What happens if you use protected cells in a light that only uses unprotected cells.

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What happens if you use protected cells in a light that only uses unprotected cells. Just bought a few lights without reading the description and didn't realize they require unprotected 21700 cells. Unfortunately I only have protected 21700 cells and aren't comfortable keeping unprotected cells in the house. Providing the lights can fit the longer protected cells is there any danger to using them in these lights either for the battery or the light itself. Any help would be greatly appreciated.
 
Providing the lights can fit the longer protected cells is there any danger to using them in these lights either for the battery or the light itself
If they physically fit OK, w/o spring over-compression etc., they should be fine if the current rating of the cell meets or exceeds the current requirements of the light. I also greatly prefer using protected cells, unless something precludes it, and that is very rare. I want over-charge protection, and know of no other way of providing that(?). I have protected cells that will fit and work great in every light I use, but I also don't use 21700, so can't speak to that.
 
If they physically fit OK, w/o spring over-compression etc., they should be fine if the current rating of the cell meets or exceeds the current requirements of the light. I also greatly prefer using protected cells, unless something precludes it, and that is very rare. I want over-charge protection, and know of no other way of providing that(?). I have protected cells that will fit and work great in every light I use, but I also don't use 21700, so can't speak to that.
Thank you so much for the information! Really appreciate you! Thank you!
 
Take this with a grain of salt, as this is my understanding of it all and parts could be wrong.
Exactly what happens if any of the protections kick in, I actually don't know.

As far as I know, there are 5 types of protection for batteries:
Over-current
Over-voltage
Under-voltage
Short-circuit
Overheating

Some of these safeties are beneficial during use, and some during charging.

TL;DR:
Buy quality flashlights, batteries and chargers, and don't cheap out on either of the things. Use common sense at all times, be extra careful at first to gain personal experience with your devices and batteries, and then unprotected cells are perfectly safe to use.

I also used to refuse to use unprotected cells. With very cheap lights, the protection inside the battery was the only way to prevent any incidents like over-discharging the cell, effectively ruining it. But in the last couple years, starting to buy custom lights, I have become comfortable with using unprotected, quality batteries. Such as Samsung 40T (21700) and Molicel M35A (18650). Using a Nitecore UMS4 charger, that has plenty of safety features built-in to protect the batteries.

Expensive and / or quality lights have several of these features built-in, and even with unprotected cells, the light itself won't drain the battery any further by simply refusing to turn on if the voltage gets too low. This is both to protect the battery from being drained too much, and also because the light would only give out a couple Lumen, which would be practically too low for most usecases. But many lights will still trickle-drain the cells (called "vampiric" or "parasitic drain"), which can and will drain unprotected cells down below their safe limits. Which is why I prefer unscrewing the tail cap on a light half a turn, to physically disconnect the battery.

Over-current protection can be an issue with protected cells in custom lights, as the LEDs and electronics in such light are made to drain, say, 10 Amps from a battery. Most top-end protected cells that I have seen can deliver up to 8 A in discharge current. Any more, and the over-current protection will kick in, effectively killing the battery to protect itself.

As a sidenote on over-currents, I prefer chargers with a low charge current. High current heats up the battery, and I don't like any charger that charge a battery with over 2 A. 1 A tops, or preferably 0.25 A. Yes, it takes longer to charge a battery - from an hour to most of a day, potentially, but I got time when I charge things, and it is much safer.

Over-voltage protection is useful when charging a battery. If you leave a cell in a charger overnight, the battery can in the worst case explode without any safeties. But while I personally take steps to prevent this, I have forgotten even unprotected cells in a charger. And this is where having a quality charger comes in. Most, if not all, quality chargers have a cut-off point, where the charger itself will reduce its output to just trickle charging when it detects the battery is fully charged.

Under-voltage protection comes in when the battery senses its voltage delivery gets too low, which is between 2.5 - 3.0 V for a typical, protected 18650 cell. If it gets drained further, it can cause permanent damage to the cell or even ruin it. But unprotected cells don't have this feature, and more care by the user must be shown to properly use unprotected cells. Though if you have common sense, and the faintest glimpse of understanding of electronics, I guarantee it will be fine.

Short-circuiting an unprotected battery can lead to spontaneous combustion and explosion shortly after. This is why proper storage and handling of unprotected cells is much more important than when using protected cells. This goes for all batteries, but make sure, at all times, that nothing made of metal can connect both sides of it.

Overheating is closely related to over-current, and can happen either when charging or discharging a battery. Heat kills electronics, and overheating protection is down the connected device to take care of, either the charger or flashlight. Again, quality devices have this protection, both to protect itself and the battery. The batteries themselves, protected or unprotected, typically don't have any sort of heat protection.

Custom flashlights can often get too hot to hold around the head, especially on Turbo (which to me begs the question why the feature even exists), and there seems to be a general consensus among the experts here on the forum never to use the Turbo feature on high-output (and with that, high-drain) flashlights. Both to protect the flashlight itself, and the batteries.

I personally HATE the integrated USB-C charging feature on flashlights and batteries. Am I expected to trust the tiny brain in a battery, when fitted inside a metal tube, instead of an open, much better controlled, dedicated battery charger with much more space for electronic safeties? Hell, no. I am not putting random current into something that is effectively a hand grenade by design and take my chances. I am always removing the battery from the device, and placing it into a dedicated, quality charger, thank you very much.
 
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The above post pretty much says it all. A management summary could be: a good charger will prevent all events where "over" must be avoided, and a good light will prevent all events where "under" must be avoided. I've done a quick sweep over my shelves and only 10 of 65-ish batteries are protected. There is just one combination where I would urge to use protected batteries. And that is in lights, like the long plunger lights, where two or three batteries are inserted in series. One of those batteries is, by definition, the weakest link in the chain. Usually those lights have no low voltage protection (LVP). The protection in the battery will prevent the two "jocks" to drag along the weakest battery until it drops to a voltage from which if can't be recharged.

For all other lights I suggest four things: 1/ if not in actual use (e.g. in hand) unscrew the cap half a turn, to physically disconnect the battery. 2/ if the highest mode does not add more lumens than the previous mode, take out the battery to check on it or recharge it. 3/ never leave a charger unattended. Not by constantly looking at it, just don't leave the room longer than a few minutes. 4/ Some people complain their charger is incapable of providing 3 Amp. Mine does, but I usually charge at 0.5 Amp, or 1 Amp when in a hurry. That is for 18650 batteries. Lower Amps for smaller batteries. And always as low as posible for 14500 (= AA = penlight size) batteries.
 
Thank you aznsx, PaladinNO, and Henk4u2! I appreciate all the help. Thank you PaladinNO for elaborating and sharing your vast amount of knowledge on this subject. Thank you everyone and take care!
 
A little bit of battery crush on the positive end of the battery is not the end of the world, but using one of the longer protected batteries in a flashlight designed for unprotected might be bad, especially if you drop the flashlight in such a way that the positive end of the battery gets hammered. The flashlights you bought, do the positive sides (at the head) touching the battery use a spring or a post? A rigid post is more dangerous if dropped wrong.

Consider how many flashlights are out there that use unprotected cells. It's not like we are hearing horror stories every day about unprotected batteries. Some people will not use protected batteries, believing they are less reliable under certain scenarios.

Like so many things in life; potential danger is all around. Make yourself informed, use common sense and pay attention to what is going on around you.
 
a good charger will prevent all events where "over" must be avoided
...until it malfunctions.

(I know little about Li-ion cells, and am as far from a chemist as one can be, but do work in systems reliability design / implementation in the electronics realm.)

From what I've gathered, some of the more catastrophic failure modes of these cells (Li-ion specifically) occur during charging.
Catastrophic events in general (not limited to electronics) often occur not because of a single failure, but when 2 or more occur.
Fail-safe design objectives typically involve elimination of single-point failures which have catastrophic results.
Most electrical systems incorporate fusing at both the source and the load, at a minimum.
A high quality properly designed / properly operating charger should never over-charge / over-voltage a cell, but given the wrong internal malfunction / failure, it can happen, and has happened.
One reason I can use Li-ion cells and sleep well at night (not during charging) is because I generally use cells with a safety module / protection circuit / PCB which provides, among other things I also want, over-voltage / over-charge protection.
In many cases, this provides the second point of failure required for the over-voltage / over-charging catastrophic failure / event to occur.
Almost nothing I know of in engineering is without tradeoffs, and I understand and work with those in a reasonable way, with a systems perspective; and I, the user, am part of the 'system', BTW. I also have a failure rate too. I use 'protected' cells unless it is impossible to for some reason, or I have another very specific good reason not to. That's on a 'case-by-case' basis, but not as a matter of general policy.
When I do use cells lacking this feature, I realize that I become the 'second point of failure' which enables the potential catastrophic event, so I don't do it lightly, or as a matter of general policy; only if / when required, and therefore I do not and cannot recommend it to others as general advice.
Risk/reward/cost/benefit: sounds like most of life to me;-)
 
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Thought about this alot;

~ between my 5~6 dozen Li-Ion flashlight batteries
~ 16 or so Lithium Polymer batteries at various voltage (Lipo)
~ several Lithium powerbanks (Li-Ion & Lipo)
~ nearing 20 Lithium power tool battery packs, including my 40 & 80V yard tool items
~ a few Li-Ion and LFP powerstations
~ ...and about 20,000 Watt Hours worth of LFP batteries that are solar charged at strong Amps

these all are in heavy use every day at my house


Expect for two 18650 I never use::: none of the individual batteries themselves are 'protected'
and I lose zero sleep over it.
 
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When i started buying "Real" flashlights i thought sure buy extra batteries, i mean how is that a bad thing?
Fast forward to learning about rechargeable batteries and i certainly have created a second career lol.
Every quarter i get my stored batteries out and put them on my quality charger at 0.5 Amp charge. They are usually at 50%-75% and i charge the ones that need it to 75% not 100%.
Then i do the same thing with the batteries in the half dozen flashlights i am currently using no pun intended.
Lights like my Maratac lights when low start flashing while i am using them letting me know it's time to charge.
Some have special lights on the button or body of the light that let you know it's time.
I definitely twist the end cap when not using the light.
 
+

Thought about this alot;

~ between my 5~6 dozen Li-Ion flashlight batteries
~ 16 or so Lithium Polymer batteries at various voltage (Lipo)
~ several Lithium powerbanks (Li-Ion & Lipo)
~ nearing 20 Lithium power tool battery packs, including my 40 & 80V yard tool items
~ a few Li-Ion and LFP powerstations
~ ...and about 20,000 Watt Hours worth of LFP batteries that are solar charged at strong Amps

these all are in heavy use every day at my house


Expect for two 18650 I never use::: none of the individual batteries themselves are 'protected'
and I lose zero sleep over it.
I do stuff in my car that most would lose sleep over and consider an unacceptable risk. I understand that risk tolerance is a very subjective, personal thing. I try to learn from the experiences of others when possible, without waiting for my own personal hard lesson. I will also greatly reduce the possibility of potentially catastrophic events when I can easily do so and lack a good reason not to. I did not post expecting any agreement here at all (I know better, quite the contrary), but EDIT: for rather for the benefit of others posting questions. I do consider it a 'responsibility' to inform them that there is more than one opinion on this matter, so they can make their own 'subjective' judgement. If anyone has a question about my post, I'll gladly answer it. I consider it irresponsible not to post this view of the facts. I've done my duty per the OP. As you were...
 
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