I have two flashlights that take single 18650 cells. I bought protected cells years ago and a soshine two cell charger. I’ve charged these batteries many times through the years. I’ve never had an issue. This weekend I have had some concerning issues and I’m u sure if the batteries are any good. I left two cells in the charger last night and when I went to get them today one showed green and one still showed red. I removed and reinstalled the red one and the light was green. It was also a little warm. I figured the cell was making a bad connection. So I stick the next two cells in for charging and a few hours later checked them. They were the same way. When I removed the one showing red and reinstalled it, it also turned green. And it was the same side of the charger that was doing it before. But the scary thing is that the one showing red this time was burning hot. I mean I couldn’t hang onto it. I set it in the plastic battery case and an hour later it is still really warm. These are all protected cells and I made sure to buy good quality cells years ago. Is my charger junk or do I have a few cells with faulty protection circuits? It was so hot it acted way overcharged but it technically should have shut off once it hit full charge so I am confused and concerned. I just need to know if I need to throw something away and buy new and I also don’t know if this hot battery is any good anymore? Not sure if it is safe to put in my flashlight. Would hate for it to explode once I turned the light on.
Hard to tell without more data. You should probably get a cheap digital multimeter so you can measure the voltage of your cells - it’s always a good idea to double check voltage on LiIons occasionally.
Sounds to me like your charger is toast though.
-never charge unattended, esp. not over night. things can always go wrong, even with a new quality brand cell
-it is probably a good idea to discard the cell that got really hot, it might have been overcharged and damaged. discharge it slowly in an old light, outside of your home if possible, before trashing it
-LiIon cells do degrade slowly but constantly over time, you might want to buy some new ones soon
Thanks. I was under the impression that if protected cells were used it was no different than leaving your phone on the charger all night.
In theory that’s correct … but weird things can always happen.
Also protected cells add more possible points of failure, they can protect against overcharge/discharge etc. but they can also fail in ways unprotected cells cannot.
An unattended overnight charging resulted in a too-hot-to-handle battery?
Please don’t do that ever again. Somebody up there really likes you. A lot!
A protected battery means less risk. Less risk is not the same as riskless!
As Valynor points out: a protected cell does not always mean: more safety.
Cells are like cars. The older they get, the bigger the failure rate.
No a few hours today did. After two and a half hours these cells are still warm
That’s definitely not normal. They have very little thermal mass and should cool down to room temp in 15-30 minutes. If they’re still warm they still generate heat … sounds like an internal short perhaps. Discard the cells, and don’t keep them in your home or somewhere they could start a fire until you do … just to be safe.
Which charger?
But anyway, the battery behaving anomalously is probably effed, I would not try to use nor charge it again – better to discard it safely and get a new one.
I can’t recall. It’s a soshine 2 cell charger
Can’t you just look at the charger and find out what charger it is?
Do you not have access the charger anymore? 
Yes but I’m not home now
Not a really reputable brand. It’s more than possible that it’s been slowly effing up your batteries. I would get a new one from a known-good brand and model along with the battery you’re replacing.
Coincidentally there are two active threads here on BLF talking about good chargers:
Good luck!
That would make sense. These batteries haven’t been used hard through the years. They should have hundreds if not thousands of charges left in them.
Li-Ions are usually specced for about 500 charge-discharge cycles, so unless yours are something special like LiFePO4, “thousands” should definitely be out of the picture…
- Get a charger with voltage read out, and/or a digital multimeter.
- Never charge Li-ion batteries at night whilst you are sleeping.
So, my opinion:
Get a decent multimeter. Do not buy cheap stuff, it can be inaccurate, and with li-ion batteries you need fairly high accuracy (something like 0.05v difference can be important). This is generally useful thing to have anyway and will last for a long time.
Do not discard cells right away. Check voltages, see if the cells are self discharging fast or are overcharged, only then decide what to do. If nothing else just for the sake of curiosity.
Check what the charger does. Make sure it stays within safe voltage and stops charging correctly. Potentially - just get a better charger.
Also… “thousands of charges”? Not going to happen. 300-500 from new would be reasonable. With standard removable cells like 18650 it makes very little sense to use them all the way to failure. They are very cheap and become less stable/safe as they age, once capacity loss becomes noticeable it makes sense to simply replace them.
I’ll take all this info into my arsenal. I work in the trades and own two Fluke meters. I’ll start checking cells with my meter and I will get a better charger. Also I’m guessing I’ve charged these cells probably less than 20 times since I’ve owned them. The lights they were bought for were used for riding my bike at nights and that wasn’t something I did regularly. We took the family on a ghost hunt tonight so I was charging all my batteries to get my lights ready.
What is it about lithium ion batteries like my Dewalt and Milwaukee tools rhat I can put them in the charger and walk away and they are ok but these protected 18650 cells you shouldn’t?
I actually wouldn’t leave power tool batteries charging overnight either. Some of the chargers have protection that’s supposed to prevent bad stuff from happening, but like has been mentioned, it’s better to be safe than sorry. I always practice unplugging all my li-ions as soon as they’re full.
Technically you should not leave any li-ion battery charging unattended. Be it loose cells, power tool batteries, phone, laptop or whatever. Any of them can catch fire and if you google for example “milwaukee battery fire” or may be “explosion” instead of “fire” you’ll surely find videos of it happening.
It is not always practically feasible though, but you should still understand that each time you do it you take certain risks.
Then there is the fact that power tool batteries usually use high quality cells and have multiple levels of protection. Phones are similar. They often use much more advanced protection circuits too, for example preventing user from shooting themselves in a foot by disabling charging when the battery is too cold/hot.
Typical protection circuit in 18650 would prevent over/under voltage and short circuit, that’s it. And unlike phone or power tool pack which have built-in chargers you are using a charger of unknown quality too, so not 100% sure that it will not do something bad.
Another question then - were they overdischarged by any chance at some point? If you store li-ion battery for a long time, especially protected one since protection circuit has parasitic drain, it can be overdischarged and once it gets below ~2.5v it is no longer safe to use and is basically ruined.
I wasn’t aware that the protection circuit created a parasitic discharge. I did not know this. Some of these batteries have been sitting on my bench for several years. I keep my batteries fully charged prior to storage and I was not aware that they would discharge like that. I typically would take them and put them on my charger to top them off if they have been sitting a while, but I never knew they would drain down to that point. It’s possible that’s what happened.