Should I throw out batteries that had been discharged down to 2.0v?

So I left my battery in my flashlight for quite sometime and didn’t took them out. The light won’t turn on so I put the battery in the charger and check for them. They were reading 2.0~2.1v but they jump back up to 2.9v within 20~30 seconds of charging ? What do you guys think about this situation? Should I just go be safe than sorry and just throw them out? is there a safety feature where battery will just go dead but will come right back to alive when charging? it’s been not even 5 mins and they are reading 3.3v. as I am editing this post

They’re Sony VTC6.

I’ve read about people reviving over discharged batteries, but I personally would not take the risk.

It was probably 3.5v when I last used the light and they just sit there for so long (I even unscrew the cap on the light ).
They all just come right back up soon as I plugged them into power

Hold on. You unscrewed the tail cap, so you’re sure parasitic drain is zero? And how long have you not used the light? If you look at a discharge curve, the voltage takes a nose dive below something like 3.0V. This means if your resting voltage is something like 2.0V, then it also takes very little time to recover to something like 2.9V. I’ve had it once with an AW cell, although it wasn’t discharged to that low level.

I would say about 3 months. It comes back to alive within seconds. I was watching my charger all time make sure it isn’t smoking or anything lol. It goes up to 2.3 right after I plugged it in and back up to almost 2.9 in 30 seconds. And now it’s charging up steadily and slowly to 3.4 as I’m typing.

It’s possible that the battery has discharged from something like 3.5V to 2.0V in three months due to parasitic drain. But you said you unscrewed the tailcap. It sounds very unlikely that it’s self discharge. What light was the VTC6 in?

It was in Astrolux MF01. I found that thing drains battery pretty quickly. Is there anyway I can check if battery are still good or not? My charger(Nitrcore) says channel status is good but idk if it means anything or not

They should be okay just he careful with them. Some high drain cells are made to be drained to around 2.5v. The reason the voltage jumps up so quick is there is no or low capacity below around 2.5v-3v

I’ve got the MF01 too. I also have the feeling that below certain voltage parasitic drain is fairly high. As a matter of fact I have one next to me right now unscrewed as well which I normally don’t do. The odd thing is that we are told not to discharge below 2.5V, but pretty much any (good quality) charger will charge over discharged cells without problem or warning.
I think you’re safe, but I see no reason to take unnecessary risks. Hopefully other people will chime in. Otherwise you might want to contact HKJ and ask for advise.

Charge them and use them. I did that many times. Except dead by age/cycles batteries should work.

I would charge them up and then let them rest for awhile. If they hold their charge at around 4 volts, they should be fine. But that’s just my opinion.

I had a similar concern with some laptop batteries that had very low voltage, around 2 volts if I remember correctly. I charged them up and they’ve been fine since then. They’re only around 1,900 mAh, but hold their voltage at around 4.1 volts.

But if you are really concerned about them, then just recycle them for piece of mind.

Regarding a safety feature, protected batteries usually turn off at around 2.75 volts so it appears yours are not protected.

Out of curiosity, what flashlight were the batteries in?

It was the MF01

If the cells are damaged internal resistance will be high. That is one way to tell.

If you have an analysing charger (Opus, Miboxer, Zanflare, etc.), you can do more tests, but in general, I don’t worry about “dead” cells.

I’ve gotten laptop pulls that’ve been sitting unused for years, some cells bricked to 0V and not taking any charge at all, but the other cells revived just fine. I still use them in my EDC light and others (headlamp, etc.) where I can keep an eye on them, but haven’t had any issues yet.

Especially after being in service for so many years (old panny-As!), they might have only 2/3 the capacity as when new, but they’re still chugging along in low-stress lights.

Unless you have the required equipment to charge them very slowly back to 3.0V(50-100mA) and they weren’t discharged that low for very long, I wouldn’t try it.

However, if that 2.0V was only for a brief period of time, then you recharged then, then it shouldn’t be a problem.

The parasitic drain in the Astrolux MF01 must be pretty high to drain 4 batteries in about 3 months. Are you able to measure it?

My Meteor M43 with a lighted switch seems to take forever to noticeably drain the 4 30Q batteries. I don’t use it that much, but it will sit on the shelf for months at a time with the switch staying at blue (meaning fully charged).

I know the M43’s drain value with button light off is ~20 μA (~80 years). With the back lit button on it still is only ~80 μA (~20 years).

I haven’t measured that of the MF01.

I have personal seen batteries that measured well below what most would consider “throw them away batteries”. These were never used that I know of but the seemed as they had just rolled off the factory line.
I would watch them very close, see if they heat up while being charged or in use. I wouldn’t be so crazy about using them in series either or asking the cell for high current. One bad cell can make a very bad situation. Parallel should be fine or single cell use. I would continue to use them but I would keep a sharp eye on them. As soon as your batteries are depleted, give them a charge even if its only a partial charge. Never leave them in a stressed depleted state.
More info here……

These battery never seen anything below 3.5v since I bought them. I’ve been using my D4 all the time since it’s small and bright so I forgot about the big guns lol. I’ll see if they can hold charges and if they get too hot or not. But yea it’s crazy how fast it got drained

I agree with sp5it. They will be fine.