Overdischarge of NiZn Cells

Greetings Everyone

There have been a few recent threads where the subject of overdischarge of NiZn cells (namely PowerGenix AAs) has been brought up. I have not done any overdischarge tests on these cells, and likely won't have time in the near future to do so. However, I was traveling today and used some idle time on the plane to look into the matter from a more fundamental perspective.

In order for any cell to be considered "overdischarged", the potential of one or both electrodes must change enough that detrimental side chemical reactions are activated. So let's look at the potentials of both the electrode charge/discharge reactions for the NiZn cells. All of the half-cell potentials given below are versus the standard hydrogen electrode (SHE):

Positive (nickel) electrode:
2 NiOOH +
2 H2O
+ 2 e → 2 Ni(OH)2 +
2 OH- E=0.45 V

Negative (zinc) electrode:
Zn
+ 2 OH- → Zn(OH)2 +
2 e E=-1.25 V

The difference between the positive and negative electrodes is 0.45-(-1.25)=1.7V. This is the open circuit cell potential.

The two side reactions that one want to avoid during discharge of a NiZn cell are hydrogen evolution on the nickel electrode and oxygen evolution on the zinc electrode. Those reactions are given below for an alkaline (pH=14) solution:

Hydrogen evolution:
2 H2O +
2 e → H2 +
2 OH- E=-0.83 V

Oxygen evolution:
2OH- → 1/2O2 + H2O + 2e E=0.40 V

So, the potential of the nickel electrode must decrease from 0.45V to -0.83V to generate hydrogen, and the zinc electrode must increase from -1.25V to 0.40V to generate oxygen.

I have not confirmed this, but it is almost certain that NiZn cells are cathode-limited, i.e. they have excess zinc. This means that on discharge, the nickel electrode runs out of capacity first and will begin to drop. Hydrogen gas will begin to be generated on the nickel electrode when that electrode drops to -0.83V, which will occur at a cell voltage of (-0.83V-(-1.25V)=0.42V!

So if the cell is discharged below 0.42V and current continues to flow through the cell, then hydrogen is generated at the nickel electrode and the zinc electrode continues to discharge as normal until the zinc capacity is depleted. Once that happens, the zinc potential will rise. If the cell is not in series with other cells, then the zinc potential will rise to -0.83V and the cell voltage will be 0V and nothing further will happen. But if the cell is in a series string and current continues to flow, then the zinc electrode will rise to 0.40V and oxygen will be generated at the zinc electrode simultaneously with hydrogen at the nickel electrode. This will occur at a voltage of -1.23V.

Let's summarize. There are three stages of discharge for a NiZn cell. Stage 1 is normal discharge. Stage 2 starts at a cell voltage of 0.42V when the nickel electrode is depleted and hydrogen evolution occurs. Stage 3 starts at a cell voltage of -1.23V when the zinc electrode is depleted and oxygen evolution occurs concurrently with hydrogen evolution.

I should point out that there are also three stages of discharge in a NiMH cell. Using a similar analysis to that given above, Stage 2 starts at -0.1V and Stage 3 starts at -1.23V (note that Stage 3 starts at the same voltage for both NiMH and NiZn because the same reactions are occurring). So for a NiMH cell, you must drive the cell to a negative voltage in order to initiate overdischarge.

Also, in a NiMH cell, the hydrogen generated in stage 2 is absorbed by the metal hydride alloy, so the internal pressure of the cell stays relatively low. There is no such internal mechanism to absorb hydrogen in a NiZn cell, so hydrogen generated on overdischarge results in an increase in internal pressure. Same for the oxygen generated in Stage 3.

Ok, with me so far? Now let's look at how much hydrogen and oxygen are generated.

The amount of hydrogen and oxygen generated is directly proportion to the current being passed through the cell. During Stage 2 discharge, hydrogen is being generated at a rate of 14ml per amp per minute. During Stage 3 discharge, oxygen is being generated at a rate of 7ml per amp per minute (in addition to the hydrogen generation). So if you are discharging your series string of NiZn cells at 1 amp, and one cell drops to <0.4V, hydrogen is being generated at a rate of 14ml/minute. If that cell is driven into reversal to -1.23V, you are generating 14ml H2 + 7ml O2 per minute (please note that I am assuming 25 degrees C and 1 atm for these gas volume calculations).

You can see that the internal pressure of the cell can rise rapidly if a NiZn cell is overdischarged. The vent may even activate, resulting in loss of water (both liquid electrolyte and as hydrogen and/or oxygen gas). You will note in the positive electrode reaction above that water is consumed in the discharge process, so if you lose water, you lose capacity.

I will now summarize this for those of you whose eyes glazed over my rambling above. Overdischarge of a NiZn cell should start with hydrogen evolution on the nickel electrode when the cell potential drops to <0.42V. This is Stage 2 discharge. If the cell is driven into reversal down to -1.23V, then both hydrogen and oxygen will be generated. This is Stage 3 discharge.

Take home message: keep those NiZn cells above 0.42V!

NiMH cells are more robust with respect to Stage 2 discharge damage because Stage 2 starts at a lower voltage (<-0.1V), and the pressure will not rise because hydrogen is being absorbed by the metal hydride alloy.

Please note that this "paper" exercise was conducted with no testing. These are only estimates of the actual voltages where overdischarge will occur. I have also made the assumption that there is excess zinc in the NiZn cells, and although I think that this is a good assumption, I could be mistaken.

That being said, I believe that the results of this exercise are consistent with the experiences of NiZn users that have been posted on various threads in this forum.

Questions? Discussion?

Hope this was all clear. I kind of did brain dump here.

Cheers,
Battery Guy

ALL HAIL THE BATTERY GUY:bow::bow::bow:
Clear enough wish i could nock out a mental wad such as that

no i got lost quick
dont most of these batteries have Loads apon loads of "plates" on both sides, not nessiarily running out of it from any single discharges , BECAUSE , them plates things are always rotting (well chemically) away into oblivion through the life of the item, so i dont see where in most of the batteries that the plates themselves start to wane away and there is no "seed" for the power flow Etc (hey its just stuff i adsorbed through my electrolytes i really dont know squat).
if there is no plates, and the actual plates arent a coating on a conductor, then there is no conduction, with no conduction the battery wouldnt last AS the plates continue to rot away into oblivion (well not really oblivion , but where they are not usefull for the elecroplating reactions as well).

so in my theroetical fantasy of miss-knowleges, the plates will never be Gone right away, neither of them, something that would take many cycles or much and much time or damages. sooo that is why i got lost fast. dont believe it, nope.

On the other hand IF the plates were gonna rot away i think your accurate that one would go long before the other for sure.

if i was to assume something in the chemical reactions was very limited from seeing this stuff apart, i would think it would be the water/liquids the electrolyte stuff, because relative to say a wet cell, these things are dry, to me they seem way too dry, but then again less to leak or spew on a vent releacing.. Bah Safety :-(

i think attempting to forumlate some precision chemical reaction that ends with all the items used up in the normal rolled battery is non-existant, reguardless of the charged or discharged states. i think it is more like a few of the chemicals are starved (starved to death). Trying to create a scenario where the chemical reaction is mole for mole exacting and loosing one of them send the whole thing into oblivion, is not taking its physical properties into existance and trying to work it out on paper. pitri dish.

In other words, yee hast hurt your head for nothing BUT your probably right about most of it anyways. i just got lost fast in the idea that there is some sort of precision to the chemical numbers, that the reactions are matched as it would be in a single test tube reaction done in class. when inside there is Lots and lots of this stuff (whatever it is) and mere drops of the other stuff (whatever it is).

Users have demonstrated that this cell type cant handle high rates of discharge, as they have internally broken some easily, when the main conductor fails that easily, and is as usually poorly connected to some psudo plate thing that is impregnated stuff in a cheap slurry cant we just assume that again the ideas on paper for making this stuff with least expencive materials , and the user using them in applications they are not designed for , that the piece of junk is just failing? or is that to easy
how many of these are not working that were treated like gold? and how many are being pushed , just because the higher voltage will push some things harder?

and better yet, why not dissasemble a ruined one, and see what is ruined? i dont think you will need an electron microscope to see what is going ary, although one could be usefull in creating more wild assumptions as to why it will fail via the USER when eyeballing it with a 30x loupe will probably tell you why it failed via the Manufacture.

Disclaimer, told ya i dont know squat, but its fun anyways.

plus just to display my vast ignorance of the subject, as soon as any oxygen starts hanging around in the cell, its going to oxidise the metals.
Which reminds me of the oxygen suckers used in freezy dried foods.
those things with basically metal shavings suck the oxygen out of stuff so agressivly as to compress the packaging they are in. where does the oxygen go?? right into the metals (i assume) turning them immediatly into metal oxide non-conductors.
WHICH
i would say was a really big problem if we already didnt know that videos showing these putzes making batteries will not do it in a vaccume
not that i want to spend $5 on a cell item, but you would think with all the precision math of the chemical reactions (again) that they would make them in a vaccume (and mabey some do).

i just think (again) that there is a lot of cheapskating going on, so they can have the CEOs millions, and the store markup, and the dealer markup and the representative and the 500 people doing nothing in a big office building, to pay the 75 laywers to protect a patent that will be obsolete before they get out of court, and toss in another percentage to make the pretty packaging on the outside , instead of making gods gift to a battery and chemical perfections. Priorities man, this world has magic technology in huge quantities, going into one thing, making that green stuff we use for exchange.

and you wouldnt need a loupe to see that even its the money isnt it.

VidPro said:

... but its fun anyways.

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Hey VidPro

It is indeed fun.

But I am concerned that the intent of my original post/rambling diatribe was not clearly stated. It was not my intent to blanket all NiZn failures as being caused by overdischarge, as I fear it may have come across.

Allow me to attempt to clarify.

I have been reading more and more posts from people who have experienced problems with the PowerGenix AA NiZn cells. There appears to be a growing concern regarding their durability, and a lot of speculation as to whether this problem is inherent in the design, variability in manufacturing quality, etc... There has also been some speculation that these cells might be damaged by overdischarge, but as yet there has not been put forth any data on what constitutes overcharge with these cells, or why they may be more susceptible than our beloved NiMH cells.

The intent of my original post was to address the overdischarge issue by putting some numbers around it, and to see if there are any fundamental reasons why NiZn cells would be more easily overdischarged and more easily damaged by overdischarge than NiMH cells.

Cheers,
Battery Guy

yes very cool.

I think i would have to create the discharge scenario and see if "running them flat" even kills them first.
and if pressure buildup of any sort is the result there are ways to observe if it has vented.
or even if its getting pressure in it.
a pressurised cell item will have very slight bulging. as can be measured with simple calipers or seen via the bottom. depending of course on container. every larger cell that i have ever had vent harshly (meaning without easy pressure releace) has had external signs.

mabey that could be observed by somebody who has a high-resistance or failing one?
bulging tailcaps in watertight light items?
hard to unscrew light as pressure is built up?
and possibility of microfarts when opening light item?

it is also more likly that a higher voltage cell item used in normal things (designed for alkies) will get a reverse charge faster than a similar lower voltage cell item, just via the maintaing a higher voltage potential on it's teammates (via the bulbs or curcuits).
Plus the cell is not a "LSD" type cell AND users have observed large capacity differances even without shelf time, meaning I can assume that occurances of reverse charge are more likly . they arent similar in capacity even in quick testing.
Plus, the users reported the energy dropping like a rock quickly at the end of discharges.
all things which would be contributors of a full on reverse discharge.

does that go along with the theory that the voltage potentials can get the gasses flowing , like your scenario? ; and then some.
it would not be the first battery that gasses, and then Really gasses when reversed. (again me not having really any idea as to what all the gases are other than slightly understading the breakdowns of water)

VidPro said:

plus just to display my vast ignorance of the subject, as soon as any oxygen starts hanging around in the cell, its going to oxidise the metals.

Click to expand...

Well, yes and no.

When a NiMH cell goes into Stage 3 discharge and oxygen is generated at the metal hydride alloy electrode, the alloy begins to rapidly oxidize/corrode. This is why Stage 3 discharge absolutely kills a NiMH cell and should be avoided at all costs.

However, all of the other metals in the cell are pretty well passivated due to the very high pH of the electrolyte. By "passivated" I mean that they have a super thin, tenacious oxide or hydroxide film that prevents further oxidation. Just like a film of chromium oxide keeps stainless steel from rusting.

VidPro said:

Which reminds me of the oxygen suckers used in freezy dried foods.
those things with basically metal shavings suck the oxygen out of stuff so agressivly as to compress the packaging they are in. where does the oxygen go?? right into the metals (i assume) turning them immediatly into metal oxide non-conductors.

Click to expand...

Dude, you just lost me with that one.

VidPro said:

WHICH
i would say was a really big problem if we already didnt know that videos showing these putzes making batteries will not do it in a vaccume
not that i want to spend $5 on a cell item, but you would think with all the precision math of the chemical reactions (again) that they would make them in a vaccume (and mabey some do).

Click to expand...

I don't know what video you are referring to.

Lithium batteries and lithium-ion batteries are made in dry, moisture free environments and the electrolyte is usually vacuum filled (they pull a vacuum on the cell, and let the electrolyte be sucked in, thereby getting out all of the nasty oxygen and moisture that can wreak havoc in lithium and lithium-ion cells).

Even your standard, disposable alkaline batteries are marvels of electrochemical science and engineering. You might be very, very surprised just how precise and pure materials need to be to work in your run-of-the-mill alkaline batteries. For example, the zinc used is 99.9999+ pure. The "alloying" elements are added at less than 200ppm. Cell poisons such as molybdenum need to be kept at levels less than 10ppb to prevent corrosion of the zinc, gassing and leakage. The manganese dioxide in the same batteries has to electroplated...yes electroplated...in near boiling sulfuric acid in order to produce just the right nano-crystalline, porous structure to work reliably. And the list goes on.

Batteries are a big business and there is a lot of competition. This competition drives cost and innovation just like any other industry. While there are still some factories in third world countries that pour raw manganese ore into starch lined zinc cans to make cheap carbon-zinc cells, these are the exception, not the rule.

VidPro said:

i just think (again) that there is a lot of cheapskating going on, so they can have the CEOs millions, and the store markup, and the dealer markup and the representative and the 500 people doing nothing in a big office building, to pay the 75 laywers to protect a patent that will be obsolete before they get out of court, and toss in another percentage to make the pretty packaging on the outside , instead of making gods gift to a battery and chemical perfections. Priorities man, this world has magic technology in huge quantities, going into one thing, making that green stuff we use for exchange.

Click to expand...

Man, I want some of what you are smoking tonight.

And I think that we are way off track of the original topic.

So, movin' on back to overdischarge of NiZn cells.....

Cheers,
Battery Guy

VidPro said:

yes very cool.

I think i would have to create the discharge scenario and see if "running them flat" even kills them first.
and if pressure buildup of any sort is the result there are ways to observe if it has vented.

Click to expand...

Keeping in mind that simply shorting the cell to zero volts is not nearly as bad as driving it into overdischarge at high currents in a multi-cell string.

The best, most reliable way to determine leakage is weight loss, but you need a good scale with mg accuracy. Bulge is probably not going to do it for you, as this is unreliable for a cell that has a resealing vent.

VidPro said:

or even if its getting pressure in it.
a pressurised cell item will have very slight bulging. as can be measured with simple calipers or seen via the bottom. depending of course on container. every larger cell that i have ever had vent harshly (meaning without easy pressure releace) has had external signs.

Click to expand...

You can give it a go. Just remember that the measured bulge will increase and decrease as the vent opens and closes. Also, when measuring bulge, be sure not to short out the cell with the calipers! You can't imagine how many people I have seen do that.

VidPro said:

...and possibility of microfarts when opening light item?

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Wouldn't that have more to do with what they ate for lunch?

VidPro said:

Plus the cell is not a "LSD" type cell AND users have observed large capacity differances even without shelf time, meaning I can assume that occurances of reverse charge are more likly . they arent similar in capacity even in quick testing.
Plus, the users reported the energy dropping like a rock quickly at the end of discharges.
all things which would be contributors of a full on reverse discharge.

Click to expand...

Well stated. I agree on all of the above. You have the amazing ability to turn on and off your lucidity seemingly at will.

VidPro said:

does that go along with the theory that the voltage potentials can get the gasses flowing , like your scenario? ; and then some.
it wouldnt be the first battery that gasses and then Really gasses when reversed. (again me not having really any idea as to what all the gases are other than slightly understading the breakdowns of water)

Click to expand...

Indeed, I think that the issues you just brought up, including fast voltage fall-off, variability in capacity and variability in self-discharge would all make it easier to overdischarge these cells. These issues, combined with the fact that overdischarge occurs at a higher voltage than NiMH, and that it has the potential to do more damage when it does occur, all brings home the message that you should be extremely careful to not overdischarge these cell.

Cheers,
Battery Guy

check out the videos of the lithium battery plates being rolled on YouTube, somewhere in the process they start wrapping it with a plastic (like) seal, but we all know how quick an exposure some metals will snag thier O2s. i seen it it was exposed, ya precisions, mabey this was the china factory and why the batts suck so bad.
check out what we can see , its out there.

sure precision everywhere, then they open a plant in malaysia , get recycled zinc from china and save a nickle per pound.
anyone who can do such amazing chemical math certannly knows about realities of life. when your TP runs out do you quit wiping i AM being real.
there is nothing that we dont agree with, sure they have all lab grade stuff, me agrees, then they have Humans , as simple as that.

pull it apart, and tell me that YOU think that is exactally how you would connect the battery up for example, stress it out and tell me if YOU making it only for yourself with no reguards to costs would not see some things in there that might be "improved". humans, corporations. reality , as real as chemical math is. do all cell phones work correctally with all thier features Heck NO! , does all software come free of bugs? LOL, is all my spelling and grammer correct Rolf. humans are real, even the ones that made enough errors to meltdown chernoble, everyday proving that flaws and humans are more normal than chemical enthropy, and "good enough" often gets the job done for many.

O2 packs for food storage, simply metal dust in them, they are amasing, and so simple. http://www.sorbeadindia.com/html/oxygen-absorber.html

i think i sort of understand the plates metals being sealed in thier electrolytes. covered, but crack it open, they seem to be much dryer than say I (knowing nothing) would make it for myself, but then i would not be restricted by 27 countries safety stuff, and billions in lawsuits, so i would make a crasy insane unsafe battery that worked better. they would make a battery for consumers that pass goverment and accident injury lawyer standards, and they can (even) morally unload on other humans. Any battery i made i wouldnt even hand my worst enemy but it could potentially be way more potent than safe.

me smoking stuff
hey you used the word competition in todays society, whos smoking what check out the first Pan in the movie "Manufactured Landscapes"
Almost all the media in america from paper to data is owned by 5 huge corporations.

how many entities go to space in america ?

we have batteries with 50 labels and one manufacture.

computers are dominated by 2 OS types, the internet is "free" it all runs through hubs that are operated by One entity.
there are 2 (count them) processors used in desktop computers.

Proctor and gamble sells food ???

the goverment has deemd that there exist competition (anti-monopoly) if ONE other entity exists, even if it doesnt represent competition for the one conglomerating and merging.

just how many NI-Zi packagers are there even, let alone manufactures?

go to a fast food corner for variety , and what corporation owns/franchises 6of the8 fast food places you have to choose from?

what you smoking, competition in todays world is "owned by the competition". and if not they are selling to them.

VidPro said:

me smoking stuff
hey you used the word competition in todays society, whos smoking what

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Fair enough.

VidPro said:

just how many NI-Zi packagers are there even, let alone manufactures

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Only one, and they are struggling to survive.

Points well taken.

...moving back on to NiZn discharge now.......

Sounds like a job for Super battery man Silverfox, to do one of his exhaustive and well documented and so precious and well reguarded long term battery tests , nobody does it better :thumbsup:

they said that ni-mhy would croak if discharged below ~.9v , but i have not found that to be true. although Stating that AS the parameters, ensures (better) that no ni-mhy in a series pack (of the many used) would be reverse charged.
the specs keep it alive always, but IMO single ni-mhys could be discharged without any huge immediate problems, just not reverse charged, and the "info" was almost "wrong" perpetuating that they would croak magically from deep discharges, they didnt As Far as i could tell.
BUT
if asked i would say .9V, because then a pack will survive.

Is it possible that a Ni-Zn chemistry does not croak the moment it droops below some specked out voltage.
Like Li-ion will it survive and work fine afterwards, but Will be hurt in the long run?
Like Ni-Cd will it thrive on it? (even though many ni-cd dischargers would never discharge that far).

i have not read any consistant USE data on failed or non failed NiZn to make any guesses about its reality (without reverse charge occurances) stuff just isnt USED like that very often, AA things are often used in teams. flash guns, multi cell lights, cameras.

here is one of the spec sheets
http://www.powergenix.com/docs/powergenix-specs-aa.pdf
do you see any warnings like li-ion has? and why isnt a Minimum discharge spec listed? or a warning or anything? or is it:shrug:

all i see is If it will not "accept" a charge anymore it is bad.
and test parameters that dont do anything Other than stop, when the cell is basically out of juice.

The Xellerion battery has five times the cycle life of these batteries, which cannot generally achieve more than 100 deep cycles. Nickel-Zinc can be continuously deep-discharged to 100% for over 500 cycles without damage to the cell. Deep discharge Lead Acid batteries will lose their rechargeability if they are discharged beyond 80% of their capacity.

this is of course Thier technology versions of it they are discussing.
http://www.xellerion.com/TECHNOLOGY_03.htm
and is about as usefull as a Press releace

http://en.wikipedia.org/wiki/Nizn
no indication of such limitations in the wiki.

sure they managed to stop all the problems that this battery has had for 100 years using the new technology . . . but really did they? on the first few rounds of consumer sales?

i would say tetering on its new legs it gives a pretty good run, but mabey it wont win the kentukey derby just yet :tinfoil:

Batou00159 said:

Clear enough wish i could nock out a mental wad such as that

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... and as do I.

impressive work, and discussion!

Thanks, Battery Guy. That was very clear, informative, and interesting.

c_c

VidPro said:

Sounds like a job for Super battery man Silverfox, to do one of his exhaustive and well documented and so precious and well reguarded long term battery tests , nobody does it better :thumbsup:

Click to expand...

No argument from me on that one

VidPro said:

they said that ni-mhy would croak if discharged below ~.9v , but i have not found that to be true. although Stating that AS the parameters, ensures (better) that no ni-mhy in a series pack (of the many used) would be reverse charged.

the specs keep it alive always, but IMO single ni-mhys could be discharged without any huge immediate problems, just not reverse charged, and the "info" was almost "wrong" perpetuating that they would croak magically from deep discharges, they didnt As Far as i could tell.
BUT
if asked i would say .9V, because then a pack will survive.

Click to expand...

I agree on this point as well. There is no reason that discharging a NiMH cell to 0.0V is any different from discharging it to 0.9V. As I pointed out in my original post, Stage 2 discharge where hydrogen is generated at the nickel electrode begins around -0.1V, so you need to take the cell into reversal before you should see degradation due to overdischarge.

VidPro said:

Is it possible that a Ni-Zn chemistry does not croak the moment it droops below some specked out voltage.
Like Li-ion will it survive and work fine afterwards, but Will be hurt in the long run?
Like Ni-Cd will it thrive on it? (even though many ni-cd dischargers would never discharge that far).

Click to expand...

Sure. Brief excursions below 0.4V may not hurt the cell at all. It has everything to do with how much and how fast the hydrogen is generated. If the current going through the cell is relatively low, and the cell does not stay in Stage 2 discharge for very long, the hydrogen generated during overdischarge will recombine with the nickel electrode when you charge the cell back up. The degradation only happens when you generate hydrogen fast enough to cause the cell to vent (thereby effectively losing precious water) or if the generation of gas in the electrode causes physical damage.

VidPro said:

i have not read any consistant USE data on failed or non failed NiZn to make any guesses about its reality (without reverse charge occurances) stuff just isnt USED like that very often, AA things are often used in teams. flash gund, multi cell lights, cameras.

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Nor have I seen any good use data. The chemistry is too new to the consumer market, and nobody has published any good studies.

VidPro said:

here is one of the spec sheets
http://www.powergenix.com/docs/powergenix-specs-aa.pdf
do you see any warnings like li-ion has? and why isnt a Minimum discharge spec listed? or a warning or anything? or is it:shrug:

all i see is If it will not "accept" a charge anymore it is bad.
and test parameters that dont do anything Other than stop, when the cell is basically out of juice.

Click to expand...

Nope, don't see any. But keep in mind that lithium-ion has been around in the consumer market for almost 20 years now. Also, overdischarge of a lithium-ion cell can lead to serious safety problems, not just performance problems. So from that perspective, specifying a low voltage cut-off is much more important.

Since the PowerGenix cells are sold to the consumer market, most people using these cells are probably going to use them in devices that take up to 4 cells in series at the most. Also, most of these devices will probably work on alkaline cells, which means that the discharge currents are relatively low. The risk of damage due to overdischarge is much lower. So it does not surprise me that PowerGenix does not mention overdischarge on the spec sheet.

However, many of the folks reading this forum are not typical battery users, as they have flashlights with 4+ cells in series with high discharge currents. The more cells in series, and the higher the discharge current, the more risk there is of overdischarge damage.

I should note that I have nothing against the PowerGenix AA NiZn cells. In fact, I love the performance I have been getting from these cells. My intent here is to shed some light on the limitations of these cells with respect to overdischarge, and perhaps explain some of the observations that have been posted with regards to performance problems linked to discharging to low voltages.

Cheers,
Battery Guy

VidPro said:

The Xellerion battery has five times the cycle life of these batteries, which cannot generally achieve more than 100 deep cycles. Nickel-Zinc can be continuously deep-discharged to 100% for over 500 cycles without damage to the cell. Deep discharge Lead Acid batteries will lose their rechargeability if they are discharged beyond 80% of their capacity.

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Keep in mind that "deep-discharged to 100%" does not mean overdischarge or discharge to 0V per cell. It means that the battery was cycled to 0%SOC, which for these NiZn cells is somewhere in the 1.0V to 1.4V range, depending on the load.

VidPro said:

this is of course Thier technology versions of it they are discussing.
http://www.xellerion.com/TECHNOLOGY_03.htm
and is about as usefull as a Press releace

Click to expand...

Yes, I am familiar with these, although I have not actually got my hands on one yet. Same chemistry as the PowerGenix, but radically different cell design. These might even be flooded batteries, similar to deep-cycle lead-acid. If that is the case, then you can overdischarge them to your heart's content because they would be designed to be topped off with water on a regular maintenance schedule. It appears as though they are positioning themselves to compete directly with the deep-cycle lead-acid battery market. If so, then people using these batteries are already familiar with the proper maintenance of flooded batteries, so requiring occasional water top-off would not be a big disadvantage.

Cheers,
Battery Guy

Battery Guy said:

......It means that the battery was cycled to 0%SOC, which for these NiZn cells is somewhere in the 1.0V to 1.4V range, depending on the load.

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This is something that is never very clear no matter where you read about it. Are you suggesting that PowerGenix cells are at 0% SOC when their OC voltage is somewhere below 1.0-1.4 Volts, or their voltage under load is in that range?

For example, I accidentally ran a single PG NiZn cell down to 0.9 Volts recently in an AA light. This was indicated by the lights own voltage checker. I didn't mean to go that far, but as has been mentioned, NiZn cells run down really fast near the end, and I'm sure it'll likely happen again. When I checked the cell voltage after removing the cell from the light, it was ~1.50 Volts. So, what I'm asking is, was this cell over discharged? I think not, but hey, these NiZn cells are new to me!

Dave

Hi BG,

I read your interestin opening post with the calculation of the Nernst equation for Ni-Zn cells, you are correct.

I'm higly sceptic that Ni-Zn rechargeable cells may find a widespread use, but not because the inner working of these cells is not understood or followed. The major problem of the Ni-Zn cells is the fact that they, indeed, contains Zinc.

I have bought some roasted peanuts jar, with a very visible marking on the label: "WARNING: Contains peanuts". I believe in full honesty that such warning should be affixed to any battery, of primary or secondary type, containing zinc.

Let me elaborate on this concept. Zinc is a common and relatively cheap metal, to express myself with simple words, containing a lot of energy. This is why is widely used in batteries. It has drawbacks, too. The major problem of the zinc metal is the fact that it is corroded both from acidic and alkaline electrolites. Scientifically is called an amphoteric material.

Hystorically, zinc was used only in zinc-carbon batteries, which used a almost neutral electrolyte (ammonium chloride) and the amphoteric behaviour was of no importance.
The major improvement in mass-marketed battery was the invention of primary alkaline batteries, which used a different electrolyte and a different mechanical construction compared to the zinc-carbon batteries. The electrolyte used was potassium hydroxide, which being strongly alkaline reduced dramatically the internal impedance of the battery, vastly improving its efficiency under heavy loads.

...but. The alkaline electrolite was "eating" the zinc inside the battery in few hours. The solution provided from the experts was adding some mercury to the zinc. Some metals, including gold, when mixed with mercury at ambient temperature, forms an "alloy" with outstanding and very desiderable physical and chemical properties if compared to the original metal. The scientific name of this "alloy" as a"amalgama".

With the "green revolution" of the '90, mercury, lead and cadmium - which are highly toxic and produces accumulative damages to body's organs and defeat the immune system - were earmarked for removal from mass-marketed items. Alkaline batteries were among the first items to be made compliant with the new green rules.
Let me state that there is no real substitute to the mercury. Manufacturers have tried the most diverse chemicals to reduce the oxydation problem (the most preeminent is the addition of zinc oxyde), together with the use of buffer chemicals, to mantain a neutral pH in proximity of the zinc.
The results have been a mixed bag. The real fact is that an alkaline battery will leak, it is only matter of when, not if. SOme manufactures mantain an expiration date of three years, some other up to seven years. In my personal statistics, mercury-free alkaline batteries are at danger of leaking two years after production date.

What is the problem with the zinc and the alkaline electrolyte? I dont' want to get in the explanation of the amphoteric behavior, which involves electronic affinities and other intricate theoretical mechanisms. The fact is that, over a long time stretch, the behavior of the zinc becomes unforeseeable.
This unforeseeability becomes compounded in nichel-zinc batteries, mainly because of two factors: the presence of impurities, and the rechargeability, which amplify the negative effects produced from those impurities.

You will say, the manufacturer of alkaline or Ni-Zn is not able to produce pure enough chemicals, so they stick firmly to the original blueprint of the battery?

The answer is: no. With zinc, in my experience, it is not possible. Even the slightest trace amount of impurity, not measurable with the most sophisticate lab equipment, will found a way to stimulate the amphoteric behaviour of the zinc.

Because of its strange electronic affinities, zinc is not the correct material to be used in a rechargeable battery. Cycling gives to each battery its own properties in terms of self-discharge and capacity, and this gap increases with the time, and with the number of cycles. And it is not under the control of the battery manufacturer.
Even with the best use of amalgamas, and not with the funny anti-oxydizers and buffers, zinc cannot provide more than two-hundred cycles.

With the onset of Eneloops and quality Li-Ions, we are accustomed since few years to reliable batteries with repeatable behavior and hundreds of cycles. My first tought, to whom feels marveled from the "strange" disparities observed in Ni-Zn rechargeable batteries, has been to stick a warning sign, as I said at the beginnign of this post "Warning contains zinc" so no further complaints are accepted...

Ni-Zn batteries are earmarked to replace lead-acid batteries in vehicles, on the day that the green lobby will beat the automakers lobby.
I don't see that day yet, because an automotive Ni-Zn battery will have a slighter low voltage, requires a more complicate charge circuit, will add 100 - 200 $ to the cost of a car, and will last two years instead of three... but. It will take tons of lead out of the environment.

This said, I personally don't see the Ni-Zn consumer cells to gain a significative share of the market, unless the manufacturers don't find a way to fix the problems which are just coming to surface now. With the established Eneloops technology, I see it as a bit of a problem.

Regards

Anthony