Hot batteries in Fenix TK40

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jirik_cz

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To use any rechargeable battery like this "series-stacks-in-parallel" is ALWAYS a bad idea, due to the enormous currents these batteries WILL deliver when only a small difference in voltage between both series-stacks exists.

If I would use 2x4 eneloops and there would be 0.1V difference between two stacks, there would flow 0.5A current at max. And that is without considering the resistance on contacts. Not a big drama imho :)

Just always use cells (rechargeable or nonchargeable) in the same state of charge and you will be fine.
 

Jackal-Head

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mabey this explains the 3 easier, they might have been being overcharged, while the poor single is being destroyed from reverse charge?

I presume it was exactly like this: reverse one cell and you got a huge (compared to normal chargers) voltage difference between the sets. Since the NiMHs (or L91s) have a low ESR and the battery carrier with its gold-plated contacts also has a low resistance, this can translate to a high current discharge of five, and a high current (over)charge of three batteries. The one thing I find odd though is that the 3 were more discharged than the other 5 in Glock's case. Maybe he got that mixed up though?

The light will initially appear to operate correctly as the light head still gets approximately 4.8V (minus some unusually high losses in parts of the carrier).

I think the "poor single one" doesn't exist; with the carrier removed from the light, the same forward (i.e. discharging) current passes through it as through the four correctly installed batteries in the other set. And if the head circuit is closed, there is less, not more current through this single battery.
 

Jackal-Head

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When using ANY kind of rechargeables, just use 4 of them, just one stack. Problems will be largely avoided then. Of course, runtime will be compromised, for this, keep 4 spares at hand.

While it's definitely safer with only 4 batteries, assuming the 8 rechargeables have the same type and charge state, the current will be low, and equilibrium will be reached quickly. Now should one battery suffer an internal short-circuit (stuff happens…) this could be more of a problem. But that's rare and can also happen with primaries. I also think in this case, the battery's terminal voltage will decrease slowly over the course of hours, and not drop rapidly.
 
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Glock27

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I an certain that 3 of 8 discharged a lot more than the other 5. I am also certain that 5 of 8 took within 20 mAh of each other when recharging. I too expected that 1 cell would have been destroyed.

With 10 - 12 cycles since my incident, the "hot" 8 are performing exactly like my other set.
G27
 

Jackal-Head

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I an certain that 3 of 8 discharged a lot more than the other 5. I am also certain that 5 of 8 took within 20 mAh of each other when recharging. I too expected that 1 cell would have been destroyed.

I can't think of a way to explain this behavior with the (simplistic) theoretical model because 5 batteries always seem to "beat" the other 3. However, the actual behavior of a galvanic cell can deviate from this model anyway. Possibly (now this is speculative though :D) 5 cells overcharged the others initially, and then, due to battery chemistry, the closed-circuit voltage on those 5 sagged so much, and possibly the voltage on the others increased, that this caused the flow of current to reverse.

Assuming an ESR of 80mOhms per battery and another 100mOhms in the carrier, 2.4V would translate to about 3.25A. Not all that much but enough to decrease the terminal voltage of the cells being discharged significantly. Ok, so I'm going way out on a limb here, but my point is just that a current reversal could happen because of the specific behaviors of NiMH chemistry.

By the way, I investigated why it's not recommended to charge NiMHs in parallel and found this: "Rechargable battery cells can and do short out, so it's always dangerous to have two packs in parallel even when not charging. The right solution is a diode bridge to isolate them"

Great… so the TK40 is a time bomb unless you take one battery out when it's not used? :banghead:

With 10 - 12 cycles since my incident, the "hot" 8 are performing exactly like my other set.
G27

Well, I'm glad to hear that your batteries survived this incident. Let's hope it doesn't happen again or there may be one continent less to worry about. ;)
 

VidPro

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By the way, I investigated why it's not recommended to charge NiMHs in parallel and found this: "Rechargable battery cells can and do short out, so it's always dangerous to have two packs in parallel even when not charging. The right solution is a diode bridge to isolate them"

Great… so the TK40 is a time bomb unless you take one battery out when it's not used? :banghead:

i really doubt that the bats at that link were damaged (that badly) due to any single issue, many misteaks were made or it wouldnt have gotten that bad.

example1: the charger he is using is dependant on V-drop, when a parellel ni-mh pack reaches end of charge the V-drop on the parellel cells does not occur simeltaneously, so v-drop is a bad thing to try and use when in parellel, one set is going up, while the other is going down, and (stupid) smart computer he is using wont work that way.
Example 2: , he tried to charge it at 1C for a single parelell item in the series set, the thing was basically enclosed, where was heat supposed to go.
example 3: once he overheated, overcharged , then he also got electical short, just to make things worse than they were, where was the isolation needed, sure he had insulation to increase the heat, but no isolation to reduce the possibility of shorting.
if he thinks a diode was his problem :) then he hasnt been watching what happens when he charges very closely. at the least he could have made his first charge monitored , before assembly.
 
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exodus125

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if anything, this post shows one of the cons of having an 8AA light.

However, we are all flashaholics here, make sure the batteries are in right before you put them in your 150 dollar flashlight.
 

Glock27

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I am hoping an unsatisfied K bin owner will take the challenge.....find a combination of cell reversal that causes a Hot pack.....and 3 of 8 cells showing more discharge.:devil:
 

strideredc

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i had a hot pack but not in the tube just when i loaded the cells, i think there was a fault with the holder, i have emailed the seller, i will let you know...
 

jirik_cz

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I'm surprised no one mentioned it yet. But Fenix released TK40 with newly designed battery springs a month ago. The new design should prevent an electrical contact of incorrectly placed batteries. This should make the light more fool proof :)

tk40srpings.jpg
 

Bushman5

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^ got a pic of the OLD battery carrier? and is there a way to ID wich TK40's have the old vs new? (ie serieal numbers or sumtin?
 

eng_driver

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No spare carriers at this point in time. I'm not sure if they'll offer it except for warranty use. Do be careful when loading these lights. Remember they
are only 1.5v each - lots of room for stuff to go wrong.

1) If you're using alkalines, ALWAYS use NEW cells from the SAME pack
and the SAME brand. Do not mix between brands, or even cells with different
remaining power. When the first cell depletes, immediately the continued use
will cause that cell to begin to be CHARGED BACKWARDS.

2) If you're using NiMH, use the SAME rating, SAME brand, same batch (purchased at the same time) and don't use those cells for other things. Keep them together for the life of the cells with the flashlight.

Discharge and charge them together. Never mix different brands even if they have the same capacity. Never put cells with different charge states. You want to keep all the cells in the same condition all the time. Same thing happens when you nearly deplete the pack - the first cell that runs out (to 0v) first will damage the cell going below 0.7v or thereabouts and when it hits 0v, it immediately begins to REVERSE CHARGE. Just a friendly warning. :)

This whole thing with matched cells is an important issue and kind of a drawback with having an 8 cell light (even though the power is fantastic.) All 8 cells should be matched, and if you draw the light all the way till one just one eneloop (or whatever) reverses, then you should replace all 8. :faint:

The lesson is.. Don't draw the cells till one or two reverse charge!
 

eng_driver

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Or rather, I meant to say, don't draw too much from cells so as to cause one or two to reverse charge!
 
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This whole thing with matched cells is an important issue and kind of a drawback with having an 8 cell light (even though the power is fantastic.) All 8 cells should be matched, and if you draw the light all the way till one just one eneloop (or whatever) reverses, then you should replace all 8. :faint:

The lesson is.. Don't draw the cells till one or two reverse charge!

While this is true, you are aware of the situation which started this thread, correct? I'll give you a hint, the reverse polarity protection goes out the window when you're using lithium AAs. I haven't heard of anyone reverse charging any quality NiMH in this light.
 

DougL

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I know this is an old thread, but I just picked up a TK40 recently, and I'm concerned about the last comments made by was.lost.... Does this mean we should not use the Energizer Ultimate lithium cells in the TK40 as it will void the built in reverse polarity protection of the circuitry, resulting in a potentially dangerous situation? Until now, I had been under the impression that the Ultimate lithium primaries could be used wherever alkalines could be used, the only potential downside being that in some flashlight models, it would cause a loss of lower modes until the cells had been drawn down somewhat.
 

Labrador72

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Nice buy! : )

I think if you use good cells, all new, I don't see why you should have a problem. In any case, you are better off using nihm than Energizer Ultimate Lithium if you want to be kind to your wallet. Of course very low temparatures and times when you need longer runtimes are just some of the exceptions when using lithium cells makes sense.
 

DougL

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Any idea why Fenix specifically recommends that we not use the lithium primaries? Is there any danger of anything, including reverse charging when they get low in charge? Any precautions I need to take aside from taking them all from the same 8 pack, such as perhaps when the highest level, or perhaps the next to highest level, or the third to highest level, is no longer available, switch them out for a new set of cells.
I don't expect to use this light much, but rather to have it as an emergency light, so the Ultimate lithiums would be best for me in that regard as I would want the light to have the cells ready in place, just untwisting the head of the light a couple of turns to avoid parasitic drain. I have plenty of other lights that I use on regular basis, mostly those that use either one CR123 or two CR123s or one 18650.
 

Labrador72

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Does it say in the user manual for the TK40 too? I don't see a reason why the TK40 wouldn't be able to work fine on the L91 and I don't think you'd be losing any modes. Which battery carrier did your TK40 come with, the old or the newer one?

If you want to be dead sure for the piece of mind, just contact Fenix service: they usually reply very quickly.

EDIT: actually somebody contacted Fenix about this years ago.
Hi, Fenix has replied my mail regarding the use of lithium batteries and also, I had permission to share this info here.

Thank you very much for using Fenix light.

For the TK40, we don't suggest you use lithium battery, usually the Lithium battery volt is higher than 1.5v which we require, so it will do harm to the light, even though the volt is not higher, as you know, the lithium battery has more dangerous factors than the Alkaline and Ni-MH, and TK40 use 8 batteries, then the power will be very great if it's Lithium, so we can't ensure whether it's safe for the light, if you don't take care, there will be the danger that we can't anticipate in advance.

So for the light's safety, please don't use Lithium battery in the TK40, meanwhile, if you don't use it for a few days, please put the battery out.

Thanks!

Well there you go.

From my point of view, I think she also meant that it will stress the electronics. Running fresh L91s would be about 7.2v. Fresh Alkalines about 6.2, NiMH has about 5v.

If I were you I wouldn't worry though: I'd just make sure to use new cells, insert them correctly and unscrew the tailcap when you don't use it!
 
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