venting Li-ion cell unless the PV is sealed, and the pressurizing gas is contained with pressure allowed to built until mechanical failure occurs..
Evaluating vented PV failure mechanisms against air or gas distribution systems operating at a few hundred PSI instead of 20,000 PSI (pressure in a 45 auto cartridge) may be more applicable.

It seems the top of the cell and clearance to the inner mod shell may limit effectiveness of holes/slots down the side.
The concern for venting up toward the top of the mod is the possibility for the cell case to swell cutting off gas flow below that point. With a cylindrical shape the hoop stress on the metal case would seem to have to exceed the yield strength of the metal used in order for it to swell in the cylindrical section. Coke cans usually hold the cylindrical shape and the top and bottom bulge before the can explodes. I would like to find examples of steel encased Li-ion cells that swelled, stretching the metal case, before I proceed.

In this rough sketch you can see that the pressure remains about the same until the first vent is encountered. Whether this is the start of a slot or the first of several holes, the pressure before the vent is higher than after the vent, continuing down the body of the PV, with pressure decreasing as the gas pressure is vented through more and more vent area. It appears that with a venting cell, that the highest pressure will always be nearest the top of the cell, duh, and the annular space between the cell and the PV case at the top of the cell may be the limiting dimension for gas flow.

Moving the upper extent of the vents down the sidewall will move the high pressure zone further down the PV but may not affect the peak pressure at the top, or by very little. This may be the most critical area to prevent an exploding PV. Annular space between the PV body and cell case seems to be the parameter with the most influence on gas flow unless vents are placed at or above the top of the cell. To help visualize this this consider a very tight fitting cell with a very small passage for gas flow at the top of the cell. At the point of the first vent, cut all the remaining PV case away. Can't vent much more than that. The gas flow/pressure from the cell will be limited almost entirely by the initial passage.

Another thing comes to mind (hear the wheels turning?) is that the head space above the cell or maybe above the first vent may allow a larger reservoir for pressurized gas storing more energy for accelerating the front end should it fail. This effect of this stored energy can be seen in the difference in the failure of gas vs liquid proof test applications.

Ok, enough for now, just food for thought.


Comment as we go along here if you want, but if you quote it, and I change it, you are quoting old stuff.
Did that make sense.

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fsors

like this?

Rocketman

I'm still thinking and revising.
Your quoting old stuff that I can't access

I don't know, not for sure. That does look pretty good, but is is better or worse?
Just plain don't know.
My guess is it doesn't take a lot to be adequate.
Wouldn't that be nice?

Little chance of a cell failure blowing an atty down your throat with that design.

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fsors

Thanks RM it's what I planned it for (safety) but it sure is nice to get assurance from someone I respect

Thanks again Buddy.