Cycling cells and cell chemistry

Cycling cells and cell chemistry
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Cycling cells and cell chemistry - 1/3/2008 7:50:22 PM

nikg

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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.

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

guver

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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.

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

nikg

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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

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

guver

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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.

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

rcairflr

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

nikg

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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.

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

guver

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

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

nikg

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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)

< Message edited by nikg -- 1/5/2008 1:06:47 PM >

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

cyclops2

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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

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

cyclops2

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

Rich

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

guver

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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.

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

nikg

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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?

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

guver

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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.

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

nikg

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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.

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