@ teacher
I had some spare time today, so I got caught up on the homework reading the information in the links you posted 
There some good information on DMM in those links.
With protected Lithium cell, when the protected circuit trips is the cell render useless or can it be reused?
When a battery is not charging right or to its max voltage, acting weird, is that when you use the refresh setting on the charger?
@ Venom
Excellent, glad you had a bit of free time from the “salt mine”… 
Glad the info helped a bit…. I learned a lot myself getting it together. 
In most cases of tripping to prevent over discharge the protection circuit can be reset with no problem.
Usually the refresh mode is used for older batteries or batteries that have been unused for several months. It kinda “exercises” them & gets them going again.
BTW, I have only ever had one protection circuit activate…… long ago I fell asleep while playing with a light and it tripped sometime during the night. :person_facepalming:
I put it in the charger the next morning & it reactivated right away.
I said all that to say this……… Protection circuits tripping on a regular basis is not something that happens if you are halfway paying attention. Falling asleep while playing with light is zero attention……
@ teacher
Just by putting the battery in the charger it will reset the protection circuit or is there more to it than just that?
@ Venom ……. Just put it in the charger and leave it alone. Give it time to do it’s thing.
Not all chargers will reset protection circuits. Lii 500 will as will most Xtar’s, Lii 100, Lii 202, and OPUS 3100 …. just to name a few I am familiar with.
Most likely it is a function you will rarely ever need or use.
From what I understand you will be more comfortable starting out with Protected LiIon cells . But after you get used to the LiIons and realize for yourself you will most likely NOT blow up if you only use a bit of precaution & common sense…. you will probably drop the protection circuit all together for future cell purchases. I know I did & have not looked back.
From what I understand, protection reset has nothing to do with the charger. The battery’s protection circuitry automatically resets itself when the condition (over-current, over-voltage, etc.) causing the trigger is no longer present.
Agreed. Although some flashlights are designed to only accept button-top batteries, and often times, to get a button-top battery means having to get a protected one.
That has certainly not been my experience when a circuit pops to protect from over discharge. No “auto reset” have I seen.
I hear you, but there are plenty of button top options in unprotected batteries now. Or a simple & easy solder boob job on a flat top will solve the problem also.
If the voltage goes to high or to low the protection circuit will kick on and render the battery useless intil it get within the recommended spec and can be used again.
Over-discharge (undervoltage) condition certainly doesn't resets by itself, if it were it would defeat its purpose. A flow of current in is required for it to reset: setting it on the charger is most common, but can also be done with any sort of higher than protected battery voltage source safely since you just need a very brief burst. For example, using a primary 9V PP3 battery in a 2S pack.
Undervoltage condition can be triggered by a bad cell connection since a bad joint's much higher resistance causes abnormally high voltage drop (V = I × Rjoint).
Cheers 
I’ve tripped dozens of protection circuits. They read 0volts like it should. Breaking the flow of electricity. Once on the charger a few seconds. It shows the real voltage of the battery after the circuit is reset
Thanks for the explanation, guys!
I did play a bit with tripping the over discharge protection here:
http://lygte-info.dk/info/DischargeProtectionTest%20UK.html
I did not see any that reset itself, but I expect it was because I did not wait long enough.
I saw some batteries that was pretty hard to get started again.
Thanks for the link HKJ…. :+1:
Good point Barkuti…… :+1:
I just mentioned the charger reset because it is the only one I have ever used.
I did see a video one time where a guy put the positive of the tripped cell against the positive of a charged cell and then very briefly connected the negatives and it reset the circuit. But I have never tried that
But I have stayed at a Holiday Inn Express………
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EDIT: Yeah, here is a picture of what I described above, taken from HKJ’s link to his review.
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If the charger cannot reset the protection, it can be reset with another battery. It is best to use partial charged battery, but a fully charged can also be used. Connect them as the photo shows. Depending on how much hysteresis the protection has, the needed time can be below a second to a minute or two.
Interesting findings. Fyi, i have a cell similar to the first one you tested, a panasonic 18650B 3400 mAh protected with EVVA sticker from mtnelectronics. It is supposed to trip at 2.5V. I accidentally left it running in a flashlight. When i finally removed it, the resting voltage was 2.51V, so the under load voltage was probably lower. Yet, it never tripped. Wondering if that PCB is working at all…
The below was also copied & pasted from HKJ’s link to review. It seems the trip voltage is 2.0 to 2.5 V.
Conclusion
It looks like 2.0 volt to 2.5 volt is very common for under voltage protection, independent of the specified minimum voltage for the battery.
The protection works both at high and low currents, but on some batteries it is more difficult to reset the protection when it is tripped, especially at low currents.
When the protection has tripped there is very little energy left in the battery.
Thanks, teacher.
I’m actually looking at the description of my battery on mtnelectronics’ website, and it states “protection will kick in at around 2.75v” Hmm…
Sure thing Pete. . Hmmm… 2.75v?? That is strange. It does seem something is out of kilter. 
Its look this thread been a little busy.
Thanks for all the replies!
Does a flashlight have low battery indicator if the battery hits a cetain volt, either under load or at resting?
Some do, some don’t. Some flashlights will also cut power at a certain voltage to protect the cell from over discharging.
For example, I just got a JetBeam R26. The power button will slowly blink when battery is below 50% charge and will quickly blink when battery is below 10%. However, it does not appear to cut power, so if you ignore the blinking and leave it running, there is a risk it could over discharge your battery, unless it’s a protected battery.