I have an dual 18650 torch which Magically discharged one battery in seconds without discharging the other at all. <!--break-->
This will be my first post on this forum so excuse me if this isn't absolutely the correct category.
This also being my first flashlight build and first flashlight so please point out if I have done something wrong.
specs:
xhp70.2 5k (manually dedomed)
5 mode 8.4v driver board max 4.5a
dual 18650 (obviously) lg MJ1 3500mah li-ion (non protected would have been I noticed they were a thing)
convoy c8 body with a flashlight extension tube
a omten 1288 switch others have used before at 10a with a reduced life span
maybe unrelated but a 42mm reflector hole cut to have an enlarged emitter hole-13mm with a 7070 insulation gasket on the led ,with a handmade 'mylar polyester' adaptor to increase the diameter of the insulation gasket to cover the entire gap from the reflector. (the reflector hole is that big to fit led and to get a 'perfect focus' I managed to do a bad job of it)
if your interested a glow in the dark o ring and a AR coated glass lens
the driver board picture is a before picture.
the problem...
to keep this simple i'll give the facts then the thoughts.
FACTS
the event: The batteries are carried in separate case during transport only put in the flashlight when in use. I wanted to look into a crack in a mine shaft cover so...
put batteries in
cycled through the modes to get to the second mode (1000lm) i click the switch on then half press to cycle (i miss it first time round so half pressed a few times till i got it)
the flashlight has been on 30s? working fine not hot.
I handed the on torch to someone else to look who managed to knock the head of the flashlight length-ways lightly and tries to shine it upon which its not on.
upon trying to click off switch there is no click feel or switch at all, secondly the switch and rear extension tube (exclusively) has become quite warm, thirdly I remove the tail and both batteries the rear one(closest to switch) being warm upon (foolishly i don't take note of which battery got hot!)
the aftermath:
one battery is at 4.12v the other 1.2v!!! both were 4.15ish before putting them into the flashlight. The one at 1.2v has a slightly melted wrap at one side, on the negative end, I'm guessing this is the one that got hot. There are also 2 marks on the terminals one at the negative end of 1.2v batt and one on positive end of 4.12v batt these may/may-not be new.
I've removed the switch ... it's in a depressed position I cracked it open it has melted so ( best guess due to switch design melted and let the spring push contact up whilst leaving the switch depressed breaking the connection an hense flashlight is off)
the driver board spring is completely squashed (permanently-above elastic limit and darkish colour, may not be related to incident just only first noticed it when taking it apart)
there are not other marks the flashlight still works using different batteries and a 12awg wire to act a switch
these are taken afterwards the batteries had black insulation tape to get a tight fit in flashlight - the tape has been removed off the 1.2v batt.
MY GUESS :
the flashlight worked fine until getting knocked causing the driver spring to compress breaking the circuit between the two batteries the 1.2v battery im guessing was at the back shorted with the aluminium body causing large current to flow over the one battery (3500mah battery so 3.5a over an hour so best case-shorted over the ~10s it was off but had switch on and batteries in it so best case is 1200a ! massive) this causing the switch to have 1200a through it melting it (not switch failing causing short as its either on or off so max 4.5a from driver over BOTH batteries cant end with one at 1.2v and 4.12v) Because neither batteries appear to have bare contact to body presumably the melted battery wrap is because it arced to the gap between the threads of the tail cap, is this possible at 3.7v 1200a over max 1-2mm?
OR
short was over both batteries leaving the marks on both terminals melting switch bypassing driver and led as contact is broken between driver spring and batteries - however the voltages would be equal as they should equalise or we could never use batts in series disproving this.
so as can be read I am no expert though I would say I have enough knowledge to see this is an odd incident. I know that replacing both the switch and batteries will fix it but could this? whatever this? may be happen again?
Any help explaining what it may be / a fix / an error in building / misuse / how a switch fails and its affect or if one battery just malfunctioned due to a slight knock is possible IT WOULD BE GREATLY APPRECIATED
Nfi what’s that light, looks like a C8 head with S2+ shorty tube and all sorts of other plumbing.
One cell cooking itself to death (low volts, super hot), while the other’s unscathed, usually means that the one cell only was dead-shorted. My guess? The cell at the tail end. If the driver works at all afterwards, then it likely didn’t handle all the current through it, and it was shorted through the body (and switch, being that it melted).
Then again, being that the driver spring cooked itself, current likely did go through that, too.
So… that’s a mystery. Current through the switch means it likely came from the tail end, and melted spring at the driver means it also came from the front end. Yet only one cell cooked.
In short, fiik.
Me personally, I’d just give the light to someone I hate.
thanks for the reply, I’m glad I’m not the only one that thinks its odd but i guess then as the light works fine with nothing abnormal with light output or modes surely the current passing through it must have fried it big time so the spring being darker is another issue and its my ‘other plumbing’ that caused the issue. out of interest I’ve not heard of a ‘shorty tube’ looked it up but i can work out its purpose.
my poor plumbing is all original with an added extension tube (or thats what it was sold as) so i could have 2 18650s to get the 8.4v to drive the led.)
That cell top looks like it does have protection, and like someone spot welded the hell out of that vent cap. Might as well peel that wrapper off and see what is really under there—it appears to be a modified cell.
The rear cell (closest to switch) has its negative connected to the flashlight tube. Its positive is connected to the front cell negative. Then if the case of the front cell gets shorted to the battery tube that is dead shorting the rear cell through the tailswitch. This is a danger that I often forget about since this risk is not present in a single cell light.
To me it seems like that is what happened with you since the rear of the light got hot. Though it seems like there might be some red herrings that point to different explanations. There is a tear in the cell wrap of the shorted cell, but if my explanation above is correct, that wouldn’t cause the dead short. It would be the front cell shorting its casing with the flashlight tube that would cause the rear cell to short. So look on the other cell and in the tube to see if you can spot where it might have shorted.
So you took out the cells right after the incident, and was one of them really hot? There’s no way 1200A flowed; 4V/0.05Ohms is 80A. I wonder if some safety disconnect in the cell got triggered and it’s not actually completely depleted. If it got shorted and drained completely it would be BURNING hot.
I have done that to both cells here are pictures I don't know what it should look like but the cells came from "18650uk":https://18650.uk/product/lg-mj1-button-top-18650-battery/ with an advertisement of a button top spot welded on.
ok so both cells wraps are gone the only mark I can find on the 4.2v non-shorted cell is this... at the top/positive of the cell there are a few small marks.
Brown scorching around the vents and top of the discharged cell, so at some point it got hot and opened the exhaust vent.
When the vent opened then some internal part may have touched the top and wall together to drain the cell. This may be a design feature since the max current is 10A and they are specified not to explode or catch fire.
The overpressure device may have been damaged by the spot welding, then when the light was longitudinally knocked it caused the vent to trigger open.
i like to figure out why they fail and have dismantled cells after completely discharging them and then using a lathe to carefully cut the top off such that the OPD could be examined. Also you could hold the cell in a vise and use a dremel cutoff wheel or hack saw to do the same.
Maybe, as an (thermo) mechanical engineer, I’m not qualified to comment on this.
But during my study I had lengthy discussions with the more capable sparks drawing variety.
I remember to keep a sharp lookout for missing an/or loose parts when dealing with electricity.
Concerning the “plumbing” I noticed a few things.
- The original tube should be fitted with the letter “C” of Convoy at the side of the head.
The driver should be fixated with a retaining ring. I can’t see one.
The first is a matter of conventions, not critical, but is handy to lock-out the light with ¼ turn of the tube.
The second might be part of the conspiracy.
- The retaining ring not only fixates the driver in the head, but also “pushes the tube back” a few mm.
- You miss those few mm. And you have already lost (2x) a few mm by putting on button tops.
And for starters, the MJ1 batteries are already (2x) a bit longer than my favourite 30Q’s.
I think what happened is:
You screwed in the tube nice and firm (as you should), and by doing so actually eliminated all play (and more). There were only two things that could give way to relieve the pressure: the spings on the driver and switch. First longitudinal, and then bending. I think the bending driver spring shorted the cell with the tube/head. And since the light was on, the poor springs and the switch got the full blast. That happened when the light got a little bump, not much, just enough.
I guess one or both springs are now “mushy” and the intestines of the switch have melted.
You need: a new retaining ring, shorter batteries, and only ONE of both batteries needs a button top.
And a new switch on a PCB, of course.
That’s a good thought Henk4U2. Maybe the spring wouldn’t even need to short all the way to the tube, maybe it shorted to a component on the driver board connected to ground. But if it shorted the cell positive to ground both cells would have been shorted, and from the description it seems like only the rear cell was shorted.
Flyinglight220, I am still not clear on what you observed when you took out the cells immediately after the incident. Was one of the cells very hot?
yes only one cell was hot when taken out as i said foolishly didn’t take note of which two cells though one was at 1.2v so must be that one.
ok so i have managed to not put the ring in but that was solved by the tube being cranked down but then left me with less room in the tube for both cells meaning when hit it pushed the spring into … causing a short (so yes the driver spring is soft - but no rear spring still feels stiff but it had a 18awg? bypass so current would go through it the wire not spring or at least most of it as has lower resistance?)
so my error was leaving out the retaining ring- seem to remember the driver wouldn’t fit with it so just cranked tube down instead.- and i needed only one button top cell and the extra capacity makes lg longer and unsuitable so try for a smaller one with same output current.
also noticed the driver spring was allready compressed in the before picture so would fit it shorting out there … i have heard some mentions of a copper pill instead of a spring maybe to prevent this problem?
You can replace the driver-spring by a short copper nub, but do realise then you loose a bit of play.
The battery can’t protrude in the tailcap very far. I assume your tailcap is an original Convoy tailcap.
Checking: place the light heads-down on a table, and the battery should NOT stick out of the tube.
If it is sticking out, you loose a lot of play in the tailcap spring (and if you bypassed that, even more).
Correct that by placing a plastic shim around the theads on the side of the tube that goes in the head.
Moment of reflection: even in a perfect world, members of the same team are not always equal.
I keep on thinking that the short went through both batteries, and only the fittest apparently survived.
ok so there is about 7mm gap from the top of some other 18650s to the top with no protection or button tops, button tops add 2mm each so the origional shorted cells in it would have 2mm clearance below the top of the tube.
so a button on the driver would be fine,
the tailcap is original though the switch was changed to the same size but to ensure quality was changed.
pics 1. replacement identical omten 1288 switch to be put in vs one that came out light.
also on further inspection only the top half of the driver spring is discoloured and soft
Not really that’s quite a distance ant the tube that screws into the head will cover that up but the spring might be able to bend to touch some contacts on the pcb for an optional switch prephaps the ground pin creating the ‘dead short’
Don’t think so (in principle). The un-anodized threads of the head are at the inside, and are in contact with the un-anodized threads of the tube. But these are on the outside of the tube.
The spring can (maybe) touch the inside of the tube, but that is always anodized. When new.
no they were bought at the same time and I’m used to li-po batteries so i always make sure that they are at the exact same charge level, wear should be the same they have only been used in the same light as a pair but i suppose as this Isn’t a perfect world there would be a a small difference in resistance/ voltage.also I’ve tried measuring the resistance of both cells but gives ever increasing resistance of one and infinite for the other and putting leads the other way give 0.9 some factor of ohms for both cells but its the same resistance as just touching the leads together so i don’t know how to do it?
