Honda Insight Forum banner

201 - 220 of 313 Posts

·
Moderator
Joined
·
6,934 Posts
It's already hard enough to quantify the damage reversals do in general. Figuring out what voltage damage starts to occur at is noble, but even I'm not going to go there, lol.

Interesting concept. I'm doubting that the damage is absent. But maybe that is the threshold for venting?
 

·
Registered
Joined
·
227 Posts
Discussion Starter #202
It's already hard enough to quantify the damage reversals do in general. Figuring out what voltage damage starts to occur at is noble, but even I'm not going to go there, lol.

Interesting concept. I'm doubting that the damage is absent. But maybe that is the threshold for venting?
Here is a reference to an Energizer technical paper on NiMH batteries:

http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf

The graph on page 10 shows what happens during deep discharge and electrode reversal. Reversal occurs in two steps. The first step occurs at zero volt, which is the ionization potential of hydrogen. If the current continues flowing in the discharge direction, the second step occurs at -1.25V, which is the potential of water electrolysis. The paper says that the battery is irreversibly damaged only after the second step.
 

·
Moderator
Joined
·
6,934 Posts
Yeah, that graph has been passed around here before.

The graph makes it seem like it's a capacity based thing, that you have to discharge 150% of the batteries capacity before you'll reverse the negative electrode. That may be the case at low currents, but I know that it can happen instantly at higher currents - it's also an IR thing.

So I wonder if IR would affect the point where a cell instantly goes to -1.25V?

Edit: I still don't think damage is absent. Remember that the chemical reactions aren't just black and white, on and off. Discharging a NiMH cell below 1.0V or so does "more damage" than keeping the discharge above that. But it seems you're right, the rapid damage that is caused by gas evolution is avoided until you hit -1.25V.

Very interesting stuff, thanks. I don't really know much about all the chemically stuff. :cry:
 

·
Registered
Joined
·
227 Posts
Discussion Starter #204 (Edited)
Yeah, that graph has been passed around here before.

The graph makes it seem like it's a capacity based thing, that you have to discharge 150% of the batteries capacity before you'll reverse the negative electrode. That may be the case at low currents, but I know that it can happen instantly at higher currents - it's also an IR thing.

So I wonder if IR would affect the point where a cell instantly goes to -1.25V?
I think they are a little muddleheaded regarding capacity, although they are correct about the two-step reversal process. There is indeed no any meaningful capacity (or reversal "buffering cache", if you will) at the negative voltages. Mike's graphs are also consistent with quick voltage drop of about 2V, and I also saw reversals proceeding fast from 0 to -1.25V at about 1A current.
 

·
Registered
Joined
·
1,738 Posts
Here is a reference to an Energizer technical paper on NiMH batteries:

http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf

Excellent tech paper.

The graph on page 10 shows what happens during deep discharge and electrode reversal. Reversal occurs in two steps. The first step occurs at zero volt, which is the ionization potential of hydrogen. If the current continues flowing in the discharge direction, the second step occurs at -1.25V, which is the potential of water electrolysis. The paper says that the battery is irreversibly damaged only after the second step.
For someone building their own battery the 2nd step could be avoided by placing a reversed biased diode across each cell. The diodes would have to be able to handle the expected reversed discharge current.

Since unknown cells are able to reverse charge at rather high pack voltages the article certainly backs up the idea of using a low current discharges for rejuvenating batteries. For the few packs I've discharged I kept the discharge current below 300 ma.
 

·
Registered
Joined
·
227 Posts
Discussion Starter #206 (Edited)
For someone building their own battery the 2nd step could be avoided by placing a reversed biased diode across each cell. The diodes would have to be able to handle the expected reversed discharge current.
This is an excellent idea. Most silicone diodes have forward voltages around 0.5V, so any 3A rated diode would work. These diodes will not interfere with normal charging and discharging, so they can be connected permanently. The most important problem with this approach is implementation. It would be difficult to add cell taps and diodes to the pack as it is. But it would be perfect for a stick discharge rig.
 

·
Registered
Joined
·
227 Posts
Discussion Starter #207 (Edited)
Results of ultra deep discharge restoration

After the ultra deep discharge, the two Hybrid Revolt sticks were fully charged and discharged as I did before. Two cycles were done with each stick, and both yielded very close results.

H154FAA0666E before after

Capacity, ah: 5.33 6.01
Ri at ~7.6V, mΩ: 26±0.4 20.33±0.03
Midpoint voltage, V: 7.18 7.57
1st dropout, V: 5.0 (@1.25A) 2.61 (@0.65A)
Open voltage, 12 h
after full charge, V: 8.14 8.43


H154FAA0669E before after

Capacity, ah: 5.04 6.74
Ri at ~7.6V, mΩ: 22±0.4 16.64±0.04
Midpoint voltage, V: 7.15 7.62
1st dropout, V: 5.33 (@1.33A) 2.94 (@0.73A)
Open voltage, 12 h
after full charge, V: 8.11 8.47

Individual cell internal resistances were within 12% of the average after the restoration.

All the parameters, including capacity, internal resistance, balance, and self-discharge considerably improved. The second stick was actually restored to like new condition. The first one did not fare so well. It might have suffered irreversible loss of 10% of its capacity, and increased internal resistance. Probably, it had a history of venting episodes and had lost water. Interestingly, since this stick is well balanced, all its cells suffered similar deterioration.

The take home is that ultra deep discharge is a very effective battery restoration tool, however, it is not a panacea and will not fix permanent damage.
 

·
Registered
Joined
·
227 Posts
Discussion Starter #208 (Edited)
Although I didn't want to theorize, I cannot help doing it because the chemistry behind the deep discharge seems pretty straightforward. To my excuse, I was a chemistry geek at high school, and took extensive courses of inorganic and physical chemistry as part of my undergrad studies. I am a PhD in biochemistry, and chemistry is a significant part of my professional activities.

So, in lay terms:

The negative (MH) electrode is of little concern from the point of view of cell's deteriorating capacity: it is purposely designed to have larger capacity than the positive (nickel) electrode. However, upon reversal down to -1.25 V, atomic oxygen will corrode it, causing reduction of its capacity. Atomic oxygen is a very potent oxidizer. Nothing can stand to it but noble metals like platinum.

Since the positive electrode is limiting to cell's capacity, its degradation is the key to the capacity loss. There are three major modes of degradation:

1. Large crystals of Ni (hydr)oxides with Ni oxidation states above 2. These large crystals form because the battery is never fully discharged in the car. Due to their low surface-to-volume ratio, they cannot support higher discharge currents, but can still support low discharge currents. At low discharge currents, these large crystals will be eventually converted into Ni(OH)2, and this way the larger crystals will be broken up into smaller entities, which, during the subsequent cycles, will be able to support higher currents. Another name for this phenomenon is memory effect.

2. Formation of γ-NiOOH. The normal isoform of the NiOOH in the cell is β, which is readily converted into the Ni(OH)2 during discharge. γ-NiOOH, which slowly accumulates as battery ages, is converted to Ni(OH)2 with more difficulty, and only after all the β isoform is gone. γ-NiOOH accumulates because the battery is never fully discharged in the car.

Note added in edit: γ-NiOOH is bad in two more ways. It crystalizes with considerable amount of water, which sequesters water from the electrolyte, making the electrolyte less conductive. The consequence is increased internal resistance. When γ-NiOOH is eliminated following deep discharge, this sequestered water returns back to the electrolyte, and internal resistance decreases.

Sequestration of water in γ-NiOOH dramatically increases its volume. Whereas the β-NiOOH<=>Ni(OH)2 transition is associated with only 1-2% volumetric change, the γ-NiOOH<=>Ni(OH)2 transition alters the volume by whopping 40%. It is therefore much more destructive to the active layer of the electrode.

3. The active Ni (hydr)oxide layer of the positive electrode may lose contact with Ni foil support, or flake off, which takes it out of commission. The charge-discharge reactions on the positive electrode involve small, yet consistent volumetric expansion and contraction of the (hydr)oxide layer. This eventually causes material cracking and flaking, which in turn reduces the capacity of the electrode. γ-NiOOH exacerbates flaking of the active layer.

Ultra deep discharge remedies conditions 1 and 2 by eliminating the large crystals and γ-NiOOH. Interestingly, ultra deep discharge may also remedy the condition 3. As the cell gets below 0.25V, the nickel metal support of the positive electrode forms a galvanic pair with the negative electrode, and conversion of nickel into Ni(OH)2 begins:

Ni - 2e- = Ni2+
Ni2+ + 2OH- = Ni(OH)2

As Ni2+ ions are formed, they immediately react with OH- ions (from KOH) to form the insoluble Ni(OH)2, which is deposited at the electrode. Thus, the lost active layer of the positive electrode is replenished.
 

·
Moderator
Joined
·
6,934 Posts
Very interesting stuff. I wish I understood it. :D

So here I go giving my secrets away again, but I'd like to see if you have any input on this.

In my experience, deep discharging doesn't do as much to actually recondition a stick as charging does. Deep discharging is great for balancing sticks/packs, but all of the magic seems to happen during charging. And not just any charging...

I've basically stumbled upon this by accident. The hobby chargers take 4-8 cycles to do what our custom home-brew equipment can do in a single cycle, and what a 350mA charge seems to struggle to do at all.

It appears that the material within a cell that hasn't been used in a long time has very high IR. The longer it has been inactive, the higher the IR. It also appears to produce excessive gas - it's very easy to vent cells when charging them in this area, if you're not careful.

A picture is the best way to illustrate this:



The notations are for Charge 1. You can very clearly see the low IR area the car had been using, because the weakest stick/cell was only allowing ~1500mAh of pack capacity. Then comes the area that hadn't been used in quite a long time, and finally the area at the top that likely had basically never been used.

This problem is self-reinforcing, because the odd IR curve affects the voltage curve, causing an earlier positive recal than you would have otherwise seen.

After reconditioning, you can see the dramatic improvement in the IR curve in Charge 2 - returning the stick to ~90% of normal.

/runs to patent office

But anyway, what could possibly be happening, chemically, here? I think it's something new, possibly unexplored and I think definitely not understood, not unlike how Huggins explores the [previously misunderstood] overcharge induced voltage depression in his book.
 

·
Administrator
Joined
·
10,799 Posts
I suppose the obvious question then Eli is what regime you used to achieve that improvement. ;)

I tend to seem most improvement when cells are aggressively cycled during charge and discharge say 1C at least, but when doing that you have to avoid cell reversal or excess heating.

On another point is cell venting only associated with excessive cell heating?
If it is, then individual cell temp detection may be required for the ultimate stick cycler.

As it's not easy to detect individual cell voltages we have sort of adopted the cheap slow low current pack discharge techniques because it is likely to cause least damage when cells do reverse. Ditto when charging slow and steady means cheap CC led chargers can be used, but it needs long periods and it is probably not as effective as it could be.

My conclusion is that when stick cycling is it needs to be at the individual cell level, with a purpose built rig, several of which we have seen on here. Separate high current supplies/load for each cell or using the bypass technique with diodes and/or relays to short cells when they reach 0V thus protecting them from going negative when discharging at 1C currents.

I suggest charge termination for each cell be based on simple cell temp detection when charging at say 1C.

e.g
Charge at minimum 1C (6.5A) and terminate when cell temp reaches 35-40C
Discharging minimum 1C (6.5A) and terminate when cell reaches 0V or 0.5V if using diode bypass.

After cycling several times (3-4) using the above then a final high current 70A~ capacity and IR test can be performed to see how the stick has fared.

We perhaps ought to design an IC stick cycling rig that can be built easily by members using some of the newer techniques discussed on this thread. Sounds like a project for the new year. :)

A single stick cycling setup is still going to take around 8-10 hours per stick for four decent cycles, so you need a lot of time or multiple rigs.

Just my two pence.. Have a good xmas.
 

·
Premium Member
Joined
·
1,069 Posts
Very interesting stuff. I wish I understood it. :D

So here I go giving my secrets away again, but I'd like to see if you have any input on this.

In my experience, deep discharging doesn't do as much to actually recondition a stick as charging does. Deep discharging is great for balancing sticks/packs, but all of the magic seems to happen during charging. And not just any charging.......
Eli,

I'm trying to understand what you are saying here, and I grasp some of the concepts....

....But after all the discussion about how important deep cycling is....

....now I have a wrench thrown into my understanding....?

Do you happen to have graphs that show the effect of deep cycling versus your proposed method? That may help.

Jim.
 

·
Registered
Joined
·
227 Posts
Discussion Starter #212 (Edited)
In my experience, deep discharging doesn't do as much to actually recondition a stick as charging does. Deep discharging is great for balancing sticks/packs, but all of the magic seems to happen during charging. And not just any charging...
Eli, you may be right here. So far, we only focused on what happens at the bottom end of the cycle, and neglected the issue that might be even more important - what happens at the top end. This forum provides ample evidence for the fact that full charging alone (better known to the members as "grid charging") improves balance and extends battery life. By analogy with deep discharge, it appears like some "crud" accumulates in the battery if it is chronically undercharged, which is the way the car uses the battery. Grid charging helps converting this "crud" back into the active matter. How efficient grid charging is in doing this remains an open question.

I tried to search for the information about what this "crud" might be, but got only some hints and not anything definitive, as in the case of deep discharge. Nickel (II) hydroxide, Ni(OH)2, is much less electrically conductive than NiOOH, and this is why cell's internal resistance is highest when it is discharged. There are 3 known isoforms of Ni(OH)2: α, β, and amorphous. α-Ni(OH)2 is unstable and, if formed, it is rapidly converted into the β isoform. β is the good stuff and the predominant isoform in a healthy battery. I couldn't find any information as to whether or not the amorphous Ni(OH)2 forms in the battery. It is reasonable to suggest that the "crud" is either large crystals of the β-Ni(OH)2, or the amorphous Ni(OH)2. The "crud" is refractory to conversion into NiOOH during charging, and special charging regimens are indeed needed to completely get rid of it.

Unlike deep discharging, which is relatively easy to do, and which poses no danger to the battery as long as high current reversals are avoided, full charging is difficult to achieve in a safe manner. The problems are lack of reliable indication of full charge, overheating, gas formation, and increased self-discharge at higher voltages. The last but not least is that overcharge causes rapid buildup of the γ-NiOOH.

I've basically stumbled upon this by accident. The hobby chargers take 4-8 cycles to do what our custom home-brew equipment can do in a single cycle, and what a 350mA charge seems to struggle to do at all.
So, seems like you have found a good full charge regimen. I thought that a rapid 0.5C charge to 9.00V followed by 10-12 hour 300-350 mA trickle is what foots the bill. During the trickle charge, stick voltage typically stays at ~8.9V, so overcharge is avoided, and long time at this relatively safe voltage facilitates dissolution of the crud, which might be a slow process.

It appears that the material within a cell that hasn't been used in a long time has very high IR. The longer it has been inactive, the higher the IR. It also appears to produce excessive gas - it's very easy to vent cells when charging them in this area, if you're not careful.
This is exactly what large crystal or amorphous Ni(OH)2 is - high resistance. The longer the battery has been inactive - in its top range - the more consolidated the crud gets and the more difficult it is to dissolve it. Trying to force the inherently slow process of dissolution of the high resistance material inevitably leads to higher voltages, buildup of O2 and H2, and venting.

Your graph represents the phenomenon nicely. The blue area is where a brand new battery still has good active material, but because it is not being cycled, it gradually turns into crud. As more crud accumulates, the BCM
keeps downward adjustment of the useful area, which leads to even more crud accumulation, and you start getting into the brown area. The process is self-perpetrating, and it is further exacerbated by the fact that the BCM brings all sticks to the lowest common denominator.

But anyway, what could possibly be happening, chemically, here? I think it's something new, possibly unexplored and I think definitely not understood, not unlike how Huggins explores the [previously misunderstood] overcharge induced voltage depression in his book.
I tried to speculate with the crud theory, but this is what it is, speculation. Please take it with a grain of salt.
 

·
Registered
Joined
·
227 Posts
Discussion Starter #213
Eli,

I'm trying to understand what you are saying here, and I grasp some of the concepts....

....But after all the discussion about how important deep cycling is....

....now I have a wrench thrown into my understanding....?

Do you happen to have graphs that show the effect of deep cycling versus your proposed method? That may help.

Jim.
Jim: Eli is suggesting (and I agree with him) that full charging may be as important, or even more important than deep discharging for the battery restoration. You might find more in my answer to his post.
 

·
Registered
Joined
·
6,196 Posts
One important aspect of the deep discharge process that I think gets lost or got lost in the 'conversation' over the last couple years is the extent to which positive results have come from balancing the cells - in a 'raw' capacity-to-capacity, Ah-to-Ah sense - vs. from a substantial, material change in the cells' chemistry... Eli mentions above how deep discharge is good for balancing - and I think that aspect is at least as important if not more important than any other changes that happen during the process...

This makes me question a few things, like the extent to which grid charging balances cells - like, if it looks like we're only getting a good balance after a deep discharge and a grid charge, why is that? Why doesn't the charging alone do it? If we grid charge a number of times, and then, we try the deep discharge and get much better results, better pack performance, can't we say it's the deep discharge that did it?

Lots of questions...

In my earlier experimentation I was mostly dealing with full sticks, not looking very closely if at all at the cell-level. You can't get a clear view of the substantial changes without looking at the cell-level - because basically you're not looking at the unit that actually experiences the changes; instead you're seeing results that include differences among the 6 cells, which isn't something you want if you're trying to understand the impact of deep discharge, or whatever aspect of how Insight NiMH works...

I did a handful of cell-level tests that seemed to support the idea that 'super' deep discharge - going below the typical 0.9V threshold - helps a lot more. But it's hard to pinpoint cause and effect with these things, because you've got to charge back up in order to discharge to test say capacity or whatever metric you're using to measure performance - so was it the charge back up or was it the deep discharge? What's more, most "0.9V" threshold cycling around here has been done at the stick level - so most of the work people do isn't even testing that threshold; it's testing a stick-level cycling regime that terminates the discharge cycles when the device sees - not 0.9V for each cell, but rather, 0.9V X 6=5.4V total (and then, that's not even accurate, because the devices have a lot of lead resistance so the voltage isn't accurate)... This brings me back to the balancing idea - that a lot of the stick cycling people used to do, using a 0.9V threshold or what-not - doesn't work very well because you're never actually getting a decent balance of all 6 cells; you're just cycling back and forth until the charger device sees your aggregate voltage thresholds... It can help, but not nearly as much as if you drained each cell and really did complete, full cycles...

Etc etc...
 

·
Registered
Joined
·
227 Posts
Discussion Starter #215
This makes me question a few things, like the extent to which grid charging balances cells - like, if it looks like we're only getting a good balance after a deep discharge and a grid charge, why is that? Why doesn't the charging alone do it? If we grid charge a number of times, and then, we try the deep discharge and get much better results, better pack performance, can't we say it's the deep discharge that did it?
According to the Crud Theory ;), there is bottom crud and top crud, which are not the same. Deep discharge and grid charge take care of the bottom and the top crud, respectively. Both are needed for the fullest restoration, and each one by itself yields only a partial result.

a 'raw' capacity-to-capacity, Ah-to-Ah sense - vs. from a substantial, material change in the cells' chemistry
I believe that this is just one thing rather than two separate things. Restoration by deep discharging, or grid charging, or both, brings about substantial change in battery's chemistry, which results in closer-to-the-original capacity, internal resistance, and other parameters, which, taken together, improve battery's balance.
 

·
Moderator
Joined
·
6,934 Posts
I think both charging and discharging are pertinent to the maintenance of these cells. You can't just do one or the other and expect good results, much like you can't just exercise and be healthy - you also have to eat a healthy diet.

"Things" happen during both that can be good, depending on the context.

There is more to this theory. I'm not sure if it's actually "unused material", because not all sticks exhibit this phenomenon; only good sticks from out of balance packs. Packs that would otherwise be quite good, without the bad players. That's opposed to a 15 year old pack that is just uniformly worn out. If it were just "unused material", then all sticks would be like this to certain extents, since the car never charges them to full.

I think it actually has to do with the way the car uses the batteries, when the pack is in this state. I do think it relates to being charged over and over and over again, in the same areas.

But I can't explain why deep discharging doesn't "fix" this, like we suspected it would. It doesn't seem to do anything to it at all; the cell IR while charging still climbs precipitously once you get into the areas that haven't been exercised in a long time...

It's really interesting stuff. And I do know the aspect of my equipment that does the reconditioning. Forgive me for not giving it away. :p Let's just say that the motion in my ocean meets small craft advisories. :D
 

·
Registered
Joined
·
6,196 Posts
....I believe that this is just one thing rather than two separate things....
In general, yeah, that's the way I see it too, or at least it's hard to separate the two. But what I'm getting at here is that, in the car environment and over time, some cells don't get charged as much as they could be charged if they were charged alone, simply because other worse-off cells have triggered charge termination. Or perhaps different self-discharge rates come into play, too. So the cells end up with this 'raw' 'capacity-to-capacity' imbalance. Just charge the cells to the same level and maybe you wipe out a good deal of the problem, at least in the short term...
 

·
Registered
Joined
·
227 Posts
Discussion Starter #218
And I do know the aspect of my equipment that does the reconditioning. Forgive me for not giving it away. :p Let's just say that the motion in my ocean meets small craft advisories. :D
I can try to guess, but you don't need to answer yes or no. It might be a desulfation type of charge - low duty cycle high voltage pulses. This is what breaks down the stubborn crud in lead acid batteries.
 

·
Registered
Joined
·
227 Posts
Discussion Starter #219
In general, yeah, that's the way I see it too, or at least it's hard to separate the two. But what I'm getting at here is that, in the car environment and over time, some cells don't get charged as much as they could be charged if they were charged alone, simply because other worse-off cells have triggered charge termination. Or perhaps different self-discharge rates come into play, too. So the cells end up with this 'raw' 'capacity-to-capacity' imbalance. Just charge the cells to the same level and maybe you wipe out a good deal of the problem, at least in the short term...
Such "pure" capacity imbalance would not be a problem, since the BCM identifies the weakest pair of sticks and then treats all other pairs as if they all have the same low capacity. The real problem is that as time passes, all the unused capacity becomes mired in the crud, and what was bona fide capacity loss in only one section becomes capacity loss elsewhere.
 

·
Registered
Joined
·
6,196 Posts
hmm, I think I see what you're saying... If the problem were only this raw capacity imbalance, then a bulk charge of the whole pack, i.e. a grid charge, should balance the cells... So maybe the extent to which grid charging doesn't help might reflect the extent to which cells are 'crudified'(?)...

One interesting aspect about all this (about limited range cycling=portions of the cell crudifying=bad) is that it supports the idea that periodic grid charging should help keep packs working better: the full range of state of charge gets exercised, doesn't go unused and crudify... [edit: actually, a grid charge wouldn't exercise the full range; it'd only exercise the range above where the car discharges to.]

YET, what do we really know about the correspondence between states of charge and areas of a given cell? What portions and why are those portions 'going unused'? If the car doesn't use the range above 75% state of charge for instance - how does that translate into an actual area of the cell going unused?

What isn't being used? - I guess that's the more direct question...

It doesn't seem like state of charge has any direct spatial relationship to the material structure of the cell, i.e. cycling within the range of say 60-70% doesn't mean you're using the same physical part of the cell over and over (maybe it does, I don't know for sure)... What about the chemistry?

Oh, one other thing I want to add - What about the separator and the electrolyte? What role might a dried-out separator play in weak cells? I know that Mike Dabrowski, IC veteran, has opened up cells and once commented that the old bad cells had dried-out separators. He once said that he thought that was probably one of the major reasons cells begin to fail... Seems like if you did have a venting event, portions of the separator would dry-out sooner than others, maybe resulting in patches... And I know the 'vast online community' believes that cycling 'redistributes the electrolyte' and that's what cycling fixes, supposedly...
 
201 - 220 of 313 Posts
Top