Fluctuating voltage of NiMHs

[shobhitk]

Hello everyone,
recently i was trying to wake my 'storage box queens' (4 NiMH GP 2700 mah, non-lsd) which had been into storage for about 9 months without a recharge. when i first started with 0.5C charge followed by a 0.2C discharge on my C9000 the results were quite bad-

Cell 1- 800mah
Cell 2- 180mah
Cell 3- 230mah
Cell 4- 1100mah

also the voltage of the 2nd cell showed as 0.23v i was quite worried so i decided to recover them using the method told in this thread. as it was a 'stubborn case' i initially went on with some fast charge and slow discharge cycles(2 cycles) and then lastly discharged them 3 times at 0.1amps to deal with crystalline formations.
after each cycle the cells improved greatly! after gradual improvement, all of them reached at about 1850mah and i have yet to do a Break-In on them, then i suppose they would touch 2000 mah which would be 'new like' performance as described in the above article.

now my question is that while performing fast charging and slow discharging i saw that almost all cells at some point of time showed fluctuations while discharging. like one cell behaved like this --> 1.04v, 1.03v,1.04v and so on. the other behaved similarly --> 0.96v, 0.97v, 0.96v and so on. i would like to know what actually is happening here. has it something to do with the breaking of crystalline formations?

also i was trying to diagnose the problem so that i can follow the correct procedure for its recovery. as i am new to battery charging and all, so kindly let me know whether the above behavior shows the normal 'voltage depression' problem witnessed after long storage of NiMHs or is it something else?

 

[Curious_character]

I don't think there's any significance to the fluctuations, as long as they're only one or two steps of the least significant digit. That could easily be due to a small amount of internal electrical noise causing dithering when the measuring circuit is right on the edge between the two values.

I don't know if you checked the capacity put into the cells when charging. I've seen low capacity on stored cells which was due to premature charge termination. Not much charge was put into the cells, so not much came out. This is usually corrected by doing a break-in cycle or a number of standard charge-discharge cycles. I've also seen cases with new or stored cells (particularly with Maha chargers) where the charger misses the termination altogether, so it's a good idea to keep an eye on things while charging and manually pull the cells if the charger is still going after the cells have gotten a normal charge. This also usually goes away after a few cycles.

c_c

 

[shobhitk]

i had read somewhere in the CPF archives that fluctaions may be related with crystal formations, but it was an ongoing debate.
yes, i had checked the charge put in during charging. they were very low and this happened because of, i think, high internal resistance, as the voltage of the cell quickly started rising and within 20 mins. it reached from 1.2v to 1.47v, the peak voltage for C9000.

so should i suppose that it is 'voltage depression' and the process i have adopted for their recovery is correct?

 

[45/70]

now my question is that while performing fast charging and slow discharging i saw that almost all cells at some point of time showed fluctuations while discharging. like one cell behaved like this --> 1.04v, 1.03v,1.04v and so on. the other behaved similarly --> 0.96v, 0.97v, 0.96v and so on. i would like to know what actually is happening here. has it something to do with the breaking of crystalline formations?

Yes, it is possible to see the voltage of a cell fluctuate like this and I think that is likely what you are seeing. SilverFox told me once that if you use an incandescent bulb, instead of using a C9000, or a CBA as I did, that the bulb will brighten and dim, back and forth as the formations break up. In my experience, you only see this when the voltage is very low, towards the end of the slow discharge.

Sounds like you're on the right track. Keep in mind that a lot of times, you can recover cells suffering from voltage depression, but sometimes they revert back to being problem cells after a while. It's definetly worth a try though.

Dave

 

[core]

0.96v, 0.97v, 0.96v and so on. i would like to know what actually is happening here. has it something to do with the breaking of crystalline formations?

As Curious_character said, this small of a fluctuation could easily just be due to measurement.

But with that said, I have seen much larger fluctuations on my storage queenies which are almost definitely due to large crystal formations breaking down. As 45/70 pointed out, you will in fact see an incan bulb change in brightness. This is a better way to handle it anyway, as stopping your discharge at 0.90V on a voltage depressed cell probably isn't doing what you want.

Oh, and one last thing: If the cell is sitting there at 0.96V long enough for me to notice such things and fret over them, I already know there is voltage depression. Without the graphs even running.

 

[Lynx_Arc]

now my question is that while performing fast charging and slow discharging i saw that almost all cells at some point of time showed fluctuations while discharging. like one cell behaved like this --> 1.04v, 1.03v,1.04v and so on. the other behaved similarly --> 0.96v, 0.97v, 0.96v and so on. i would like to know what actually is happening here. has it something to do with the breaking of crystalline formations?

it is quite possible the digital register on voltage has a hidden digit beyond what is shown like 0.964 and 0.965 and most of the time this hidden digit could fluctuate slightly up and down and you wouldn't know it until it borders a 5 and the is subject to being rounded up to the next digit. Thus a 0.964 would be displayed as 0.96 and 0.965 would be displayed as 0.97. Likewise a 1.034/1.035 slight wobble could make for a reading of 1.03/1.04 so you see a change of 0.001v and roundoff could be the cause of this.

 

[shobhitk]

Hi Dave,
good to see u yeah i had read that incandescent method of SilverFox. but i dont have that setup with me right now, so i think i'll have to stick with my charger for the time being. to achieve similar effect i am discharging it 3 times down to 0.9v thru my charger, i hope it works.

hi core. you r right but i am a bit more careful here as these r NiMHs and they r quite sensitive to deep discharge and not as robust as NiCads which can be taken down to 0 volts. incandescent bulb technique may discharge the NiMHs to a point of no return... and they may get damaged beyond repair...so am a bit cautious... but thanks for the suggestion.

and yes Lynx_Arc that might be the case...

but am happy that after 2-3 cycles they have recovered to 1800mah and now am running break-in on them...am waiting to see the results...

 

[TakeTheActive]

...recently i was trying to wake my 'storage box queens' (4 NiMH GP 2700 mah, non-lsd) which had been into storage for about 9 months without a recharge...

...but am happy that after 2-3 cycles they have recovered to 1800mah and now am running break-in on them...am waiting to see the results...

Before you get too excited, remember that crystal growth on 'High-Capacity' (aka 'Thinner Separator') non-LSD cells has the potential to poke permanent holes in the separator. :mecry:

...i am a bit more careful here as these r NiMHs and they r quite sensitive to deep discharge and not as robust as NiCads which can be taken down to 0 volts. incandescent bulb technique may discharge the NiMHs to a point of no return... and they may get damaged beyond repair...

If you're not already, regarding 'Deep Discharge of NiMH Cells', you may be interested in reading:

Interesting... but not recommended...​

Not the best title for future SEARCHes, as I pointed out several days ago...

 

[core]

Yes, what he said. That was an interesting experiment indeed, although that was with LSD.

I'd just like to add though, that at no point did I say to drain it down to 0.0V here. But if under load it's still in the flat portion of the discharge curve at 0.90V then you obviously haven't discharged it. If the internal resistance is that high then that necessarily shifts all of your numbers.

 

[TakeTheActive]

Yes, what he said.

You sound like my wife - she will regularly mention SEVERAL females by name during a conversation and then suddenly switch to SHE and expect ME to know which one she's now referring to... :sigh:
(i.e. QUOTEs are useful when replying...)

...That was an interesting experiment indeed, although that was with LSD.

I'd just like to add though, that at no point did I say to drain it down to 0.0V here...

Those ignored 'Storage Box Queens' are all probably *CRAP* now (NOT to say that they necessarily should be recycled - they're still useful for remotes, thermometers, clocks, etc...), so, LSD or non-LSD, 0.9VDC or 0.0VDC, an 'Experiment' isn't going to cost much.

I've been discharging non-VIBRANT cells way below 0.9VDC with single cell LED and incandescent lights for years now with no noticeable ill effects and sometimes some positive effects.

... But if under load it's still in the flat portion of the discharge curve at 0.90V then you obviously haven't discharged it. If the internal resistance is that high then that necessarily shifts all of your numbers.

Please explain to the average 'Smart Charger/Analyzer' owner (i.e. without voltage-logging capability) how they can determine if their cells are "...still in the flat portion of the discharge curve at 0.90V...".

Thanks!

 

[core]

You sound like my wife
(i.e. QUOTEs are useful when replying...)

Hey now, I was just trying to be cute. I see that I must have succeeded, if you are now comparing me with your wife!

Please explain to the average 'Smart Charger/Analyzer' owner (i.e. without voltage-logging capability) how they can determine if their cells are "...still in the flat portion of the discharge curve at 0.90V...".Thanks!

It's not that hard to get a rough and dirty estimate. Even at a slow 100mA discharge rate, a healthy cell which has been drained to that point will go from 0.92V to 0.90V quickly enough that if you go get some coffee you will miss it. (And if you're using a LaCrosse you may use some 4 letter words if you wanted it left discharged.)

But if it just hovers there forever, that should be a pretty good clue.

The other alternative is to go buy some better equipment? Never anything wrong with that.

 

[TorchBoy]

It's not that hard to get a rough and dirty estimate. Even at a slow 100mA discharge rate, a healthy cell which has been drained to that point will go from 0.92V to 0.90V quickly enough that if you go get some coffee you will miss it.

Effectively, rough differentiation by coffee. Excellent!

 

[shobhitk]

hi TakeTheActive.
you were right, my break-in cycle results were extremely BAD! :shakehead initially their internal resistance showed at around 1.61volts on an average and after the break-in, all of them were at around 1.88volts , one of them even touched 1.99v! they had also become quite hot! and their after break-in capacities were around 600mah only!! what exactly happened here??

that article on deep discharge was quite informative, thanks!
many questions pop in my mind now...pls guide-

first, i dont have a single cell LED or incandescent bulb discharge setup. i only have multi cell incandescent bulb setup. in that case how should i discharge them below 0.9 volts without reverse charging them? and how would i measure the voltage in this setup and to what voltage can i take it down?

secondly, whats the role of shorting + and - terminals with a wire? is it the same as discharging it in a single cell incandescent bulb setup? is it safe? how is it helpful?

thanks

 

[shobhitk]

bump...

 

[Mr Happy]

my break-in cycle results were extremely BAD! :shakehead initially their internal resistance showed at around 1.61volts on an average and after the break-in, all of them were at around 1.88volts , one of them even touched 1.99v! they had also become quite hot! and their after break-in capacities were around 600mah only!! what exactly happened here??

It's possible that your break-in cycle "overcooked" the cells.

We have to remember that a break-in cycle overcharges the cells and overcharging can be harmful. The break-in cycle is only safe when applied within the operating limits of the cell.

For instance, your first tests recovered the cells to a capacity of about 1850 mAh, although the label says 2700 mAh. Now it is my opinion that if you run a break-in cycle on those cells you should enter the capacity into the C9000 as 1800 mAh, not 2700 mAh. The cells are not proven to have 2700 mAh capacity yet. You did not say what break-in capacity you used?

What might have happened was that the charging current applied during the charge part of the cycle was too much for the cells to handle and they vented gas, drying out the separator. I normally watch the break-in process carefully to make sure everything looks normal: the voltage should not exceed 1.5 V and the temperature should remain cool. If something looked like it was going wrong I would pull the cells immediately.

 

[shobhitk]

oh! i entered it as 2700mah :shrug: i think ur right...it cooked the cells!

this recovery process is sure a tough job... but thanks to u ppl here at CPF who r always there to extend a helping hand :twothumbs

pls i want to discharge it below 0.9volts without reverse charging them...i dont have single bulb/LED setup...what should i do?
is shorting method similar to the above method? kindly help

 

[Mr Happy]

Once a cell is discharged down to 0.9 V by other means I don't think it would be too harmful to short it directly with a piece of wire. But I don't think it will do much to help recover missing capacity either...

 

[shobhitk]

i c....i wanted to do this so as to break down the crystals because of severe voltage depression.

 

[45/70]

shob, the best way for you to try to do this is to just use your C9000. Granted, it's not the best means to accomplish this, but it can be done.

Set the discharge rate to 100mA, then discharge the cells until it finishes. Then charge the cells at a ~1C charge rate for about 5 minutes, then stop the charge. Then discharge the cells at 100mA again. Repeating this process several times will "sorta" accomplish what you are trying to do. It would work a lot better if the C9000 didn't pulse the discharge at 1 Amp (it uses PWM) to achieve the 100mA discharge rate, but that's all you have to work with.

Also, keep in mind that a lot (most?) cells that a recovery attempt is made, are "toast" anyway. A lot of the time recovery is only temporary, and the cells soon revert back to a poor state.

Another note, if cells that have been charged for 5 minutes, and then are put on discharge and immediately show "DONE" on the C9000 when discharged at 100mA, this is a pretty sure sign that these cells are "toast" and most likely aren't worth bothering trying to recover.

Dave

 

[TakeTheActive]

hi TakeTheActive.
you were right, my break-in cycle results were extremely BAD! :shakehead initially their internal resistance showed at around 1.61volts on an average and after the break-in, all of them were at around 1.88volts , one of them even touched 1.99v! they had also become quite hot! and their after break-in capacities were around 600mah only!! what exactly happened here??

It's possible that your break-in cycle "overcooked" the cells...

...your first tests recovered the cells to a capacity of about 1850 mAh, although the label says 2700 mAh. Now it is my opinion that if you run a break-in cycle on those cells you should enter the capacity into the C9000 as 1800 mAh, not 2700 mAh...

...I normally watch the break-in process carefully to make sure everything looks normal: the voltage should not exceed 1.5 V and the temperature should remain cool...

oh! i entered it as 2700mah :shrug: i think ur right...it cooked the cells!

YOU cooked the cells - the C9000 did what you set it to do. You didn't MONITOR the progress of your 'Experiment'.

shobhitk,

What is your goal - what are you attempting to accomplish?

...i wanted to do this so as to break down the crystals because of severe voltage depression...

You mistreated some fragile, temperamental cells and they failed. I.E., you stored charged High Capacity non-LSD NiMh cells for 9 months, crystals grew, and more likely than not, some crystals broke irreparable holes in the thin separator. My WAG of the heat produced when charging them for 16 hours @ 270mA is due to shorts between the layers. You could try ZAPPING them to remove the shorts, but you'll never close the holes.

In all probability, these cells will NEVER be VIBRANT again. But, you can 'Experiment' with them.

...this recovery process is sure a tough job... but thanks to u ppl here at CPF who r always there to extend a helping hand :twothumbs

Not only is it a 'tough job' (with GREATLY varying results), it's also not something that can be learned via exchanging a few E-Mails/posts. It's more about the reading of CPF Archive posts discussing theory and action/reaction, trying some 'Experiments' and then, when you reach 'a fork in the road', possessing enough knowledge to make an educated guess on which way to go. *IF* you chose the wrong fork the first time, you now have the experience to use in the future.

In Post #1, when you initially charged them @ 0.5C (1300mA?), you didn't mention anything about heat. Since the C9000 has a MAX VOLTAGE termination @ ~1.47VDC, you should have noted how much CHARGE actually went into those cells. I.E. if 3240mAh (120% of 2700) went in, without heat, that's good. But, if right after a DISCHARGE @ 100mA to 0.9VDC only ~1000mAh went back in, you KNOW that you have a problem. The fact that they got hot @ 270mA sometime during the 16 hours, and you didn't notice it, is unfortunate. Hopefully, this is a 'Lesson Learned' for a future 'Experiment'.

IMHO, it's to avoid these situations that many CPF Gurus reply with short, textbook answers - non-VIBRANT -> RECYCLE IT!. 'Experiments' are the responsibility of the Experimenter (a person who enjoys testing innovative ideas).

...that article on deep discharge was quite informative, thanks!

You're welcome!

...many questions pop in my mind now...pls guide-

Nope. Please READ more of my Sig Line LINKs and continue to build your knowledge base.

...first, i dont have a single cell LED or incandescent bulb discharge setup. i only have multi cell incandescent bulb setup. in that case how should i discharge them below 0.9 volts without reverse charging them?

shob, the best way for you to try to do this is to just use your C9000. Granted, it's not the best means to accomplish this, but it can be done...

:shrug: "...the best...", "...not the best...", "...the best...", "...not the best..." :thinking:

IME, the C9000 is the WORST tool (in MY arsenal) to use for 'Deep Discharge' - on cells with very High Internal Resistance, I can ALWAYS get more current out of them on my BC-900 (sometimes up to ~100mAh), then my Superman 1 AA Incandescent Flashlight (up to ~30 minutes) , then my Sears 1 AA LED Solar Lawn Light (overnight).

...Also, keep in mind that a lot (most?) cells that a recovery attempt is made, are "toast" anyway. A lot of the time recovery is only temporary, and the cells soon revert back to a poor state...

:thumbsup: The majority of my recovery efforts have been on 700mAh non-LSD NiMH AAAs and 1300mAh and 1600mAh non-LSD NiMH AAs usually ignored (i.e. 'Storage Box Queens'), sometimes overcooked (Rayovac PS1, PS3), with THICK separators. I still don't understand what went wrong with my Rayovac 2000mAh non-LSD NiMh AAs, but, every now-and-then, I'll try another 'Experiment'. I wrote-off my La Crosse 2400mAh non-LSD NiMH AAs and Digital Concepts 2500mAh non-LSD NiMH AAs as *CRAP* (aka false advertisement) from day one, so besides a few 'Experiments', I never worried about them. They reliably power my GPS (for measuring the distance of and mph during my walks) even at ~75% Capacity (which appears to remain steady). As for my LSD cells - their purchase coincided with my INTENSE reading of the CPF Archives (starting in Nov 2008) so I don't expect to have any problems with them.

...Another note, if cells that have been charged for 5 minutes, and then are put on discharge and immediately show "DONE" on the C9000 when discharged at 100mA, this is a pretty sure sign that these cells are "toast" and most likely aren't worth bothering trying to recover...

But they're still better than putting alkalines into a seldom-used remote.

...pls i want to discharge it below 0.9volts without reverse charging them...i dont have single bulb/LED setup...what should i do?

1. BUY one.
2. BUILD one.
3. GIVE UP.

...

1. and how would i measure the voltage in this setup
2. and to what voltage can i take it down?
3. whats the role of shorting + and - terminals with a wire?
4. is it the same as discharging it in a single cell incandescent bulb setup?
5. is it safe?
6. how is it helpful?
7. and etc...

I prefer to GUIDE people to the answers to their questions (and encourge them to THINK

) rather than EXPLICITLY / REPEATEDLY EXPLAIN / TEACH the same topic over-and-over. :sigh:

There are, IMHO, LOTs of good answers in my Sig Line LINKs.

Good Luck! :wave:

BTW, we're not using SMS on cells phones, counting the number of characters being SENT and RECEIVED. Please use full words and proper grammar in your posts.