Cycling cells and cell chemistry (Full Version)

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nikg -> Cycling cells and cell chemistry (1/3/2008 7:50:22 PM)

I am trying to get an idea of the capacity and voltage numbers for some loose cells so I can construct some packs that are matched relatively well. I have many to test and i don't want to spend a month trying to get info on them. They are GP 3300's and I am form charging them 6 at a time at 210ma for 24 hrs. I then intend on taking them individually and discharging them to see what each cell reads for voltage and run time. Is discharging at 10A OK or should I go more like 5-6A?

I know to get full capacity the cells should be cycled about 3-5 times, but this will take me 3-4 days for each 6 cells, and I don't want to spend that much time if I don't have to. I'm not exactly sure how battery chemistry works but here are my three theories.

Let's say cell A shows 3000mah at 1.15v on the first discharge and cell B shows 2500mah at 1.12v on the first discharge.

THEORY 1--- Both cell A and cell B continue to gain capacity (and voltage?) by 10% on second cycle, 5% on third and 2% on fourth cycle. Cell A thus gains more each time because it started at higher capacity (and voltage?). This leaves cell A having an even greater difference in capacity (and voltage?) than cell B from the first cycle, even though they both improve.

THEORY 2--- Both cell A and cell B continue to gain capacity (and voltage?) in each cycle, but it is more linear. For instance, they both gain 200ma (and .02v?) on second cycle, both gain 100ma (and .01v?) on third cycle and so on. This still leaves cell A with more capacity and voltage than cell B, but the difference is the same as it was after the first cycle because they both improve in the same amounts.

THEORY 3--- Both cell A and cell B continue to gain capacity (and voltage?), but the opposite of theory 1 occurs and cell B "catches up" to cell A by the third or fourth cycle. In the end they wind up having similar numbers.

I guess what I'm really trying to ask is, are any of these theories correct?(I'm leaning towards theory 1 intuitively) Can I get decent comparative information from just one cycle, or must I run them through 2 or more? I'm really just looking to match them up relatively well compared to each other so the packs drain the cells evenly.



guver -> RE: Cycling cells and cell chemistry (1/3/2008 7:56:45 PM)

Here's my take on it (if they are used cells)

Your charge is adequate and your discharge rate is ok at 5 or 10.

Match them up by capacity first , then voltage, 1 cyle is adequate and each cyle will show similar numbers.



nikg -> RE: Cycling cells and cell chemistry (1/3/2008 8:51:12 PM)

Thanks Guver. 1 cycle being adequate tells me that either theory 1 or 2 is probably correct, right?

They are not used cells. They are new. Some of them were thrown in a matcher and have some labels with numbers that are way off because the matcher false peaked. Others have match numbers that I would like to confirm for comparitive purposes. Again, my main goal is to get comparative information (even if it doesnt reflect true final numbers) so I can match the cells in packs.

I have two "dumb" wal-warts that I will use to charge. 210ma for 24 hrs and 300ma for 16 hrs because I know this is safe for form charging of new cells.

Is there any damage/consequence to discharging at higher rates for new cells? I have read through about 30 threads that suggest anywhere from C/10 to C/5 for discharging, but will it damage them to discharge at more like C or 2C? I will be discharging individual cells, not packs, down to .9v or 1.0v per cell.

Also, if new cells were initially charged at a very high rate then are they permanently affected? Diminished capacity? Or will forming them a few times after the first harsh charge bring them back? Still looking for answers to my other post where this question was.

Thanks again



guver -> RE: Cycling cells and cell chemistry (1/3/2008 9:39:00 PM)

I think that even though they are new, all 3 theories are not right. If they are 3300's and have been charged/labeled then they must've been setting a long time. My theory may be wrong as well. In fact you could really wind up with anything. It wouldn't surprise me if you wound up with mahny that followed each theory,lol.

The best thing to d may be to simply test until it is repeatable. A couple variables that should be constant is temp, time between charge/discharge, ect. And I'd make sure to leave them on the 210 ma or the 300 ma for PLENTY of time. It won't hurt them at all and will ensure that they are full.

Interesting project, how many cells are you doing? You may even find a number of bad ones or ones that fall off in voltage when pushed to 20-50 amps or will self-discharge very fast.



rcairflr -> RE: Cycling cells and cell chemistry (1/3/2008 10:36:14 PM)

deleted



nikg -> RE: Cycling cells and cell chemistry (1/4/2008 3:06:59 AM)

I have over 100 cells.

Any info about whether high discharge rate is damaging to them or not? I would like to try to discharge at 1C to 3C if possible without hurting the performance of the cells. Again, the charging will be C/15 or so.



guver -> RE: Cycling cells and cell chemistry (1/4/2008 8:15:57 PM)

No, 5-10 amps discharge won't hurt at all. GP 3300's can do 30-50 with no sweat.



nikg -> RE: Cycling cells and cell chemistry (1/5/2008 3:27:44 AM)

If this is the case, then I guess I just can't get it through my thick skull why people advise such slow discharge rates upon breaking in a pack. Why recommend C/5 discharge that takes forever when you could discharge at up to 1C, 5C or even 10C?

I am assuming the advice is mostly for PACKS and not individual cells, and the reasoning is similar to the charging recommendation. Discharging at the higher rates might cause one cell IN A PACK to radically overdischarge compared to the others thus causing problems (like cell reversal?) This would be similar to overcharging a single cell IN A PACK on the charge cycle of the pack causing overheating or venting in that cell.

I would like to continue with my experimentation, so please let me know if this reasoning rings true.
It makes perfect sense to me, but I'm no engineer or chemist. (Although I'm getting there with the help of Guver, Red and some others from this site. Thanks)



cyclops2 -> RE: Cycling cells and cell chemistry (1/5/2008 10:39:29 PM)



Post this same question in the RC car & truck group and your mouth will not close for a week.
They charge at normal rates and race at full normal rates. They do not treat them lightly. EVER.

Rich



cyclops2 -> RE: Cycling cells and cell chemistry (1/5/2008 10:44:15 PM)

Where did & why would you buy so many unknown Quality batteries ??

Rich



guver -> RE: Cycling cells and cell chemistry (1/6/2008 12:28:05 AM)


quote:

ORIGINAL: nikg

If this is the case, then I guess I just can't get it through my thick skull why people advise such slow discharge rates upon breaking in a pack. Why recommend C/5 discharge that takes forever when you could discharge at up to 1C, 5C or even 10C?

I am assuming the advice is mostly for PACKS and not individual cells, and the reasoning is similar to the charging recommendation. Discharging at the higher rates might cause one cell IN A PACK to radically overdischarge compared to the others thus causing problems (like cell reversal?) This would be similar to overcharging a single cell IN A PACK on the charge cycle of the pack causing overheating or venting in that cell.

I would like to continue with my experimentation, so please let me know if this reasoning rings true.
It makes perfect sense to me, but I'm no engineer or chemist. (Although I'm getting there with the help of Guver, Red and some others from this site. Thanks)


Maybe we should make a distinction between "breakin" or "forming" AND "cycling for matching pupose"

I too would take it easy for the first charge/discharge. What you say about the packs and overdischargeing one cell in apack is true for sure, but probably has nothing to do with the recomendations.



nikg -> RE: Cycling cells and cell chemistry (1/6/2008 2:18:10 AM)

Cyclops 2---got them from Team Scream for a good price. I was assured that they were good but just needed to be cycled to bring them back. I'm just trying to get them back from sitting around a bit.

Racers I'm sure never take it easy on their cells, but to them batteries are a part of the expenses and they just come to expect buying new ones all the time as products improve. I am a basher, so I want mine to last as long as possible. That said, could I start safely peak charging at 1A? Should I set the peak detect at .03 or should it be higher to prevent false peaking the first few times around?



guver -> RE: Cycling cells and cell chemistry (1/6/2008 11:31:42 AM)

I think that a "forced" charge of any rate is what you want (at least for the first part of the charge) The rate may not matter at all. The important thing is that they are full charged and a c/10 rate is real nice becuase it is plenty high to charge them, yet will not damage them if/when overcharged.

In otherwords if you used 1-5 amps and stopped the charge when full it may be more of a guessing game than if you charged at 200-400 ma and allowed them t overcharge plenty.



nikg -> RE: Cycling cells and cell chemistry (1/7/2008 2:41:04 AM)

Thanks, Guver. I may well start to use that technique to gain a little time advantage. Maybe put them on my Turbo 30 and peak charge at 1-2A then throw them on the smallest 210ma trickle wart for a few hours.

A question:
First, what should the peak detect be on a single NIMH cell? My charger only goes in increments of .01. I set it to .02 for a single cell for this initial charging process. Is that too high? What should it be for 6 cells? I have it at .03. I know that number is commonly used for packs normally, but I thought I saw somewhere that it should be set higher for initial charging.

A development:
I trickle charged three sets of 6 cells each and then discharged individual cells at 5A (my machine goes no lower than this). It seems in each batch of 6 only one cell held any kind of capacity (2200-2400mah). The others puttered out really quickly (70-500mah). Why is this happening? Are the cells that held some capacity probably just better off to begin with or is there something inherently flawed in my setup? Voltage on all cells before first discharge was 1.39-1.41, but like I said only a few had any actual capacity.
I ran them through a second cycle, but this time I did it on my Turbo 30 at 1A cahrge rate. Now many of them are coming out in the 2300+ range for capacity, and one is at 3280mah. Is there something about charging at higher amps that is waking them up better, or would these results have likely occured if I had trickled them again? I don't know why, but it just seems like not much happened from 24 hours of trickle on 210ma. Sure, the voltage went up but the cells didn't have any capacity. If the grand total was only like 3000-4000mah total for all 6 cells, then where did all the wart's energy go?

I think my next step will be to do some initial charging with the Turbo 30 set at 1A and peak detection. I will monitor the time and voltage manually as well just to make sure they don't get terribly overcharged. Again, I need some advice on a peak detect setting for initial charging for these cells.




SilverFoxCPF -> RE: Cycling cells and cell chemistry (1/7/2008 2:41:07 AM)

Hello Nikg,

I believe the #1 reason NiMh cells fail is because of separator failure. Let's take a look at what can be done to prevent this.

The cell is made up of a + and - electrode separated by a separator. The electrolyte is smeared on the separator and the whole assembly is rolled up and stuffed into a can.

In order to get the best performance from the cell, the electrolyte needs to be evenly distributed, the spongy electrodes need to be fully charged, and, for high current applications, we prefer the crystals formed during charging to be small in size.

If you start out with a brand new cell, or one that has been in "storage" for awhile, and subject it to high speed charging and discharging, you will end up with a damaged separator and an under performing cell. The reason is that the electrolyte within the cell has not had a chance to saturate the separator and be evenly distributed throughout the cell. During high charging and discharging rates, local hot spots will form within the cell and those hot spots will damage the cell.

This is why you should take the time to form, condition, break in, or cycle the cell, or whatever you want to call it. Charging at 0.1C for 16 hours allows for a very complete charge, and the slight overcharge works to evenly distribute the electrolyte in the cell as well as allow the separator to soak up the electrolyte. Next, if you happen to be interested in how the capacity of the cell compares to its labeled capacity, you can do a 5 hour discharge and compare the numbers. I have found that you can get very similar results by doing a 2 hour discharge, and high quality cells will maintain capacity even under a 1C discharge.

If you are looking for the best performance from your cells that have been stored for awhile, first, individually discharge them to 0.9 volts per cell at 1C. Follow this by another discharge to 0.9 volts at 0.5C. One more discharge at 0.25C should give you a pretty much completely discharged cell, with all the large crystals dissolved. Now charge at 0.1C for 16 hours, followed by a 0.5C discharge. Next charge at 1C followed by a 1C discharge. At this point your cell should be pretty much formed, conditioned, and broken in. You may want to do a couple of additional 1C charge/discharge cycles for good measure, but I don't think it is absolutely necessary.

At this point you can charge and discharge at your "normal" rates and do your matching.

If this all seems like a lot of work... it is. Lots of people take various short cuts with varying degrees of success.

If I were dealing with your 100 cells, this is how I would do it.

1. I would discharge pairs of cells with my CBA. This short cut requires some monitoring of the discharge to make sure nothing strange is going on, but it should half the time to complete this step.

2. I would clamp 10 cells in series and charge them all at the same time at 0.1C for 16 hours. You may be able to take a short cut here by charging at 0.5C terminating the charge on capacity put in. If these are 3300 cells, you may be able to charge at 1.5 amps and terminate when you have put 2500 or 3000 mAh into the cells. You would then continue with your 0.1C charge for 7 - 8 hours more.

3. Next I would discharge all 10 cells clamped together at 1.5 amps down to 1.0 volts per cell.

4. Next I would charge them at 3 amps and pay close attention to the end of the charge to keep track if any cells are getting hotter than the others.

5. Now comes a discharge at 3 amps.

6. Now it's time to test. Charge 1 cell and as soon as it is charged, do a discharge test at the current you are wanting to use. Take notes and repeat for all 10 cells.

Repeat this 10 times and you will have all 100 cells done.

Tom



nikg -> RE: Cycling cells and cell chemistry (1/7/2008 3:18:13 AM)

Tom---Thanks for the very detailed reply.

I have one problem, though. My Turbo 30 can only discharge at a rate of 5A and no lower than that. You refer to discharging with a CBA. What is that?

Is there any way for me to easliy discharge at a lower rate? Something I can buy that won't break the bank? Is discharging at 5A likely to cause problems in the seperator? I'd like to avoid this if possible and get the most from these cells.

How about discharging in parallel? If the Turbo only goes down to 5A but is hooked to 5 cells in parallel and set it to .9v, then would each be effectively discharging at 1A? I know that the greater the number of cells in parallel, the less accurate that .9v is because now many cells are contributing to the equation. Is that too risky for this situation? Can this even be done, or will the parallel configuration screw something up in my machine? Logically I'm thinking 5 cells in parallel would act like one cell with 5 times the capacity.

You refer to discharge breaking up the large crystals in the cell. Does discharging, especially at lower rates, really help break up larger crystals that degrade capacity? Have you heard of or used the Spintec battery manager? I've heard many people singing its praises. Apparently it dicharges at 35A but only in bursts with rest periods in between so the cell can recover. Apparently this device is very good at breaking the crystals also. Any comments?



SilverFoxCPF -> RE: Cycling cells and cell chemistry (1/7/2008 4:48:48 AM)

Hello Nikg,

I believe discharging in parallel is OK, as long as you have good connections and keep an eye on things.

I have heard of the Spintec, but don't have one... I believe it is used more for maintenance and preventing large crystal growth from getting out of hand. Large crystals take time to dissolve. This is the reason behind the slow discharge rate.

Here is an example. I was just sent some 2000 mAh cells. The report stated that they would not charge, and had almost no capacity during a discharge. I ran a discharge on them and got about 20 mAh then the cells dropped below my 0.9 volt cut off. I tried to charge them, but they would not take a charge. The voltage would shoot up immediately and terminate the charge.

I then decided that this seemed to be a classic case of voltage depression, often incorrectly called the memory effect. I then did a slow discharge using 0.5 volts as the cut off. The voltage of the cell started at 0.6 volts, rose up to nearly 0.9 volts, then steadily dropped down to end at 0.5 volts. The discharge capacity was around 1300 mAh. I ran a couple of more slow discharge cycles, making sure that the cell rebounded to above 1 volt after the load was removed, then proceeded with a couple of 0.1C 16 hour charges separated by a 5 hour discharge. Once this was done, I ran a 0.5C charge and discharge and noted that the capacity of the cell had come up to just under 1900 mAh. At a 1C charge and discharge, everything is still looking good, so I charged them up and am letting them sit for a couple of days to see if they will develop a high self discharge rate.

At any rate, you can see that most of the capacity was originally there, but was only available at a much lower voltage. I have no idea if I will be able to revive these cells, but I am hopeful.

Tom

Edit: Oops I forgot to add that the CBA information can be found at www.westmountainradio.com .



nikg -> RE: Cycling cells and cell chemistry (1/7/2008 6:13:07 AM)

Tom,

Thanks again for the info. Why were you sent those 2000mah batteries? From a friend or something, or is this part of your profession? Does CPF in your title stand for something?

So any suggestions about a peak detect setting for 6 cells during charge cycles? Same .03v as usual for regular packs?

How about a low current discharge option? I don't want to spend a lot. Hook up some light bulbs or an electric motor or something?



Red Scholefield -> RE: Cycling cells and cell chemistry (1/7/2008 1:08:16 PM)

quote:

ORIGINAL: SilverFoxCPF

Hello Nikg,

I believe the #1 reason NiMh cells fail is because of separator failure.

True

Let's take a look at what can be done to prevent this.

The cell is made up of a + and - electrode separated by a separator. The electrolyte is smeared on the separator and the whole assembly is rolled up and stuffed into a can.

The electrolyte is added to the cell after the roll is placed in the can. It is the consistency of water and is dripped in with a hypodermic kind of needle as the cells progress through the filling station.

In order to get the best performance from the cell, the electrolyte needs to be evenly distributed, the spongy electrodes need to be fully charged, and, for high current applications, we prefer the crystals formed during charging to be small in size.

There are no crystals formed during charge or discharge. Any crystal formation is the result of extended overcharge at low rates. At high rates the heat build up damages the separator if not terminated.

If you start out with a brand new cell, or one that has been in "storage" for awhile, and subject it to high speed charging and discharging, you will end up with a damaged separator and an under performing cell. The reason is that the electrolyte within the cell has not had a chance to saturate the separator and be evenly distributed throughout the cell. During high charging and discharging rates, local hot spots will form within the cell and those hot spots will damage the cell.

Actually the damage is gassing from exceeding the hydrogen over voltage potential, hydrogen will not be recombined and the cell will vent. Venting upsets the electrochemistry of the cell to the point where performance is compromised.

This is why you should take the time to form, condition, break in, or cycle the cell, or whatever you want to call it. Charging at 0.1C for 16 hours allows for a very complete charge, and the slight overcharge works to evenly distribute the electrolyte in the cell as well as allow the separator to soak up the electrolyte.

The active material in the cell is not fully formed, the slow charge accomplishes this without gassing.

Next, if you happen to be interested in how the capacity of the cell compares to its labeled capacity, you can do a 5 hour discharge and compare the numbers. I have found that you can get very similar results by doing a 2 hour discharge, and high quality cells will maintain capacity even under a 1C discharge.

If you are looking for the best performance from your cells that have been stored for awhile, first, individually discharge them to 0.9 volts per cell at 1C. Follow this by another discharge to 0.9 volts at 0.5C. One more discharge at 0.25C should give you a pretty much completely discharged cell, with all the large crystals dissolved. Now charge at 0.1C for 16 hours, followed by a 0.5C discharge. Next charge at 1C followed by a 1C discharge. At this point your cell should be pretty much formed, conditioned, and broken in. You may want to do a couple of additional 1C charge/discharge cycles for good measure, but I don't think it is absolutely necessary.

At this point you can charge and discharge at your "normal" rates and do your matching.

Tom


Comments added in blue by the Battery Clinic - 30 years of cell manufacturing experience with major supplier.



nikg -> RE: Cycling cells and cell chemistry (1/8/2008 1:33:14 AM)

Thanks Red.

Any advice on the following:

*Peak detect for initial charging? (per cell or per pack)
*Parallel discharging OK? Calculations?
*Is 5A discharge OK to use when forming or is that too high for initial break in causing "hot spots" and seperator damage?
*Cheap method for slow rate discharge?
*Any comments on the Spintec Battery Manager? I'm very interested in it.

Thanks

(Not to discount you, Tom. I just like having lots of information and verification from several people.)



Red Scholefield -> RE: Cycling cells and cell chemistry (1/8/2008 2:41:27 PM)


quote:

ORIGINAL: nikg

Thanks Red.

Any advice on the following:

*Peak detect for initial charging? (per cell or per pack)

Initial charging of unformed cells can result in premature peak leaving the cells undercharged. If at a high enough rate it could also cause gassing (venting).

*Parallel discharging OK? Calculations?

Parallel discharging of packs gives you the capacity of both packs. If a 2000 mAh pack is discharged in parallel with a 1500 mAh you will have 3500 mAh. Both packs must be of the same chemistry and same number of cells.

*Is 5A discharge OK to use when forming or is that too high for initial break in causing "hot spots" and seperator damage?

Better to discharge at C/5 as this is what the manufacturer uses to check the capacity.

*Cheap method for slow rate discharge?

If your charger doesn't have a discharge function, use a #44 light bulb and watch the voltage with a meter shutting it off as it reaches 0.9 volts/cell. This will give you a discharge of 250 mA as I recall.


*Any comments on the Spintec Battery Manager? I'm very interested in it.

Never heard of it until you mentioned it here, but now from reading the specs it looks like a pretty nice unit

Thanks

(Not to discount you, Tom. I just like having lots of information and verification from several people.)





cyclops2 -> RE: Cycling cells and cell chemistry (1/8/2008 3:50:24 PM)

I hate to be a pooper.

But if you can only get 1 cell out of 6 to putout some power, You have been sold a lot of old batteries. chemical batteries lose capacity just sitting on a shelf. They never have the same or more power with age.

Send the batteries back...........Period...........Do not take any of their Ideas.......They sold old or used batteries to you.

Get the money. Paid by credit card I hope. Tell your credit card company you want a FULL refund. Nothing less.

Rich



nikg -> RE: Cycling cells and cell chemistry (1/9/2008 1:56:36 AM)

Well, I must admit I was very disheartened after the first charges as well. I thought I got completely screwed, but I am currently discharging a cell that initially held about 9mah after the first charge. The way things are going so far it appears that it will crank out at least 2500-3000mah upon this discharge cycle. I expect it to be even better after one more cycle. I'll let you know exactly what it churn out when the discharge is done.

I am still mystified, though. Like I said, many of the cells held just about nothing even after 24hrs at 210ma. What would cause this to happen? What makes it so a cell won't take a charge? I used a plain old DC converter (wal wart), so does this have anything to do with it? It was not a battery charger, just a power supply really.

I have put a second charge into the initially crappy ones, and like I said I am discharging one of them now. I did charge at 1A the second time around, and I used my turbo 30 with the charge termination based on peak detection and not time. I will discharge the others to see if they held anything the second time around. I am hopeful that they all show some decent capacity and just needed a good cycling. Not ready to give up quite yet until I run a few charges into them.



nikg -> RE: Cycling cells and cell chemistry (1/9/2008 2:27:09 AM)

That cell gave me 2503mah. Not bad considering the first cycle of 9mah. I think these cells just need to be cycled.

I am discharging at 5A with cutoff at .9v My machine can't go below 5A, but if it could it might show even more capacity.



Jazzy -> RE: Cycling cells and cell chemistry (1/9/2008 4:03:32 AM)

It sounds like your wall wart is not operating correctly. I have seen cells that don't "wake up" unless you charge them at a high rate. After that, they are generally fine.

I spent the time to match used cells from Dewalt drill packs. Some were excellent, some were bad. The thing is, after a handfull of cycles (up to around 10) in my stadium truck two out of three packs had a bad cell. They were matched though!!
I slow charged them. Cycled them. Slow charged them and let them sit for 24 hrs. Checked voltage and discharged again. Matched them by capacity and voltage and made the packs. Talk about time consuming! Last time I do that.

What I'm trying to say is that you've got "new" cells. Don't bother trying to match them up. After a while at hard use the weaker cells start to drop out and a pack goes south. You can't always find the weaker cells by initially cycling them but you can find dead ones. It sounds like they are all comming around anyway. What I would do is make the packs you want. Cycle them a few times and run'em hard! Try and go easy the first cycle or two. I know you can't discharge lower than 5A. That is OK. You probably would see more capacity at a rate closer to 1/2C. 100 cells makes a LOT of packs! That is a great deal of time cycling and matching! Then you have to make the packs!

Maybe make a few quickies and match up a few for 'special occasions'? I just hate to see you spend all that time and effort when you could be having fun.

Good Luck,
Jeff



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