Maybe I’m overlooking it or something but I’m searching for a very basic rundown on the ins and outs of protected vs unprotected batteries. I was expecting to find a sticky but I haven’t or a related thread on the subject at least not in layman’s terms. Also I’ve heard references to dangers on unprotected but to what point and howso? Would someone help a new guy out with the basics, Thanks 
Safer for you and the cell plain and simple. :bigsmile:
If you discharge a unprotected cell at too low of a voltage it can damage the cell permanently. If you direct short a unprotected cell for any length of time it can and more than likely will produce a harmful explosion with a very intense and hot fire. If left on a charger (usually a cheap china charger) that over charges the cell, it can build up heat until it goes into thermal runaway which will produce the same explosion with the fire again. Protected cells are meant to prevent these hazards. Care has to be taken by the user, when using unprotected cells to make sure none of these events happen. Protected cells take some of burden off the user, not having to worry so much about the dangerous events, even though they should always try to monitor and apply proper usage. Electronics do fail, and so could the protection circuit, not very common but it does happen.
Good info there. Here is another well recommended site for reading.
I’m going to take advantage of this thread (hopefully not hijack it 
) to ask a related question: Will I ruin my protected Panasonic 18650 cells or permanently trip the protection circuit by running the battery down until the protection kicks in?
I ask because I have some older protected XTAR 18700 cells that I once slowly drained in a flashlight on the lowest mode. The protection finally kicked in (dropped to 0 volts), but the charger refused to charge them until I “jump started” them by resetting the protection circuit with pulses from an old cell phone charger, which I didn’t like at all.
No, should not hurt the cell if you don’t leave the cell in this low voltage state for a great length of time.
Some chargers will just refuse to reset the PCB. Most chargers apply a small voltage at the beginning of the charge that usually resets the PCB, some don’t. You can take a charged cell and parallel it with the tripped cell for a second or two (basically applying voltage to the tripped PCB) and it should reset the PCB tripped cell, then the charger will charge the cell.
If a li-ion is left in a low voltage state for too long it can self discharge into a very low voltage state which can damage the cell permanently.
Usually when discharged at a high current rate the protection kicks early because of the greater voltage sag. There is still usable capacity left in the cell at lower discharge rates. The pcb kicks when the set low voltage is detected. The higher the discharge rate the more voltage sag and the lower the voltage the cell can provided. Which leads the pcb to kick sooner.
Ah, so in the case of my XTARs the tripped protection circuit probably didn’t reset because of the charger, not because of the circuit itself? That was with an XTAR WP2 charger. Now I’m using Nitcore Intellichargers (I2 and I4). Any idea if the Intellicharger resets tripped protection circuits? Incidentally, the Intellicharger actually refuses to charge completely drained Eneloops (NiMH obviously, and no PCB), so that I have to use my old cheapo $4 “Omnipotent” charger to give the Eneloops a minimal initial charge and then complete it the rest of the way with the Intellicharger.
Which version do you have SB of the i4? In HKJ’s review, he does say the i4 can reset the pcb on li-ions. I have the the i4 v2. I have yet to try it though.
It’s at the bottom in the heading “Summary”.
What about the converse of this? If I run a light on moonlight mode at 5ma for weeks is it possible that the protection circuit might not notice as the voltage slowly drops and eventually let the cell get dangerously low? Because now that I’m thinking about it, it seems that I once had that problem, again with my XTAR 18700 cells, where the protection circuit wouldn’t kick in when the light was on the lowest mode and it started to get extremely dim, but the circuit was still closed.
As far as I know it’s identical to the pictures in his review. It’s not the older “SysMax” version that he links to.
Don't you dare to do it, or I'll contact Mr. Admin! 
No you won't. That's what the protection circuit is for.
However, in some rare situations, you might be able to ruin your cell. For example running down to protection circuit and forgetting the cell for a year before charging might do this (because of self discharge)
I have also heard of cases where the parasitic drain would have drained the cells very slowly and the undervoltage protection would have not worked. I haven't seen any real evidence of these cases yet..
Maybe your charger isn't good for resetting the protection?
Back on topic:
I use unprotected (only) in case the protected doesn't fit to a light and the light has undervoltage protection. That's "pretty safe", but one day last week I had a close call with unprotected KingKong: I accidentally shorted the light simply by tightening the bezel (don't ask!), tried to turn the light on and wondered why it doesn't work any more.
While wondering, I noticed the light becoming very hot and finally understood what was happening.. So I quickly turned the light off and started unscrewing the battery tube (to avoid possible explosion if the cell would continue venting by itself!). When I got the tube open, the cell was smoking hot (and really smoking!) and the driver side spring was glowing red - hot enough to get unsoldered from driver.. So it dropped out of the light, straight to my carpet, burning a small hole. So I extinguished the carpet with my foot and ran out with smoking cell in the (now open) tube.
So I left the cell out to calm down and started to think what could have happened.. I was literally seconds away from tube explosion and I could have burned my house as well.
So.. What should we learn about this..
Here's what I have gathered so far:
1. Only use protected cells
- protection circuit would have tripped and saved my carpet (and possibly also the driver)
2. Be prepared to the worst
- Think this kind of situations before they happen so that you know how to act when it happens (you have literally only seconds to act!)
3. Use DMM to check light after any modifications
- I would have noticed this if I would have used DMM (Tailcap measurement) -> nothing would have happened (except burnt fuse in DMM)
- This has been my standard procedure with all new lights and after real modifications, but I simply didn't think that tightening something would cause a short.. (Fail!)
So: LFMF (Learn From My Fail) and don't burn your house. Please.
Ah. So I think I might have an anecdotal example of this, in my previous post (#8).
Re: your unprotected cell experience... Wow! Good grief, that was intense. Glad everything turned out OK in the end. Must have been a terribly harrowing experience. This re-affirms my personal preference to only use protected Li-Ions, and to never give a Li-Ion flashlight to an inexperienced user as a gift.
Yes, that can happen. That’s why the protection should not be completely relied upon. Better to still monitor cell voltages even though they are protected. Low current discharges drain the cell down, all the way down to a low voltage state because there is very little voltage sag under a low current discharge. If the pcb kicks at say 2.5v at a very low discharge then the battery voltage is probably very close to 2.5v once the pcb kicks, if it does kick. Left for to long and self discharge will kill the cell in no time. If it doesn’t kill it, it has still sustained irreversible damage. At high current discharges, once the pcb trips, the cell voltage bounces back a great deal, usually well above 3v. Voltage sag is a protected cells friend in this instance.
I see. Thanks for the clear explanation. So for some upcoming Li-Ion flashlight reviews I’m going to be doing, I probably shouldn’t do my usual lowest mode week-long battery runtime test. Maybe I’ll do it in medium mode so the draw is high enough to trip the PCB on time and yet not too much to overheat the light.
They key is to not let the battery stay in a low voltage state. The cell can be taken to the manufactures suggested low voltage discharge. Just don’t leave it there, charge it up some or fully charge it. Monitor the voltage under a low discharge current, pull it when low and charge it up and all should be good.
A bit of a sidetrack, but I have also had my I4 V2 charger refuse to charge a dead AAA Eneloop once. I stuck it back in two or three times after it gave me the error then it started charging, I think that it boosted the voltage just enough in the cell to make it work.
My problem is that several of the highest output cells are unprotected. Efest, Sony, Panasonic, all make unprotected cells that will do from 10 amps up to 50 amps. I hear that they are unprotected, because the protection circuits can't take that much amperage, but I ask myself, Why can't they make a protection circuit that will handle it?
The problem is that during a long runtime test that lasts days or weeks, if the current draw is low enough, it could theoretically drop below the safe voltage (I gather it’s somewhere around 2.5V ?) and still keep going because there’s still plenty of energy to keep the LED running on its lowest mode, albeit unsafely. I’ve basically just accepted the fact that Li-Ion flashlights are mainly useful for short term usage of incredibly powerful light. For the dimmer long-term applications that I more frequently use, a NiMH or alkaline compatible flashlight continues to be a lot more practical and fun because I don’t have to worry about the voltage.
Thanks, I’ll try this next time it happens to me.
I would assume since the charge detects which cell has been inserted, that it see’s the very low voltage or no voltage, thinks it is a tripped pcb then applies a small voltage and reads the cell again. If the voltage doesn’t come up to say 2.5v for a li-ion or .7v or so for a Nimh then it reports a bad cell. Reinserting the cell a couple of times may bring the voltage up enough to make the charger happy. If it doesn’t the cell is probably toast.