At what point is internal resistance too high? Does that depend on the current rating of the cell?
It gets overheated during charging is a pretty good indicator.
I had some used laptop pulls that worked great for years but then all started overheating badly.
All of my batteries are older than this, heck i have some that are almost 10 years old (and still testing at near 100% capacity).
I’d love to have your old cells for some testing and use if they are still good.
I have batteries older than 10 years. Does not mean I would recommend it. 3 years and 80% life is gone. There are external factors that a DMM cannot account for.
If they were $1000 each, I would stretch every minute out of them. But not for $5. I have better things to do than try to bring dead batteries from the dead.
80% life gone?
How is that happening?
It’s all zeros and ones. If you get too many zeros, it reduces the life.
When :
A) They self discharge
B) Refuse to take a charge - ( See C )
C) Voltage sag too much after re charging
D) DO a thermal run away … ( Try to explode ) I think I have had two of those over the last 15 or so years . Usually my fault cos I caused a short .
In English?
My MiBoxer and Vapcell chargers measure the IR and charge accordingly. So no way I can see them overheating if the chargers are just pushing a few Watts. But I do have noted the IR when new and periodically check. If I see a 50% increase there, I re-test and confirm. Then they are tossed in the low bin (just in case I do need a low valued cell for testing a circuit or light). But that scenario hasn’t come up yet with my better OEMs, only those “economical” pulls from miscellaneous power packs.
Yes, most definitely if a cell became very warm or even hot to the touch, that would be an immediate indicator of a dangerous cell ready for recycling. But do all cells do that once they’re worn out?
As an aside, I found this interesting page where the author claims that when cells go into deep discharge and most battery chargers will reject them, there’s a way to possibly recover them. Bring them back up slowly to the voltage threshold (might be like 3.5v) and a charger will then be able to charge back up to maximum capacity. LINK.
Interesting charger but it would just hide this problem.
I would personally use a regular charger at least occasionally so you can spot this problem on old cells. Aim for 0.33-0.5C charging on this charger so a very low charge rate doesn’t hide the problem.
Below 2.5V cells can start forming internal dendrites, the may be fine once recovered for months or years then burst into flames of these dendrites grow and puncture the cell internally.
So once it goes below 2.5V the cell is done, no matter how old or new.
I use an age based replacement system, not a symptom or status based system. Works for me. A battery can one day save your life so I like to leave as little as possible to chance.
As i said i would be interested in testing your used batteries to see the 80% drop, even my heaters were at about 50% capacity.
Would a protected cell make any difference?
?!?
I’ve got ancient laptop-pulls (mostly panny-As), from laptops that were used Every Single Day and probably charged accordingly, and while capacity might be down, they still don’t roast when charging, and have at least half their capacity left.
In low-stress lights, they’re perfectly fine.
I’ll usually charge at 1A, and if the cell doesn’t get too warm, it’s fine in my book.
I have had cells (usually those recycled/rewrapped “free” cells from craplights) that did get quite warm, so nope, they’re gone.
Yeah, I’m kind of at a loss as to how a cell might need to be tortured’n’abused to get that worn down.
Too many zeros. 
Why would I use a cheap charger? And the whole purpose of having a charger measure IR and charge consequently is for hassle-free operation. Those economical chargers seldom do more than 1 amp which is too low for my cells and my patience. I mean charging 4 x 30Qs or VTC6s individually at 1 amp was taking me some 12 to 24 hours. And the trouble of monitoring and changing the cells. That is the whole purpose of having a 4 bay unit to quickly bring these up.
And as my collection grew, I didn’t want to spend time charging the various cells I used in the day so I got a second charger. Again, for simplicity and speed.
Are you implying a ⅓C charge rate (1 amp w/3000mAh) can heat a worn-out cell? Or the fact the charger changes the rate of charge if the cell heats up. But the latter I didn’t mention. Geez, I’ve been charging at 2 to 3 amps (auto selection) and if I was to see less than 1 amp, I’d check the IR.
I’m not trying to extend battery life. A cell is to be to my expectations and when they show fatigue, they’re replaced.
BTW this is in the context of the 18650s, the 21700s, and the 26650s. My smaller cells auto-charge rate. I expected them to not last as long in cycles being less capacity. But my Sanyos (14500s) are doing fine after 2 years at about 2 recharges per week. In 2 more years will see if capacity is lost.
I get it you may. have the time and inclination to pamper your cells. My working lights have to be dependable and I don’t skim on the batteries. There is an underlying psychology of turning the light on/off from one peak to another, mentally conditioned to extend battery life. The introduction of rechargeable batteries with high capacity and no memory effect should alter this behaviour. I turn the light on and when temporarily not in need, lay or cover the face. I don’t play with the switch. If the light becomes warm, tone down the intensity. I work with gloves and 50ºC is acceptable. If the light gives a low battery signal, I replace it or pull out another light. Always have a backup.
If the protection cuts the cell output before its dangerous then it should be fine.
Though i would toss it on the charger quickly becasue keeping it empty for long periods is not good for it (according to common wisdom).
A 4 or 8 bay charger is not pricey these days.
Oh yeah.
If your cells are not overheating at 2-3 amp then it should not overheat either at 1A.
To each their own.
If you have money to spend then you can push your cells as hard as you wish and replace when the time comes.
Though please recycle your old batteries.