Self-healing miracle battery.

So I got a couple of unprotected Sanyo UR18650ZTs from FastTech the other day and I noticed that one or two of them were heating up significantly when charging, and wouldn't charge up to 4.2V, or even 4.0V for that matter, so I quickly ripped them out of the charger and put them aside.

But aside wasn't aside enough, and I'm always super lazy, so I must have accidentally put them in the charger again and then put them away when they were fully charged. I noticed that my dodgy batteries had disappeared into my battery storage area and quickly went to test all the batteries of the same type. All of them tested the full 4.20V.

So I discharged all of them down to around 3.3V, all with very consistent run times, and then recharged each one monitoring temperature very carefully to single out the dodgy ones. None of them heated up a single bit, all full to 4.20V.

I definitely don't condone trying to "fix" bad batteries, but has anyone had this happen to them before?

If this were to happen to me again, I'll definitely instantly chuck out the bad ones still. I'm just glad my house is still standing.

A cell would heat up when recharging because it has a high internal resistance. From what I understand, it’s not a property that comes and goes. With a high internal resistance, it would also heat up if discharged at a high enough current. What’s the charge current of the charger? When you discharged the cells, how did you do that? Can you monitor temperature then too? Do you have any hobby charger than can measure internal resistance?

Something has changed - and I’m betting it’s not the cells. Perhaps something to do with the charger?

Perhaps you’re a relative to my boss. He likes to put bad parts in his “magic closet” and few weeks later they are “brand new” :open_mouth:

It's just a standard Nitecore Intellicharger 4 V2 750mA max.
Discharged cells with a 3.04A P60 drop-in.
Nope no temp monitoring, but when I took them out they were warm to the touch. Not uncomfortable.
No hobby charger, but using the formula floating around with a parallel resistor, internal resistance is ~23-28mOhms for all of them. Seem perfectly fine?


i have actually had the same thing happen sort of. they are old laptop pulls. cells dated to 2006 if i remember correctly.
every time i charged them, they got hot. so i started charging them last weekend and disposed of the ones that got hot. only got around to charging a pair. now this weekend i charged all my new pulls from the B&D pack, slightly warmer than room temp. then i got around to the other 4 laptop pulls… warm to the touch…?
i know it was more than just the two getting hot on the charger as i rotate my batteries…
marked all my new pulls 1-10, and marked the old pulls 1x-4x so i can better monitor them at a later date. i can already tell that at least 3 of the 4 are going to be recycled, just for the fact that i charged them last night, and only one is holding at 4.12v so far… yes, my charger has recently decided i dont need fully charged cells… it is consistent at least :stuck_out_tongue:


Possibly something to do with the temperature of the cell before you put it on the charger. Was it possible that the cell was very hot or very cold before putting it on? The only time I have had problems of this sort is after I have received them in the mail from someplace where the cells were allowed to freeze. I have honestly considered only buying cells in the spring and fall so they ship in more mild temps.

Recently I received two trustfire flames 18650’s and one wouldn’t charge at all. I remembered that the cells had been out in a freezing mailbox. I let them sit a day and tried again. I have had no problem with them since then. This isn’t the first time either.

A change in temperature would change the resistance of the battery, wouldn’t it?

Try it.
Leave a battery in the fridge at 35F to 38F or the freezer at 0F to 10F for a half hour and check the discharging or charging Rinternal, then repeat for 95F with the battery being brought up to temp by placing in a plastic bag and submersed in water that is slowly being heated. The water transfers heat better than air so you don’t need to wait a half hour with this method.
Apparently batteries being discharged while in use get hotter than this.

The built-in intelligence of the charger and of the batteries can interfere with you knowing what is going on on the volts, amps and ohms level.

LiIon cells dont really care about temperature, unless you try to charge them. Never charge below 0°C/32°F, some cells even higher.


When I got my work van it was stocked with parts from the last guy. I would find a box with a circuit board in it and it would say on the box “good ?”. Or “maybe works”.

The previous employee had a bad habit of trying to save used parts and not labeling them. I have spent the last year finding every part that is used or not working and throwing them away and ordering replacements. After putting a few bad boards on, I decided anything that is opened or doesn’t look 100% brand new gets tossed.

He probably had a system but when he was gone, I was left with a nit mare of used parts. To make matters worse, he had a habit of opening up all his parts to check they were what he ordered so everything was opened. It was impossible to tell when a board was good or bad in some cases. He would also save used ice cube relays!