Accidently overcharged my cells :(

[MarioJP]

It started when I realized that I did not use my mobile charger because normally my phone would be dead by the time i got home but this time i had available juice left and used my home phone charger to charge my phone instead so I don't have to worry about charging these cells and be ready for the next day. EXCEPT that I totally forgot and I took the batters out of the mobile charger put it in the charger and set the current charge rate without checking twice if the display would say full.

Short after during charging I started to hear hissing sounds and that's when it hit me "oh crap" I immediately reset the charger set it in a full discharge mode. Hopefully this will stop cells from becoming damage any further to what already has happen.

It did not hiss a lot but the internal pressure is what is worrying and hopefully it does not develop a high self discharge in the long run. Any hope, or good news from all of this??. Did i save it in time from becoming too damaged. I know there is some damage but hoping its not badly damaged. These are my ansmann 2850

 

[Mr Happy]

I'm having trouble following what you said. Apparently you took some AA NiMH cells (Ansmann 2850) that were already full, or nearly full, and you put them on to charge (on what charger, at what charge rate?). And instead of stopping, the charger overcharged them so they vented?

 

[MarioJP]

no I did stop the charger and immediately set it to do a full discharge. I am using la crosse charger bc 9009. It was charging at 1A.

Problem was that for some reason I just thought I have used these cells and were discharged already.

That's the problem.

 

[Mr Happy]

Even if you put full cells on to charge, the charger should have stopped within 5 or 10 minutes before the cells vented. The problem is with the charger. How long were the cells charging before you noticed something was wrong?

 

[MarioJP]

10-15 minutes or so. I am trying to figure what happened here???. Normally when the cell is fully charged the charger should terminate before venting happens. In any case I just hope these cells are still usable still and for the long run as well. I am currently discharging the cells to 0.9v because of this.

 

[Justintoxicated]

try them out, but i suspect that they will never be the same. If they are good enough mark them with a sharpie and keep them as backups.

 

[MarioJP]

Only time will tell i did a good discharge on these batteries first phase was at 500ma discharge rate until the voltage hit to 0.90. Second phase was I reset the charger and did a 100ma discharge on all four of these cells.

I read somewhere in these forums that overcharging can lead to rapid crystal growth.

Hopefully these cells are still good to keep using after what has happened.

Nimh has to have some sort of tolerance hopefully?.

 

[Mr Happy]

I don't suppose that discharging would make any real difference to how badly the cells were harmed. You can't "undo" the effects of overcharging by discharging. If it happens again the best thing to do is just to remove the cells from the charger and let them cool down for several hours.

If the cells were hissing it means they were venting, and that is irreversible. Any damage done is done.

But really, the charger has been shown to have a weakness here. The charger should have noticed the cells were already charged and stopped charging them before they started venting. In fact, this is one argument for charging at lower rates. Venting and overheating are less likely at 500 mA than at 1000 mA.

 

[MarioJP]

will see the outcome if this would affect the capacity and how the cell performs in the long run. I would not be too worried if I did not notice any degradation of the cells.

Strange about the temps on the cell it was not overheating. So i am guessing it was by 10-15min overcharge after I put them in the charger.

Lucky that I was able to notice before it really started to leak. I inspect the cells and the charger and did not see any chemical spills.

About the safety vents on nimh cells. does the vents closes after the excess gas has been released??.

I should of checked the display before I pressed anything first because when fully charged cells is placed in the charger. The charger will automatically display "full" and would go into trickle charge right away. BUT you have a 5 second window to override before it locks. So half of it I take part of the blame because I did not check the display and I assume that the batteries were in a discharged state.

 

[qwertyydude]

Don't obsess with crystal growth. Crystal growth is a very rare occurence and is usually the result of leaving batteries alone. If your batteries hissed that's not the sound of crystals growing, it's the sound of the battery losing electrolyte. Losing electrolyte means increasing internal resistance of the cell so you lose some discharge capability, maybe some capacity. But I've had it happen before with some nimh. Usually if the battery isn't burning hot to the touch after you hear some slight hissing you haven't done too much battery damage. Just run a capacity test and if it's good keep using it with the knowledge that if used in series with a pack that the cell may go bad eventually as the weak cell in a pack is usually the first to get reverse charged and die an early death. And why does your phone use nimh batteries?

 

[MarioJP]

My phone doesn't. It is my mobile booster or charger that I use to charge my
phone does.

Now this is not making any sense anymore. I nomally charge these cells at 1.8A
charging rate. I had no problems whatsoever.

Problem happened when I started to charge these cells at 1A. That is also the
same charging rate I used when I accidentally charged my full cells too.

Now what is not making sense now is that when I charged these cells at 1A I hear
minor popping noises when nearing its charging cycle. The current input readings
on the display was only around 2700mah went into these cells before I pull the
plug

Why are they making popping sounds now???

The only thing changed was that I started to charge these cells at 1 Amp.

Starting to wonder that was a bad ideal to charge below 1.8A. After all the
label does say "fast rechargeable"

Second reason is because it is now winter and it gets cold arond here 34 degrees
to be exact. I was thinking since these cells gets very cold, I should start
charging them less than 1.8A. Not even sure if I should try at 500ma. Going to go back to 1.8A charging rate.

 

[PeAK]

I'm having trouble following what you said...

This is what I think he meant to say. Additions in red and deletions in silver:

"It started when I realized that I did not use my AA powered mobile charger (because normally my phone would be dead by the time i got home but this time i had available juice left and used my home phone charger to charge my phone instead so I don't have to worry about charging these cells and be ready for the next day)to charge my cellphone. (EXCEPT that) I totally forgot about not needing the mobile charger and I took the fully charged AA batterys out of the mobile charger and put them in the home Lacrosse charger and set the current charge rate without checking twice if the display would say full.

Short after during charging I started to hear hissing sounds and that's when it hit me "oh crap" I immediately reset the charger set it in a full discharge mode. Hopefully this will stop cells from becoming damage any further to what already has happen.

It did not hiss a lot but the internal pressure is what is worrying and hopefully it does not develop a high self discharge in the long run. Any hope, or good news from all of this??. Did i save it in time from becoming too damaged? I know there is some damage but hoping its not badly damaged. These are my ansmann 2850 "
​

In another thread, there was talk that battery chargers need some sort of "null zone". This is a a time where the charger put out current and ignores termination signals. When it comes out of this phase, it assumes the battery is NOT fully charged and that the signal to stop the charge is about to come. The problem in your case is that it already passed and your charger will never see "the signal" and overcharge your batteries.

Once batteries are overheated/misused, the signal to stop charging weakens and is more likely to result in future overcharging...like a bad recurring nightmare.
One way to improve the "stop signal" is to increase the charge current but this also has the liability of an "extremely bad nightmare" should the signal still miss and the result will be crazy, crazy, crazy hot batteries. Can you say the words ABUSE.

 

[qwertyydude]

One other thing about nimh is that too low a charge rate will actually prevent the voltage drop from occuring. The higher the charge rate the more pronounced the voltage drop at the end. It tends to be also batteries that have constantly seen a 1C charge rate will tend to "get used" to that 1C charge rate and not properly show a voltage drop at lower charge rates, I've noticed that in my cheaper cells after a couple hundred recharges.

 

[MarioJP]

One other thing about nimh is that too low a charge rate will actually prevent the voltage drop from occuring. The higher the charge rate the more pronounced the voltage drop at the end. It tends to be also batteries that have constantly seen a 1C charge rate will tend to "get used" to that 1C charge rate and not properly show a voltage drop at lower charge rates, I've noticed that in my cheaper cells after a couple hundred recharges.

Even at 1A is not enough for the charger to terminate the charge?

 

[qwertyydude]

For me it depends on the condition of the cell but yes some of my older or cheaper cells will only show a proper voltage drop at 1C which is 2 amps. If not I notice that the batteries will actually overcharge more and get hotter at .5C than at 1C charge rates.

 

[MarioJP]

Seems like the hissing and popping has stopped after 3 cycles. Yet I did not change anything on the setting kept it charging at 1A. Pretty strange how this happened.

And these are new cells not that old either.

 

[Turbo DV8]

... battery chargers need some sort of "null zone". This is a a time where the charger put out current and ignores termination signals. When it comes out of this phase, it assumes the battery is NOT fully charged and that the signal to stop the charge is about to come. The problem in your case is that it already passed and your charger will never see "the signal" and overcharge your batteries.

Maybe it is due, in part, to the characteristic of a feshly charged cell, as opposed to solely a charger issue. How about this? A charger needs to see a rise in voltage before a fall to assume it is the negative delta V? If the cell is so fully charged that it does not present a substantial rise in voltage followed by a fall, the charger will not terminate. The ability of any given fully charged cell and/or charger to charge higher than the fully charged cell's starting voltage could be influenced not only by the cells impedance, but also by the limits of the charger's power supply. Just throwing thoughts out there...

 

[SilverFox]

Hello MarioJP,

Mark the cells that have given you issues, then continue to use them. I would guess that they will be OK for awhile, but will take the journey to becoming crap cells faster than your other cells.

There are times when the vents on NiMh cells don't work properly. It sounds like this may be what is going on with your cells. Each pop you hear indicates that a little of the electrolyte has escaped. Once the vent seals again, the popping should stop, but the damage to the cells is irreversible.

Tom

 

[MarioJP]

are you saying that the vents on one of the cells is not working properly?. I have not figured out which cell is doing the hissing and popping as they are 4 cells charging at the same time. Plus when the cells were hissing i immediately just stop the charge. I just hope the cells don't explode.