RE: Cycling cells and cell chemistry

RE: Cycling cells and cell chemistry
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RE: Cycling cells and cell chemistry - 1/10/2008 2:57:22 AM

nikg

Posts: 406
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From: , NC, USA
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Yeah, I was starting to think the same thing. It's part effort and part fun actually. I really like learning new things and getting to the root of understanding. That's why I ask so many darn questions. I think I'll experiment with some of them, and if numbers start to all look the same or similar, then forget it. I'll just make the packs without too much concern for matching. They are GP's after all, so I would expect some measure of consistency from them. Should be far better than generic paper wrapped cells.

Hate to disappoint, so here is another annoying question. Both Red and Tom said parallel discharging would be OK, but is overdischarging one cell still possible or likely? If 1 cell is weaker than the others will it overdischarge in parallel? Is it more of a problem in series?
I guess what I'm asking is given the choice between discharging 6 cells in parallel down to 0.9v or 6 cells in series down to 5.4v, which would be better? Which of these options produces the most balanced discharge among the cells? Or does it not make a difference?

(in reply to Jazzy)

RE: Cycling cells and cell chemistry - 1/10/2008 4:04:46 AM

Posts: 55
Joined: 1/5/2008
From: Bellingham, WA, USA
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Hello Red,

Wow, a rare opportunity to learn from the "Master..."

Thank you for your comments.

I do have a question...

I understand that it is rare for crystals to form during normal use, but I just ran into this situation that seems to support crystal break up.

I checked out some consumer AA NiMh 2000 mAh cells. When I measured the voltage, they were at 1.3+ volts. I ran a discharge and they gave me around 20 - 30 mAh, discharging at 400 mA to 0.9 volts. I tried to charge them and they would not take much of a charge at all, and the voltage shot up very fast.

I then did a discharge with the low voltage reduced to 0.5 volts. This time the cells gave me around 1300 mAh of capacity, but the voltage was in the 0.6 - 0.9 volt range.

After the discharge, I did a 0.1C charge for 16 hours followed by a 0.2C discharge. This time the cells gave close to 90% of their initial capacity.

I followed with some 0.5C charge and discharge cycles, but 90% seems to be about as good as I am going to get.

My question is "What happened?"

I have seen similar results when trying to recover NiCd cells, and think that I was breaking up something within the cell and getting it back into solution so the cell would behave more like normal.

Unfortunately, I don't know the history of the cells. I can only speculate that they may have been left on a charger for an extended period of time.

Thanks for any help.

Tom

(in reply to nikg)

RE: Cycling cells and cell chemistry - 1/10/2008 4:16:00 AM

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From: Bellingham, WA, USA
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Hello Nikg,

In a parallel circuit, all the cells will remain at the same voltage. In a series circuit, the "weak" cell will drop voltage first, and there is a possibility that it could be driven into reverse charge.

When discharging a series pack, you should terminate the charge when the voltage drops to about 1.0 volts per cell. Under heaver current draws you can go a little lower, but if your cells get out of balance and you are driving them hard, you run the risk of ruining a cell.

I would say that it is safer to discharge cells of unknown condition in parallel. If your cells are reasonably close to each other, and if you are not driving them hard, it probably doesn't make any difference.

I am use to balancing after a full charge. I am not convinced that balancing in a discharged state will equal a balanced set of cells when they are fully charged, but your mileage may vary...

Tom

(in reply to SilverFoxCPF)

RE: Cycling cells and cell chemistry - 1/11/2008 4:25:57 AM

nikg

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From: , NC, USA
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Thanks, Tom. I thought that is how it worked. Sounds like both charging and discharging in parallel is safest as far as protecting from overcharge and overdischarge.

I'm interested in that AA cell story of yours. You said that after the discharge down to 0.5v you charged them at .1C for 16 hours and then discharged at .2C to get 90% of the rated capacity. I'm assuming that means you got 1800mah, but did you terminated this discharge at 0.9v or the same 0.5v that you had previously used to get 1300mah? I'm assuming it was 0.9v.

Is there any danger in discharging down to such low voltage? (0.5v or lower) It seems to have helped bring you cells back to life. Why does everybody stick with the 0.9v per cell? I'm assuming again that this is general advice for packs in order to reduce the unbalance and possible cell reversal. I've seen plenty of threads where racers discharge their cells down to almost nothing, but will this enhance performance? (maybe at the expense of shortened cell life?)

(in reply to SilverFoxCPF)

RE: Cycling cells and cell chemistry - 1/11/2008 12:46:49 PM

Red Scholefield

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Joined: 12/8/2001
From: Newberry, FL, USA
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quote:

ORIGINAL: nikg

Thanks, Tom. I thought that is how it worked. Sounds like both charging and discharging in parallel is safest as far as protecting from overcharge and overdischarge.

Charging Ni-Cds or Ni-Mh in parallel is not recommended, since the cells are not identical, one cell will go into overcharge first, heat up, voltage tries to drop, that cell demands more current to maintain the voltage equal with the rest, gets hotter and petty soon (if the charger has enough oomph) you have thermal runaway.

I'm interested in that AA cell story of yours. You said that after the discharge down to 0.5v you charged them at .1C for 16 hours and then discharged at .2C to get 90% of the rated capacity. I'm assuming that means you got 1800mah, but did you terminated this discharge at 0.9v or the same 0.5v that you had previously used to get 1300mah? I'm assuming it was 0.9v.

Is there any danger in discharging down to such low voltage? (0.5v or lower) It seems to have helped bring you cells back to life. Why does everybody stick with the 0.9v per cell? I'm assuming again that this is general advice for packs in order to reduce the unbalance and possible cell reversal. I've seen plenty of threads where racers discharge their cells down to almost nothing, but will this enhance performance? (maybe at the expense of shortened cell life?) If you follow the lead of racers in treating your batteries you are going to eat up a lot of them. As for the discharge level, it makes little difference between 0.5 and 0.9 volts as there is essentially no capacity left in this range. The further you discharge a series pack the higher the risk of reversing one or more cells, so 0.9 has been kind of a standard in the industry for years.

_____________________________

Red S. AMA 951
R/C Battery Clinic
www.rcbatteryclinic.com

(in reply to nikg)

RE: Cycling cells and cell chemistry - 1/11/2008 8:05:58 PM

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Hello Nikg,

Please note, as Red pointed out, that charging NiMh or NiCd cells or packs in parallel is not recommended. In my post I was referring to discharging in parallel.

After then initial low voltage discharge with my AA cells, subsequent testing was done with a 0.9 volt per cell cut off.

Tom

(in reply to Red Scholefield)

RE: Cycling cells and cell chemistry - 1/12/2008 2:26:18 AM

nikg

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Thanks guys. Intrigued by the discharging OK in parallel but not charging. What Red said makes total sense though.

Tom---I understand your post was referring to discharging. I mistakenly assumed the same would be true on the charging end as well, so I threw that one in there.

So it is settled. Discharging in parallel is OK, but charging is not. Charge in series. Good thing I didn't go ahead and build two parallel jigs like I was going to. I will build one in parallel for discharge and one in series for charging.

(in reply to SilverFoxCPF)

RE: Cycling cells and cell chemistry - 1/26/2008 2:30:15 AM

nikg

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An update---It had been bothering me that while discharging the cells I had they only read .96-1.05 for average voltage. I hooked up my Turbo 30 to some cells in some SMC 3300 packs and some unmatched IB 3800 packs just to see what would happen. Well, those voltages turned out to be low as well (1.03-1.05 average range). I figured it was just my discharger not working correctly because those SMC packs and IB packs run very well in my E-savage.

Enter the voltage sensing leads on the Turbo 30. The manual says that they give more precise voltage information but do not affect the charging or discharging of the pack/cell. Well, they are MUCH more accurate and it has a tremendous effect on the pack information. I used to watch as the cells dropped to about 1.10v within the first minute and then continue to drop to 1.05 1.04 and so on. Now with the sensing leads hooked up the voltage stops dropping at 1.22 or so and then proceeds down from there. Average voltages are now 1.17-1.18. I've only run a few cells through it with these leads hooked up, but I'm glad I decided to do that. I had a feeling something was wrong. Now I am feeling much better about these cells.

(in reply to nikg)

RE: Cycling cells and cell chemistry - 1/26/2008 12:30:22 PM

Jazzy

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From: Erie, PA, USA
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Nikg,
Your last post begs me to remind you that when discharging cells/packs there will, without fail, be a voltage sag. The higher the discharge rate the greater the sag. (This is, of course, dependant upon the cells ability to maintain voltage under load.)
Glad you hooked up the sensing leads and discovered their proper use!

Enjoy your packs!

_____________________________

Aww $#!^.... I left the )@#& transmitter at home!

(in reply to nikg)

RE: Cycling cells and cell chemistry - 5/6/2008 10:27:18 PM

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Joined: 4/16/2008
From: Chippewa Falls, WI, USA
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I have just finished reading about 100 postings about cycling NIMH batteries in this forum. A thought came to me. These NIMH batteries ,must be very amazing power sources. They seem to be able to take all the abuse we can give them, and they keep on powering our toys for years. For instance, someone might say that he cycles his batteries after every use, been doing it that way for years, another says he does it after about every third flying session, and he's been doing it that way

for years. A third person says that he never cycles his NIMH batteries. He just enjoys playing with his toys. He also has been doing it this way for years. I guess we could borrow that old slogan that the TIMEX WATCH COMPANY coined: "It takes a lickin', and keeps on tickin' ". The reason I took the time to pen this note is that to my way of thinking, at least two of those three battery uses are abuse. And I wonder, which two are they?

(in reply to Jazzy)

RE: Cycling cells and cell chemistry - 5/7/2008 5:50:02 PM

richrd

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From: Waterford, PA, USA
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LOL yea go figure. It's like their all true.

BUT! if you what max capacity AKA current then cycle more often.. That last 5% is so important to some people. NiMH have very very little memory effect if at all detectable so after pack has a few cycles on them then average usage no cycling is neccessary but if left used for many months them a few cycles is a good suggestion to wake them up, see where the confussion can come in..
Rich

(in reply to pacifier)

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