[jime]

Hey, eq1. I think you may be on to something.

You made a remark in another thread, which I cannot locate, that you had some sticks that had been abandoned in a corner for a couple of years and that on retesting, you found that they had improved significantly.

I am having the same experience. I have about 10-12 sticks abandoned about 2 years ago, some of which are now testing fair-good. Is it just possible that something improves with long rest times and complete self discharge? Since we are dealing with chemistry, possible the dendrites or whatever goes wrong somehow dissolves with long rest time.

(I'll be happy to merge my comments into your thread if you will point me to it)

 

[S Keith]

Hey, eq1. I think you may be on to something.

You made a remark in another thread, which I cannot locate, that you had some sticks that had been abandoned in a corner for a couple of years and that on retesting, you found that they had improved significantly.

I am having the same experience. I have about 10-12 sticks abandoned about 2 years ago, some of which are now testing fair-good. Is it just possible that something improves with long rest times and complete self discharge? Since we are dealing with chemistry, possible the dendrites or whatever goes wrong somehow dissolves with long rest time.

(I'll be happy to merge my comments into your thread if you will point me to it)

I certainly think so. I mentioned buried in there somewhere that a HCH2 pack that sat for 2.5 years in the as-removed state showed similar results. The as-received pack voltage was 59.6V. All sticks came in at 5300-5500mAh, and there was no capacity improvement with cycling. The pack is performing very well in a car, but it's only been a few weeks.

 

[Hugh-Falls]

Hydration

Dry cells are not "dry ". The water in the electrolyte is the medium which transports the ions The loss of water will render the cell useless. Mike has pointed out that failed cell are dried out at the electrodes. Venting dries out cells.

During use the water tends to be driven away from the higher resistance areas by the heat generated and it tends to accumulate in the cooler areas. Over years of non use (rest) the water distribution in the electrolyte (what remains of it) will tend to be equalized again.

Whether this is so could be determined by doing an ionization/conductivity test on a micro level throughout the electrolyte of deteriorated cells as compared to recovered cells. A direct measurement of the water content of the electrolyte at the micro level would be a more direct method. Either method is difficult to do.

 

[eq1]

Hey, eq1. I think you may be on to something. You made a remark in another thread, which I cannot locate, that you had some sticks that had been abandoned in a corner for a couple of years and that on retesting, you found that they had improved significantly....

I've peppered that around in various places, but the main place is the "impact of deep discharge" thread; the linked page below has the most of it:
http://www.insightcentral.net/forum...pact-deep-discharge-prior-grid-charge-10.html

I've been more or less working from the research conducted by some dude (Robert Huggins) at Stanford, who wrote a book titled "Advanced Batteries," debunking in one small section myth and conceptions about 'memory effect' in the positive nickel electrode. So, everything he explains seems to be consistent with what I've found; the problem that a deep (super deep) discharge fixes is 'memory effect' or 'voltage depression' - it's all about the electrochemistry. The voltage depression is caused by the formation of an 'amorphous' phase of nickel, hydrogen and oxygen during overcharge, and the way to get rid of this is to drop the voltage ("well") below that stuff's potential (0.78V). It just disappears upon recharge and the cell's capacity (and behavior) returns to normal.

It's always been a bit unclear to me, though, just how deep is necessary. The sticks sitting in my garage for a year were pretty much totally discharged - 0.63V per cell - so discharged that sticking the voltmeter probes on the stick would drop the voltage [edit: actually, it was the charger leads, not the DVM probes]. At one point I had been thinking that dropping voltage just below 0.78V, at whatever current, would be good enough, but I think it takes a bit more than that. I HAD done that, just below 0.78V, but I didn't see the radical improvement/transformation that I've seen - definitely in these totally self-discharged sticks - but also likely in a full pack I dropped to 22V as well as individual sticks for which I dropped each cell's voltage to something like 0.6V at 1.3A. This latter threshold still might not be enough, as the results on those 20 sticks seem to be a bit mixed...

 

[jime]

It's always been a bit unclear to me, though, just how deep is necessary. The sticks sitting in my garage for a year were pretty much totally discharged - 0.63V per cell - so discharged that sticking the voltmeter probes on the stick would drop the voltage [edit: actually, it was the charger leads, not the DVM probes]. At one point I had been thinking that dropping voltage just below 0.78V, at whatever current, would be good enough, but I think it takes a bit more than that. I HAD done that, just below 0.78V, but I didn't see the radical improvement/transformation that I've seen - definitely in these totally self-discharged sticks - but also likely in a full pack I dropped to 22V as well as individual sticks for which I dropped each cell's voltage to something like 0.6V at 1.3A. This latter threshold still might not be enough, as the results on those 20 sticks seem to be a bit mixed...

I'm obviously confused also. I was frankly surprised at the improvement in sticks which had been given the "bad" call. Not all the sticks in that pile returned to operational status, but most did.

I had tried a progressive discharge to 0V in the case of a couple of other sticks, just by placing a 12 ohm resistor across the terminals. One of those sticks showed significant improvement, but the other was killed. I think that probably shows that there are multiple failure modes.

 

[Cobb]

Maybe JimE you would be interested in my new toy? Its a smart battery charger for common house hold cells, aa, aaa, c and d.

I used it on all my solar lights. Some of the batteries it said right away were dead, others took a few hours to be declared dead, others were fully restored. The 3 solar lamps that showed water damage were dead right away. The few that had fallen over were considered dead after some cycles or attempted cycles. The rest were somewhat upright and worked for some length of time.

 

[eq1]

....I had tried a progressive discharge to 0V in the case of a couple of other sticks, just by placing a 12 ohm resistor across the terminals. One of those sticks showed significant improvement, but the other was killed. I think that probably shows that there are multiple failure modes.

I don't know, I'm not a big bible-banger of the 'multiple-failure-modes' rubric. I mean, sure there's multiple ways that cells can under-perform, fail, etc. Obviously they age and get worn down. But, it seems like 'memory effect' or 'voltage depression' as I have come to know and try to explain it take the largest, most consistent bite out of performance/capacity... Reflecting on the data I've generated with my original pack and a new 'betterbattery' stick, it looks like a little less than half the performance lost since new is simply due to age, normal wear and tear, whereas the other half is voltage depression (2500mAh vs. 4300 vs. 6500; or 1200 vs. 3000 vs. 5200 useable discharge capacity in car, voltage depressed vs. deep discharged vs. new '8Ah' stick, respectively). Granted, I still have a lot of sticks to work through, aspects to check, and real results would require a random sample of used sticks and this and that, blah blah blah etc...

 

[S Keith]

I'm obviously confused also. I was frankly surprised at the improvement in sticks which had been given the "bad" call. Not all the sticks in that pile returned to operational status, but most did.

I had tried a progressive discharge to 0V in the case of a couple of other sticks, just by placing a 12 ohm resistor across the terminals. One of those sticks showed significant improvement, but the other was killed. I think that probably shows that there are multiple failure modes.

I wanted to discharge sticks to .78V/cell in slow yet automated fashion and minimize voltage rebound when the load is removed. I also didn't want to have to fiddle with it and watch it obsessively.

I used an 18 Ohm 5W resistor with a 4.7V 5W zener diode. The idea was that it would continually draw current until the voltage dropped to 4.7V where the zener diode would shut off the current flow. What I didn't account for is operation in the breakdown region and current bleed.

The end result was that sticks first run to 1V/cell at an 11.5A load were further "trickle-discharged" to 0.6V/cell after about 24 hours, and they held there when the circuit was removed. 3.6V is about 80% of 4.7V and starts to get into the portion of the breakdown region where very little current flows.

After this process, sticks testing to only 4100mAh with typical 1V/cell cycling were restored to low to mid 5000mAh.

I've attached a pic with my "trickle dischargers" on a few sticks. If I had to do it over again, I'd put a 5-6 Ohm resistor on it to accelerate the discharge.

 

[eq1]

I wanted to discharge sticks to 0.78V/cell in slow yet automated fashion and minimize voltage rebound when the load is removed....

How did you come up with the 0.78V per cell threshold? I thought it was roughly 0.78V, too, but actually now, after reading and re-reading the material that got me started down this path, I think the author was saying 'well below 0.78V per cell' and possibly as low as 0.19V. It's based on a type of diagram (a 'Gibbs triangle') I only marginally understand. BTW, that material, most of it, is linked in the 'impact of deep discharge' thread, if anyone's interested, in one of the first few posts...

 

[Jivko57]

Hi I may have killed my Pack last night. I was discharging it with a 60w bulb from full. After 8-10 hrs it got to 147 volts. Then it took ~ 2 hrs to get to 140v. I went to sleep for 3 hrs thinking I will find it down to 130v . However after less than three hrs the bulb was dark and the pack was down to 1.8v!!! I left it bounce to ~14 volts and put on the charger. it went up to 50 volts in half a minute on the charger and in 10 hrs was up to 160v now 1s around 14 hrs of charge and it is up to 167.5v. I hope it can get back to 175 (it got to 172v on the initial charge) and behave OK. But also I am sure I inflicted some irreversible damage by bringing it down to 1.8v.....


Seems the described memory effect at 0.78v does not look like a big problem for NIMH batteries as my pack did not show much charge between 140v and 1.8v . at 200mA discharge it took it 2 hrs and 45 min total .

 

[eq1]

....Seems the described memory effect at 0.78v does not look like a big problem for NIMH batteries as my pack did not show much charge between 140v and 1.8v . at 200mA discharge it took it 2 hrs and 45 min total .

So, you're generalizing your one-shot results to all NiMH batteries? Seems a little odd. But in any event, if I understand the rest correctly, I'm not sure it works that way. Sounds like you're expecting a lot of capacity under 0.78V, is that right? If so, it kind of seems like that's the way it should work, that the capacity is 'all there' still; it's simply locked-in at a lower voltage than normally useful. But I haven't seen this in any of my tests, and it may be a misreading, misunderstanding, of the research...

 

[S Keith]

How did you come up with the 0.78V per cell threshold? I thought it was roughly 0.78V, too, but actually now, after reading and re-reading the material that got me started down this path, I think the author was saying 'well below 0.78V per cell' and possibly as low as 0.19V. It's based on a type of diagram (a 'Gibbs triangle') I only marginally understand. BTW, that material, most of it, is linked in the 'impact of deep discharge' thread, if anyone's interested, in one of the first few posts...

My description was misleading. .78V/cell was the goal based on the available voltages of zener diodes. In my head, I was thinking 0.8V/cell, but I was willing to go a bit lower. I got more than I bargained for at 0.6V/cell, but I'm very pleased with the accident.

Steve

 

[S Keith]

Hi I may have killed my Pack last night. I was discharging it with a 60w bulb from full. After 8-10 hrs it got to 147 volts. Then it took ~ 2 hrs to get to 140v. I went to sleep for 3 hrs thinking I will find it down to 130v . However after less than three hrs the bulb was dark and the pack was down to 1.8v!!! I left it bounce to ~14 volts and put on the charger. it went up to 50 volts in half a minute on the charger and in 10 hrs was up to 160v now 1s around 14 hrs of charge and it is up to 167.5v. I hope it can get back to 175 (it got to 172v on the initial charge) and behave OK. But also I am sure I inflicted some irreversible damage by bringing it down to 1.8v.....


Seems the described memory effect at 0.78v does not look like a big problem for NIMH batteries as my pack did not show much charge between 140v and 1.8v . at 200mA discharge it took it 2 hrs and 45 min total .

Good luck with your pack.

NiMH cells dump the vast majority of their capacity between full charge and 1.0V/cell. Driving the pack/cell lower isn't about getting more juice out of them, it's about driving the voltage lower, which redistributes the electrolyte throughout the cell thereby increasing its capacity. It doesn't correlate to the juice you get out of it when discharging at lower voltages.

 

[retepsnikrep]

Jikov

I say relax it probably did it good.
I've discharged packs down to zero near enough with no ill effects.

 

[eq1]

....Driving the pack/cell lower isn't about getting more juice out of them, it's about driving the voltage lower, which redistributes the electrolyte throughout the cell thereby increasing its capacity....

I saw Cobb say something similar. That's different than the theory I'm going on. Isn't the electrolyte liquid? If so, why wouldn't it already be distributed evenly throughout the cell?

 

[Jivko57]

Hi,thanks for the hope, and sorry for generalizing based on one accident .
The pack is looking good so far , it went back to 176.3 volts, after 20-25 hr recharge where before the "discharge" it stopped charging at 172.5 volts. Will drive with it today. I wanted to go to~120v as it was the first discharge, and eventually to go down to 80v - below the 0.78v described in the excerpt eq1 provided in the other thread on the deep discharge.
There the memory effect was described as shifting capacity under 0.78v with time. So the charge of an affected cell does not disappear it just remains hidden and unusable for the consumer at the second "Platoe " 0.78v .


I mentioned here because such a deep discharge resembles leaving the sticks aside for long time and the deep discharge that happens in those cases.
I had a pack that I assembled from old sticks after cycling them. 3-4 times each and the the pack sat for many months with the sticks disconnected. When I finally put I together it was at 118v. A full charge brought it up to 176.9 v . However I cannot see much how it performs as it is in my wife's cvt.

 

[jime]

I mentioned here because such a deep discharge resembles leaving the sticks aside for long time and the deep discharge that happens in those cases.

Well, yes and no....
When the sticks self discharge on their own, they do that at very low current over a long time. The current used for most "active" means is much higher. I don't know that there is a difference, but it is perhaps worth noting.

I had a pack that I assembled from old sticks after cycling them. 3-4 times each and the the pack sat for many months with the sticks disconnected. When I finally put I together it was at 118v. A full charge brought it up to 176.9 v . However I cannot see much how it performs as it is in my wife's cvt.

I think you must be very careful using pack voltage as some sort of indicator of pack condition. It is generally felt, I think, that bad packs plateau at higher voltages while good packs plateau at lower voltages. The rest voltage, after some hours off the grid charger, may not be as clean an indicator.

As eq1 says, there is considerable lack of statistical rigor in all of this. We have multiple people, trying multiple techniques, on sticks of unknown multiple conditions. It is beginning to look like deep discharge really does improve things, as Mike has found out with his Genesis automated routines, but it doesn't seem to me that the exact mechanisms are yet know.

I have a kind of prejudice against doing it at the pack level, because I have a reluctance to "abuse" the good sticks within the pack, but perhaps time will show that this "threat" shouldn't really be a concern. I still do everything at the stick level, one factor being plenty of time on my hands

 

[jime]

Hi,thanks for the hope, and sorry for generalizing based on one accident .
The pack is looking good so far , it went back to 176.3 volts, after 20-25 hr recharge where before the "discharge" it stopped charging at 172.5 volts. Will drive with it today. I wanted to go to~120v as it was the first discharge, and eventually to go down to 80v - below the 0.78v described in the excerpt eq1 provided in the other thread on the deep discharge.

I think folks who are experimenting with this deep discharge stuff need to remember that the stick voltage drops very, very rapidly when it is totally exhausted of charge. This what seems to have happened in this case. It is by no means a linear thing. This phenomenon makes it difficult to set "target" voltages while using a manual discharger.

I mentioned here because such a deep discharge resembles leaving the sticks aside for long time and the deep discharge that happens in those cases.

I respectfully disagree. I think these two approaches are probably fundamentally different. One is slow, the other quite rapid. The slow approach may/might allow the chemistry to readjust itself more fully - just an idea, not even a theory yet

 

[S Keith]

I think folks who are experimenting with this deep discharge stuff need to remember that the stick voltage drops very, very rapidly when it is totally exhausted of charge. This what seems to have happened in this case. It is by no means a linear thing. This phenomenon makes it difficult to set "target" voltages while using a manual discharger.


I respectfully disagree. I think these two approaches are probably fundamentally different. One is slow, the other quite rapid. The slow approach may/might allow the chemistry to readjust itself more fully - just an idea, not even a theory yet

I was frustrated with deep discharging even at 100mA loads. When the load is removed, the voltage would eventually creep back up by a significant amount.

What you express was my thinking with the "trickle discharger" - the current continually decreases as the stick voltage decreases. Any rebound that would occur when a load is removed is bled off as the battery chemistry produces that current. I like to conceptualize it as a circuit that accelerates the self-discharge process with continually decreasing load.

The diodes are rated for 5 microamps of current at 1V. There is no data for bleed in the breakdown region, but at 80% of the zener voltage of 4.7, the slope of the current vs. voltage line is nearly zero, so I suspect it's very low at 3.6V, and that's the reason the sticks seem to settle at 3.6V. Next time, I will check it with the ammeter.

Steve

 

[S Keith]

I saw Cobb say something similar. That's different than the theory I'm going on. Isn't the electrolyte liquid? If so, why wouldn't it already be distributed evenly throughout the cell?

I'm using that description very generally. I'm not using it like Cobb's post indicating something to the effect that the electrolyte "pools" at the low spot due to orientation. I never intended it as a physical redistribution, but a chemical redistribution with/by/of the electrolyte.

I think it's more accurate to say that it's changing the reactions and composition at the terminals by interaction with the electrolyte under deep discharge conditions.