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Discharging to .9v/cell - 10/28/2004 8:53:22 PM   
Mike01


 

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From: Brooklyn, NY, USA
Status: offline 		When people say "discharge to .9 volts then (charge, store, whatever)", do they mean .9v under load, or .9 volts resting voltage (as in seriously drained cell)?

If under load, what load is acceptable? (.9 volts under a 5 amp load leaves the cell with more capacity than .9 volts under a 2a load)

Just curious.
       Post #: 1

RE: Discharging to .9v/cell - 10/28/2004 8:56:37 PM   
Red Scholefield



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Joined: 12/8/2001
From: Newberry, FL, USA
Status: offline 		The manufacturers rate is C/5 discharge to 0.9 volts to determine capacity, if that is any help. Naturally the higher the load the sooner you will get to 0.9 volts and the more residual capacity remains.


quote:

ORIGINAL: Mike01

When people say "discharge to .9 volts then (charge, store, whatever)", do they mean .9v under load, or .9 volts resting voltage (as in seriously drained cell)?

If under load, what load is acceptable? (.9 volts under a 5 amp load leaves the cell with more capacity than .9 volts under a 2a load)

Just curious.


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RE: Discharging to .9v/cell - 10/28/2004 9:07:16 PM   
exeter_acres



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Joined: 9/22/2004
From: Johns Creek, GA, USA
Status: offline 		Follow up question.........

My cycler, by default, discharges to 1.1v per cell.... would .9v per cell be better/worse/or make no difference??

(in reply to Red Scholefield)
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RE: Discharging to .9v/cell - 10/28/2004 10:36:22 PM   
Mike01


 

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Status: offline 		Worse!

Your cycler is not discharging to 1.1v per cell...it's discharging to 1.1 volt multipled by the number of cells.

When a pack dies, one cell always dies first. So, if you set it to .9v per cell, and you cycle 6 cells, your cycler will quit at 5.4volts. When that one cell dies first, it will dump rapidly, and if the other cells are over 1.08 votls, that one cell will reverse polarity and start taking on a reverse polarity charge which will damage it permenantly...the lower your discharge amperage, the higher the chance this will happen.

1.1v per cell is a very conservative, very good setting.

Red,

Thanks for your reply. I was specifically wondering, for those peopel that recommend discharging NiCD cells to .9v per cell...what exactly they mean by that. Resting voltage, or under load. Any thoughts?

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RE: Discharging to .9v/cell - 10/29/2004 8:05:26 AM   
guver


 

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Status: offline 		Mike I think the idea is to discharge under load only to .9 to keep from reversing just like you described. Then to put on a tray to slow the discharge of each cell down to .8 or .9 so that it will rest at .9. per cell.

As soon as it is removed, it will start coming back up. Some are even recommending now to discharge each cell to zero which is ok, as long as none get reversed., I'm not sure what the claim is for an advantage though.

I guess to answer your question it is both....

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RE: Discharging to .9v/cell - 10/29/2004 11:14:26 AM   
Red Scholefield



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Status: offline 		FWIW
Taking a cell all the way to zero volts (this is for individual cells) is a good way to pick out cells with incipient shorts (bad separator or separator deterioration) as they will not recover voltage when the short is removed like a good cell will. We used it in industry as a standard practice, called it the short down spring back test, done on a lot sample basis for production.

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RE: Discharging to .9v/cell - 10/30/2004 12:12:08 AM   
karolh


 

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From: Mandeville, JAMAICA
Status: offline 		Mike, my 4 cell Rx battery pack when charged by my Accu Cycler measures 5.2v on my Hobbico digital voltmeter with load. Athough advertised to discharge to 1.05v/cell, the, same unit when used to cycle the pack only discharges it to 4.9v as measured with load. Is this good or bad. If bad, what should I do.

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RE: Discharging to .9v/cell - 10/30/2004 1:10:29 AM   
Mike01


 

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From: Brooklyn, NY, USA
Status: offline 		karolh,

From your description, I'm assuming you're measuring your pack after it was discharged. As soon as a load is removed from a battery, the voltage immediately climbs to over 1.2 volts per cell (nimh and nicd), unless the pack is so seriouly depleted that the resting votlage (voltage when load is removed) is below that. That's measuring votlage is really only useful to tell you if the pack is fully charged or is completely discharged....anything in between can't really be measure by reading voltage.

Your cycler is discharging to 1.05v under load, not 1.05 resting. I am unaware of any cycler that can do that. btw..the lower the discharge amperage, the more capacity will be sucked out of the pack before it reaches that voltage. So if you want to completely cycle those packs, do so at a very low discharge setting.

To sum it up, your cycler is doing it's job.

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RE: Discharging to .9v/cell - 10/30/2004 9:19:44 AM   
JohnMuchow


 

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quote:

ORIGINAL: Mike01
If under load, what load is acceptable? (.9 volts under a 5 amp load leaves the cell with more capacity than .9 volts under a 2a load)
Just curious.


Any load that doesn't harm the pack is acceptable really. It all depends on how fast you want the discharge to go, the impedance of the cells, etc. But, the faster you go the less efficient the discharge is...always a tradeoff somewhere.

If this info helps, we just did a 6-cell Sanyo N3000-CR pack discharge test today with a first discharge of 20A (about a 6.5C rate) to 5.4V immediately followed by a 3A (1C) discharge again down to 5.4V. We got another .29AH out of the pack for the second discharge.

But, for longer pack life we recommend a discharge current level that won't cause the cells to get more than very warm (1C-3C) with a followup discharge at 0.2C-0.5C to complete the discharge. If you're using a tray discharger, you only need to do the first 1C-3C discharge.


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RE: Discharging to .9v/cell - 10/30/2004 2:16:39 PM   
karolh


 

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Status: offline 		Mike
Your assumption was correct. My cycler has discharge rates of 250 and 500, what do these numbers represent.

< Message edited by karolh -- 10/30/2004 3:03:42 PM >

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RE: Discharging to .9v/cell - 10/30/2004 4:05:39 PM   
Silent-AV8R



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From: SOCAL, CA, USA
Status: offline 		Since your discharging device is by design using a load to drain the cells, the voltage is measured under load. As Red pointed out, Nicad and NiMH cells are rated at a C/5 load to 0.9 volts per cell. So the 0.9 volts per cell is determined by the C/5 discharge rate.

Example: a 5-cell NiMH 2,700 mAh pack should be discharged at 540mA until it reaches 4.5 volts (0.9 per cell) under that load. I discharge mine at 500 mA load.

I personally like the Sirius Super Test PRO Cycler. Does rates of 125, 250, 500, 750, 1,000, 1,500, and 2,000 mA. Cutoff voltages of 1 to 14 volts in 0.1 volt incrments. Nifty product.

http://www.siriuselectronics.com/

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RE: Discharging to .9v/cell - 10/30/2004 5:17:07 PM   
Mike01


 

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Status: offline 		John,

As you know, I have a futaba CDR-5000, which monitors individual cell voltage. As such, I have learned something disturbing about discharging cells to 5.4 volts per cell at low amperages.

When one cell dumps (as it always does), the others may still be fairly high in their voltage, resulting in the discharger not cutting off (because total pack voltage will still be over 5.4) and that one cell reversing under load.

Just try it with a volt meter...wait until the pack is almost done and monitor the weekest cell...you'll see it reverse. (I believe this was how I damaged my packs, from our earlier discussion).

If you're discharging at 20 amps, then it's fine...all of the cells will be battered in terms of voltage and by the time one cell starts to dump, the rest will be low enough for the discharger to cut off before it's too late.

Mike

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RE: Discharging to .9v/cell - 10/30/2004 5:20:05 PM   
Mike01


 

Posts: 643
Joined: 12/29/2003
From: Brooklyn, NY, USA
Status: offline 		karolh,

I believe those numbers represent miliamps, which would be the discharge rate of your cycler. So 500ma would be .5amps, and so on.

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RE: Discharging to .9v/cell - 10/30/2004 7:23:00 PM   
JohnMuchow


 

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From: New York, NY, USA
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quote:

ORIGINAL: Mike01
When one cell dumps (as it always does), the others may still be fairly high in their voltage, resulting in the discharger not cutting off (because total pack voltage will still be over 5.4) and that one cell reversing under load.


While this can be true for a pack with unbalanced cells, it doesn't have to be the rule for all packs.
Keeping the cells balanced as much as possible (by conditioning new cells, occasional slow charging and using a tray discharger, or at least, fully discharging the pack) can go a long way towards preventing this from happening. It isn't foolproof but we've only had one cell damaged from causes other than severe overcharging/ultra-high current discharging since 1992, and we've only been using pack dischargers.

I just did a discharge test of 3-1/2 year old 2-cell and 4-cell N-3000CR packs (in series, to form a 6-cell packs) to check the voltages of each cell during a 0.3C discharge (approx.). I used a combo of two old smaller packs that I had charged at 5A yesterday for another test (sitting around for a month before that, only pack discharged, never tray discharged) because each has a wildly different history of use and would encourage any cell reversal.

I measure the following voltages at cutoff (5.469V):
2-cell pack:
0.676
0.951
4-cell pack:
0.953
0.943
0.974
0.972

Looks like one cell is unbalanced with the rest, but is still over 0.6V at cutoff.

While cells can certainly get unbalanced badly enough for one to undergo reversal during discharge, it doesn't have to happen...even with older packs. I think you may have hit the nail on the head when you mentioned that's how the one cell got damaged. I'm curious how it could get so unbalanced so quickly though. Your tests of the packs when the cell is replaced will be very informative, I'm anxious to hgear the numbers. :-)


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CamLight Systems

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RE: Discharging to .9v/cell - 10/31/2004 6:57: