Dual 26650 flashlights seem to discharge front battery first?

LED Flashlights – General Info 26650 Flashlights
1

About four years ago the place I was working at handed out LED aluminum flashlights that use dual 26650 batteries. They are branded with several names, all Chinese I assume, but they work great for the most part. They’re a huge improvement over the LED Maglites we had.

The issue I’m seeing is that in use, after 3 or four hours of use the front battery discharges long before the rear battery, on every one of these lights.
I recharge mine every day, so it rarely gets low enough to be a concern but I’ve popped the batteries into the charger and had the front battery as low as 2.55v, while the rear battery was still 3.7v or more. (They gave us Nitecore chargers and the batteries are eFest IMR4200 3.7v unprotected 26650 cells)
I’m pretty much assuming that these lights have no battery protection, and if they do, they have no way of reading each battery separately.
I’ve gotten into the habit of swapping the batteries around if I’ve been using the light alot each night and that seems to help.
What got my attention is that one of the batteries rusted away from the inside out, it still held a charge just fine, but the case of the battery looked like swiss cheese. Nothing leaked out, but the can itself rusted. The battery tests 100% otherwise. I chocked that up to a bad battery, then a few others started to do it too, and its always the front battery, and the corrosion happens fast, like overnight. The older lights have green eFest batteries, the newer lights have purple efest cells. So far none of the purple cells have failed.

I did convert one of them to use dual 18650 cells. I tried to fit protected cells but most measure just over 70mm and are too long so its stuck with unprotected cells. (The added length means the rear button switch is being compressed and won’t work with the longer batteries).

Is this a common issue with this style light?

2

I recall this being a noted phenomenon in all series lights, perhaps not to the extent you’ve observed. There will be some threads on here or CPF, I vaguely recall the difference in voltage was small but measurable.

Without finding the old posts I can only guess, but I think size/chemistry of the cells is irrelevant.

There may be additional problems going on with your light(s) which explain the huge voltage difference.

3

Caveat: I’m an electronics professional, but know near zero regarding chemistry, or other areas of physics (thermal dynamics, etc), and my knowledge of Li-ion cells is limited to the very basics.

I’ve heard similar accounts in the past, but never attempted to investigate further, and do not have a solid answer. I am however fairly good at speculating, so let me throw out some of that, and perhaps others more knowledgeable can either shoot this down, or develop it further. I’m interested in understanding this myself. Although I’m not currently affected by it, I easily might be in the future.

Generally, I’ll speculate that what you’re seeing may be a function of temperature; specifically the difference in operating temp between the 2 cells

Li-ion cell capacity is affected by temperature. The extent of that effect is influenced by specific cell chemistry, and likely anode material. Real world factors would include the typical mode of operation (hand-held or in free air) specific cells being used, ambient temp, duty cycle, and perhaps in particular and most fundamentally, the heat soak / dissipation characteristics of the specific flashlight design, materials, and construction.

If I were investigating, I think the first thing I’d do is run one of these lights in fairly typical fashion / conditions for long enough for temp to come into play. When it’s nice and warm, I would then shut it down, remove the cells, and immediately measure the temp / differential) between the 2 cells. If my speculation has any merit, that might provide some actual data as a starting point for possibly developing my idea toward a concrete explanation.

If you can provide specific info on the light / manufacturer / model, that would also be productive for someone to analyze the thermal / heat soak / dissipation of the specific light (at least in general / relative terms), which may play a big role in this if it is a function of temp.

Again, this is based on speculation.

2 Thanks
4

Temperature a good suggestion, sounds like OP swaps which cell is towards the head end though.

It’s not clear what cells are currently being used, anything that drops to 2.55v and corrodes is a big red flag for me, cells are cheap compared to burning your house down or medical bills if the thing explodes. 2.55v resting voltage is LOW. When it’s under load in the light it’ll be even lower.

loosescrew- sounds like a dumb question but are both cells the same voltage when they start?

An additional test is charge both fully, then leave them out of the light,easier the resting voltage the day after charging then a week later- the voltage will fall slightly naturally. Large drops in voltage suggest a problem with the cell.

If you’ve a suitable charger, you could try capacity test both cells.

5

welcome
thanks for sharing a story with such great details

About four years ago the place I was working at handed out LED aluminum flashlights that use dual 26650 batteries.
…
the front battery as low as 2.55v, while the rear battery was still 3.7v or more.

My guess is it is not brand related, it has to do with the dual battery design, and the use pattern of high drain rate and long duration.

Consider Retraining to avoid high drain rate. Use a lower output.

one of the batteries rusted away from the inside out, it still held a charge just fine, but the case of the battery looked like swiss cheese. Nothing leaked out, but the can itself rusted.
…
others started to do it too, and its always the front battery, and the corrosion happens fast, like overnight.

im guessing corrosive gasses finally ate thru the shell
it seems to take 4 years…

new batteries every 3 years might be prudent

So far none of the purple cells have failed.

they are only 2 years old… :wink:

6

Valid point about the cell rotation, but OP stated this is happening on “every one of these lights”. I’d hoped that eliminated the likelihood of a bad / marginal single cell issue, but of course hope is not a strategy, and that cannot be ruled out.

The rotation of cells as a ‘workaround’ to compensate for the fundamental problem wouldn’t be an acceptable workaround for me to avoid potential safety issues, so I’m focused on root cause of the voltage differential. That’s what bothers me, as I’m not even comfortable using unprotected Li-ion cells in series at all, because particularly with no ‘overdischarge protection’, I hear things could potentially reach an unsafe condition (cell “reversal”, etc.).

As you followed up, eliminating potentially bad / marginal / mismatched cells should also be addressed, as it could certainly be a factor if it exists. If the cells are not in general being used as ‘matched pairs’ by everyone, that could be a major factor in itself, as the people using these lights may be unaware of that ‘best practice’. They may be mixing them up.

In terms of ‘mitigation’, I’m wondering about availability of shorter protected cells, but availability of such may be limited (particularly in that size, which I’ve never used). To further complicate things, all users may not even be using the same cell rotation practice as the OP, which would make things even worse, which is another reason I would take no comfort in that at all.

1 Thank
7

There is no heat issue, I don’t think I’ve ever felt any heat from the batteries or light, they’re not often on all that long unless its unintentional. I think the batteries would die before they got hot.

You can tell when the batteries, either one or both is low, the light gets dim fast. and the light becomes useless. At that point the batteries are generally between 2.7 to 3.2v. The couple I found down to 2.5v wouldn’t light at all.
The GTF batteries will just stop lighting the LED, even though they show over 3v in the charger. I only tested two that were dead voltage wise, and those read zero volts and the charger just read ‘BAD’. Those were green efest cells.

The rusty efest batteries all still work, nothing leaks out, and they charge and operate just fine. but they get tossed or set aside regardless but they’re not dying of any internal failure, the case is just rusting from the inside out. The one pictured was found rusty when it was fairly new. Its been in an ammo can since then.

Most guys will use their lights till they start to dim, then charge the batteries. Most will just grab a new light if their batteries fail, its partly why I have so many myself, most just toss them and grab a new one. The company considers them safety equipment so they don’t hesitate to hand them out.

The efest cells are 65mm long, the GTF cells measure 65.6mm.

When you buy Li-Ion cells, how do you know how old they are when you buy them?

I cant say for sure where they bought the batteries, but the last batch came from a scuba dive shop the owner deals with.

26650 lens
26650 lens892×604 84.6 KB

Lens measures about 45mm and has no chrome reflector, just a copper ring and domed lens.

26650 2cell
26650 2cell947×616 112 KB

This one is black anodized with no brand, just the Cree logo on the one side.
I have five different color variations of this light, but the brand name almost always varies depending on which box they came from.
They come in a brown box, 12 per box, each light is in a plain white box and each light comes with a cheap two cell charger and cord. No one uses the oem chargers. They handed out smart chargers. The chargers are branded Nitecore, Boruit, and Keenstone.

GTF 26650
GTF 266501140×796 126 KB

These are half the weight of the efest cells but have not given any issues in at least 5 years.
These are all at least 5 years old, but they likely went into ‘use’ at varying times. The office always has a box or two of new batteries on hand and they were likely all bought at the same time.

IMGP0003
IMGP00031920×1440 255 KB

These started showing up early this year, they weigh the same as the older green efest cells

IMGP0015
IMGP00151920×1313 200 KB

The cell on the left is an example of one that’s rusted through, there’s some signs of the other one rusting near the neg end as well but its hidden by the wrapper.
Both of these are charged and holding 4.2v. After sitting for over a year in a can outside, they had only dropped to 4.12v. Whatever is going on inside has not affected their functioning at all.

8

The battery closest to the top gets hotter faster, because it’s closer to the light and driver.
That’s why.

Even a few degrees makes a difference.

9

That makes sense but this isn’t just a minor difference.
If I don’t rotate the batteries front to back, at the point the light is noticeably dimmer, the front battery is down to 2.7v, and the rear battery is still showing 4.03v.

I did a quick test after my last post, I took two brand new, fresh off the charger, never cycled batteries and put them in the flashlight shown above. I let it run on high till the light started to dim.
It took nearly 3 hrs. I just pulled the batteries and I can’t really feel any difference in their temperature, neither is hot, or any warmer than anything else in the cab. I checked the volts on both batteries, the front battery is down to 3.03v, the rear battery is at 4.10v.
15 minutes after shutting it off, the front battery voltage rose to 3.55v, the rear to 4.18v.
Its as if the rear battery isn’t being drawn down much at all.
I have two other LED flashlights, ones that I bought at a boat show which use two 18650 batteries each.
I put two Panasonic INR18650B batteries in them about two years ago. The other has two Westinghouse branded batteries. Both of those sort of do the same thing but twice now the first battery was not able to take a charge. Neither sees much use, usually only on weekend fishing trips and such. It sits charged and ready on the kitchen counter when not in use.
Those came with really light no name batteries, of those four batteries, three died in the front position. At first I found both almost unable to turn on only a few days after a recharge, one front battery was at .9v, the other at 2.5v, the one at 2.5v took a charge and was fine, the other lost all voltage withing an hour of being removed. It’ll charge up but loses the voltage like someone turned off a switch in about 10 minutes.
I took the two good remaining batteries and put them together in one light and put new batteries in the other. The front battery died again a few months later, that one reading zero volts and it would not charge. That too then got new batteries.
The Panasonic batteries don’t see to be as affected by sitting like the cheap batteries were, and they weight about double as well, 22g vs 45g.
When they were new, and for several years though they worked fine, but I do realize that the Panasonic batteries last at least an hour longer and lose less volts on the shelf.
The Panasonic batteries take a lot longer to charge though, sometimes taking all night to charge vs only an hour or so for the cheap batteries.
The light that now has the Panasonic batteries in it gets warm when used for a long time, but not the one with the Westinghouse batteries. The light with the Panasonic batteries is slightly brighter too and it stays brighter longer but not by any huge margin, maybe 10 or 15 minutes or so difference.

Is the answer to stay away from double battery lights?

The dual 26650 light above is about 50% brighter than any of the single cell lights I’ve got, including one high dollar dive light. (The dive light has a white reflector around the LED, while all of my other LED lights have a copper colored reflector).

10

You’re using some sketchy batteries and some not so great practices, there’s a lot to unpack here, but none of them explain that much of a difference with brand new cells at an identical state of charge. If the problem keeps happening even with brand new batteries, and you can even reverse the order of the brand new batteries and the issue always follows the battery up front, there’s something wrong with the flashlight. The only other thing I can think of is that there’s a short in the flashlight somewhere that mostly bypasses the second battery. There’s gotta be something seriously wrong with either the flashlight or the charger.

Those 22g batteries are fake btw you can toss those. Also shouldn’t take all night to charge anything. Rusty batteries you can definitely toss. That flash rust is very concerning. Implies something corrosive something is getting on them or some fast reaction taking place. Some of them were leaking or shorted at some point and some probably still are. Rusting from the inside out is leaking, kinda. In reverse. There was an opening for moisture to get in and react. Water reacts very quickly if it gets inside a battery and the byproducts of the reaction would be very corrosive and could definitely cause flash rust like that.

11

I noticed the same with multicell lights, regardless of brands, first cell always the one that trips under voltage safety, or have lowest voltage. it happened with 18650, 26650, 32650. I even marked cells with numbers, first cell always seem to have greater load. idk, why, but this is how it is.

12

recharging a battery that has gone below 2.5V is very risky if they are ICR chemistry… slightly less risky if they are INR or IMR chemistry, and are recharged promptly after overdischarge

yes because double battery lights overdrain the front cell.

yes because mixing batteries that have different resistance and use cycles is considered unsafe

yes because rotating batteries with mismatched Voltage is unsafe

yes because using batteries with corroded and perforated cases is unsafe

1 Thank
13

The heat theory is a good one but i think there is more to it. You get a little voltage drop due to the load resistance (driver+led) but only the first battery “sees” that voltage drop so it’s working harder to bring the voltage back up. The second battery “sees” the voltage of the first battery and outputs to the load.

Voltage drop after the load

Btw welcome to the forum and nice first question :wink:

14

I have noticed that in flashlights with two batteries in series one battery often has a lower voltage. It was a difference like 3.0V and 3.5V. If one cell is over 4V I would suspect that there is a big difference in internal resistance between the two batteries. It would be possible that the springs are not stiff enough and the contact between the batteries is not good. You can either use protected cells or recharge the batteries once the flashlight gets dim. Flashlights with good drivers also have a low voltage protection and start blinking once the battery is low, for two batteries below 6.0V. Also, never use any batteries that seem too light or claim more than 3600mAh for 18650 batteries and more than 5500-6000mAh for 26650 batteries.

I would definitely recommend you to get a different flashlight. These are no-name Chinese zoom flashlights that are not bright, not waterproof and have bad leds and drivers. They may be cheap and you can throw them away after use but it would be a better practice to get a good flashlight.

There are quite a few brands with good value for the money like Wurkkos, Sofirn or Convoy. They are way brighter, more efficient and durable. You can also get single cell flashlights or flashlights with three or four parallel batteries. If you have a good flashlight with a good battery it will beat every cheap Chinese no-name flashlight in both runtime and brightness.

15

I agree,

can you make some specific recommendations to help the OP?

I dont know which one to suggest

16

It is difficult to recommend a flashlight without knowing the different requirements like price, size, runtime, brightness, thrower or flooder,…

Generally, I would recommend something like the Convoy C8+, the Sofirn C8G or the Sofirn SP33S. The Convoy Z1 would be a zoomable flashlight with a similar beam like the one OP uses.

A few shop links:
Wurkkos Shop (wurkkos.com)
Convoy Shop
Sofirn Shop

If ordering from China is not an option, some of these are available also on Amazon.

1 Thank
17

It has nothing to do with a flashlight itself, I had it happen with several different lights. leds as well as hotwires.

18

It seems to me that if unequal heating, resultant unequal capacity / voltage, and progressive cell imbalance in operation are perhaps indeed inevitable in such multi-cell series applications, the only acceptable way to mitigate those effects somewhat and maintain an acceptable level of safety would be the strict observance of all best practices (many mentioned in this post already). Those would include use of quality cells and flashlights in the first place, using and maintaining ‘paired’ cells, and last but perhaps most importantly in the case of Li-ion cells specifically, exclusive use of protected cells (and lights which will accommodate them) to avoid overdischarge (which is inevitable in the situation described).

The current circumstances sound like a potentially dangerous situation waiting to happen. It often takes more than one problem / issue to create a bad situation, and I see several here (red flags / bad practices). The company may consider the flashlights and cells to be ‘safety equipment’, but current practices are in fact creating a safety problem.

ICBW (and often am), but that’s my take.

1 Thank
19

That is why it is not recommended to use unprotected cells in multicell flashlights, thou i have used imr cells in multicell lights (3 to 5 cells), had no issues so far. about 10 years ago i bought a solarforce l900 that came with unprotected solarforce cells, i used them for at least 4-5 years with no issues. not sure if they were imr cells, it did not say on the wrapper.

20

The issue is that protected cells don’t fit. The unprotected 65mm long cells are about all it’ll take without the spring bottoming out and denting the back of the cells.

I can’t fault these lights much, out of the dozens, or likely hundreds they bought, I don’t think I’ve seen a single flashlight fail. They get run over, dropped, rained on, even dropped in puddles all the time and the keep going. I do know that the one pictured is water tight. I left it on in a pickle jar full of water for two hours and it didn’t take on any water.
I don’t dive, so that’s water proof enough for me.

The lights like the one above from work have been around for about 6 years, I’ve got my original one and its still working fine, its batteries have been cycled daily 5 days a week for those 6 years with the green Efest batteries above. Another light that someone discarded at work has the GTf batteries and it lives in my pickup truck as an emergency light. I keep two or three of them in my personal truck and both still have the GTF cells in them.
They don’t seem to have as big an issue with the front battery as those with the higher capacity cells.

As far as a safety issue, if they were a hazard I think there would have been at least one catastrophic failure in the 6 or so years they’ve had these now.
Most guys run them stone dead, I suspect some don’t bother to charge the batteries at all, they just go grab new one’s. If I’m lucky, I find the old batteries before the trash gets dumped.