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Thread: Overdischarge of NiZn Cells

  1. #31
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    Default Re: Overdischarge of NiZn Cells

    Quote Originally Posted by Battery Guy View Post
    You make good points about replacing AA lithiums with NiZn in many applications. But I would ask why not replace the AA lithiums with Eneloops? Another way to ask the question is how many consumer applications would the PowerGenix be better than Eneloops? My guess is not very many.

    In my mind, it is more of a "PowerGenix vs Eneloop" question than a "PowerGenix vs Lithium AA" question.
    I never understood this "PowerGenix vs Eneloop" comparison. If Eneloops work for your device, then sheesh just use them and quit trying to make up arbitrary silly contests between an apple and an orange and whining when the apple loses.

    The point is NiMH cells don't always work properly, especially in older devices. I dunno, maybe all of you replace all your electronics every 2 years. I tend to run things until they die. My digital camera does 1024x768 (wow, that was a lot of pixels back in the day!) and it will not operate worth a darn with NiMH. My brand new 6ch hobby radio will also warn me of low voltage after practically no use. I could go on and on and on with real examples that I own.

    You will say, "But... but... that means your devices aren't draining your alkalines down to the last drop! They are faulty!!" Umm ok. I'll agree that they don't drain the cells down to the last drop, but whether that means the designers committed some great sin in requiring a minimum cell voltage in order to be able to use 4 cells rather than 5 (!?) is debatable.

    And this may come as a shock to some of you here, but not everybody has a regulated light. Hell there are also these things called incandescent bulbs.

    As I see it, comparing a AA LSD NiMH to a AA NiZN is about as valid as comparing it to a LiIon 14500. Unless you are designing your own device or hacking an existing one, it is a meaningless comparison, and hardly falls within the bounds of "consumer application".

  2. #32
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    Default Re: Overdischarge of NiZn Cells

    Quote Originally Posted by VidPro View Post
    A fat Zinc Air primary would be cool in standard sizes too :-)
    You mean like this?

    But before you get your hopes up, you should know that I have never actual seen one of these batteries. Also, this website has not been updated since I discovered it two years ago. I did see a product brochure for these once, and I seem to recall that they had about 3x capacity of a standard alkaline, but only at very low discharge rates.

    Quote Originally Posted by VidPro View Post
    just like 45/70 there is some stuff that the extra voltage is SOOO usefull.
    ...long list of useful applications snipped...

    Fair enough. I think that PowerGenix needs to hire you to lead their marketing department.

    Cheers,
    Battery Guy
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  3. #33
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    Default Re: Overdischarge of NiZn Cells

    Quote Originally Posted by VidPro View Post
    A fat Zinc Air primary would be cool in standard sizes too :-)
    More on those Aeternus zinc-air batteries that are seemingly unattainable can be found here.

    Cheers,
    Battery Guy
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  4. #34
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    Default Re: Overdischarge of NiZn Cells

    Quote Originally Posted by Battery Guy View Post
    You mean like this?

    But before you get your hopes up, you should know that I have never actual seen one of these batteries. Also, this website has not been updated since I discovered it two years ago. I did see a product brochure for these once, and I seem to recall that they had about 3x capacity of a standard alkaline, but only at very low discharge rates.



    ...long list of useful applications snipped...

    Fair enough. I think that PowerGenix needs to hire you to lead their marketing department.

    Cheers,
    Battery Guy
    We made battery packs from AA sized cylindrical zinc air cells for the Land Warrior program a number of years ago. Unless you get a fair number of cells in a pack with series / parallel construction, they just aren't very usable at high discharge rates.

    Also, there's the fact that your device would need a bunch of big holes in it for the cells to get the air they need to work properly...

  5. #35
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    Default Re: Overdischarge of NiZn Cells

    Quote Originally Posted by Battery Guy View Post
    More on those Aeternus zinc-air batteries that are seemingly unattainable can be found here.

    Cheers,
    Battery Guy
    thanks, i figured they would not have high output, but mabey it would beat stacking a rack of the small ones.
    there is an app(lication) for that , mabey i can sell them on Nitch marketing :-)

  6. #36
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    Default Re: Overdischarge of NiZn Cells

    Quote Originally Posted by core View Post
    I never understood this "PowerGenix vs Eneloop" comparison. If Eneloops work for your device, then sheesh just use them and quit trying to make up arbitrary silly contests between an apple and an orange and whining when the apple loses.
    I didn't think I was whining. I thought we had finally honed in on a useful discussion.

    Quote Originally Posted by core View Post
    The point is NiMH cells don't always work properly, especially in older devices. I dunno, maybe all of you replace all your electronics every 2 years. I tend to run things until they die. My digital camera does 1024x768 (wow, that was a lot of pixels back in the day!) and it will not operate worth a darn with NiMH. My brand new 6ch hobby radio will also warn me of low voltage after practically no use. I could go on and on and on with real examples that I own.
    The discussion in this thread somehow turned towards the topic of whether or not there is a real market for NiZn AA cells. I didn't think that there were many applications where the PowerGenix cells would be advantageous, but VidPro offered several that I had not considered.

    If you have applications for NiZn, then great! I want this technology to succeed and be improved upon. I just fear that in its present form, there is not going to be sufficient market to keep a company afloat. Hopefully I am wrong.

    The discussion was not "which is better, Eneloop or PowerGenix". Eneloop and NiMH are established chemistries. For a new battery chemistry to get a foothold in the market, it needs to have clear, obvious advantages over the established competition. Or the company needs to have a fantastic marketing machine behind it. I just don't think PowerGenix has either of these. Like I said, I hope I am wrong.

    Quote Originally Posted by core View Post
    You will say, "But... but... that means your devices aren't draining your alkalines down to the last drop! They are faulty!!"
    Nope, I will not say that.

    Quote Originally Posted by core View Post
    Umm ok. I'll agree that they don't drain the cells down to the last drop, but whether that means the designers committed some great sin in requiring a minimum cell voltage in order to be able to use 4 cells rather than 5 (!?) is debatable.

    And this may come as a shock to some of you here, but not everybody has a regulated light. Hell there are also these things called incandescent bulbs.
    My mods are mostly incans, and I love my Mag 2D mods that use 7 NiZn cells in series with a 1185 bulb.

    Cheers,
    Battery Guy
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  7. #37
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    Default Re: Overdischarge of NiZn Cells

    Quote Originally Posted by Battery Guy View Post
    For a new battery chemistry to get a foothold in the market, it needs to have clear, obvious advantages over the established competition.
    I think "Hey, my device works properly with these, I can finally use rechargeables with it!" is a pretty clear obvious advantage. Are there enough of these devices out there to keep a company afloat? Maybe not. If you browse the reviews on Amazon you can find a nice sampling of people who were overjoyed that their cameras were working.

    These favorable reviews were likely written by folks who hadn't owned them for any length of time though.

    And you're definitely right about them not having a marketing machine behind it -- when I have to find out about something like that through a relatively unnoticed thread a forum, that says something.

    I fear you may be right about them not staying in the market for long. Which is really too bad, because I need these. Badly enough to pay $20 every other month replacing destroyed cells.

    And I didn't mean to say that you personally were whining. But there isn't hardly a NiZn thread created without somebody making a direct comparison with Eneloops, whether it be self-discharge rate or capacity or current, and saying something like "seems I should stick with Eneloops".

    That's missing the point. As printed in a big bold font right on the PowerGenix cell, the whole deal is HIGH VOLTAGE. Everything else is secondary, the good and the bad. Lithium primaries would be great too... if they weren't primaries.

    There should be a "Godwin's Rule of NiZn Cells".

  8. #38
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    Default Re: Overdischarge of NiZn Cells

    Quote Originally Posted by core View Post
    I fear you may be right about them not staying in the market for long. Which is really too bad, because I need these. Badly enough to pay $20 every other month replacing destroyed cells.
    Core,

    So I am curious. It sounds like you use the PowerGenix cells a lot, and replace them relatively frequently (every other month). Any comments on what you see as the biggest reliability issue? Do you think that it is simply inherent to the design/chemistry, manufacturing quality problems, the device usage profile, etc....

    Another question: do you have any "old" PG cells that have been used in a similar way, but still operate fine? If so, this might indicate that the design and chemistry is good, but the manufacturing process needs to be improved.

    Oh, and with respect for your need for these cells, you might want to consider buying them in larger numbers and more frequently, and storing your extras in the fridge. I know that it might sound goofy or old fashioned, but the fact of the matter is that chemical reactions occur slower at lower temperature, and this includes the side reactions that cause performance degradation.

    Just don't freeze them. You run the possibility of damaging the seal and the vent.

    Cheers,
    Battery Guy

    Cheers,
    Battery Guy
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  9. #39
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    Default Re: Overdischarge of NiZn Cells

    BG if the original problem with the zinc and the electrolytes exists still, would they fall apart over time even without use?
    i dont (ever) totally understand the interactions destroying the plates or the electrolytes getting contaminated with the metals , but if that problem had not been totally solved (whatever it was) would the problem exist in an unused cell?
    that the internals would degrade over time?

    or was most of the problem returning of the metals back? and the cycles are the biggest issue of the original problem.
    hard pulse charging :-) i'd get them suckers running back quick enough.
    i would try a staccato pulse charge on the thing, 10%on 90%off 10ms 5+C or higher CV charging, and see what that could accomplish. (slam it hard but dont overheat) but i would have to know the max amps through the connections before damage occured. And it would have to be done on every charge not when it is too late.

    could you speculate on a "storage" voltage ?? high low or medium?
    like they say charge every month?, and the more charged the less stuff in the electrolytes or something?
    Last edited by VidPro; 07-03-2010 at 05:55 AM.

  10. #40

    Default Re: Overdischarge of NiZn Cells

    Well Its now official, cell #4 has lost all it's capacity by just sitting there for 2 days. When I took the measurement, voltage idles at 1.28v.

    Furthermore,I did a short and the current went from 3A to 500mA less than 10 second. Cell #4 really took a beating I don't know if this was caused by a polarity reversal during use or something.

    All I can say is cell #4 has been declared end of its life. However I did decided to go ahead and completely drain this cell down to 0V and see if any improvements would happen. Won't find out until couple hours from now.

  11. #41
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    Default Re: Overdischarge of NiZn Cells

    Mario-

    There is no need to go through the trouble draining your cell down to 0V. It will get there on it's OWN in a couple more days!

    Battery Guy would know better than I would, but I'd say there is absolutely zero to be gained by even attempting to do so. I'd say you'd just be damaging it further, but it's basically up a creek now anyway.

    How are you going to charge it afterwards? The PowerGenix charger won't do it. If you connect it to a good cell, I'm actually not sure how long that would take to bring it up to minimum voltage. That thing is going to get hotter than HELL, I would not want to be holding it.

    And it's going to get even hotter still on the charger. My failing-but-not-dead-yet ones get hot enough to cause a warm plastic smell from the charger. Not worth cooking a charger for just yet another bad cell.

    There is no thermal cutoff on the white charger, contrary to what you read in the datasheet.

  12. #42

    Default Re: Overdischarge of NiZn Cells

    I still have the other 3 going but they get hot fast when charging. I am starting to look at these cells as "niche". These cells are beneficial for devices that need the extra voltage,which are probably pulsed drain on those devices like a flash camera split second burst.

    Maybe then these batteries will last as advertised. As for electric motors will also benefit but not necessary as these cells will give the motor that extra punch, and are gentle on the cells if the motor is not stressed.

    Now when you put them in devices that ARE designed for NiMh cells. These devices has a higher potential to wreck havoc on nizn cells, as the cut off voltage is too low resulting of polarity reversal if cells in series are uneven.

    I think that is what happen to cell #4

  13. #43

    Default Re: Overdischarge of NiZn Cells

    Quote Originally Posted by core View Post

    How are you going to charge it afterwards? The PowerGenix charger won't do it. If you connect it to a good cell, I'm actually not sure how long that would take to bring it up to minimum voltage. That thing is going to get hotter than HELL, I would not want to be holding it.
    Quite simple actually, when its charger won't do it the La-crosse can lol.

    When I put cell #4 in the charger the display read 0.64v (don't ask me how that was possible, charger detecting a cell at that voltage, all i know it saw the cell lol.) left it at its default of 200mA, voltage went from 0.64 to 0.85v stood there for about a min.

    Then it gradually start to climb to 0.94v stood there for 5 minutes. 3 minutes later voltage hit 1v stood there for 2 minutes. After a minute or so voltage went up to 1.10 and then started to skyrocket and it went like this

    1.10v> 1.3v> 1.5v> 1.6> 1.7> and finally stabilized at 1.82v.

    Since then voltage been sitting there for an hour charging and I think it just hit 1.83v as i am typing this post lol,and has dumped total of 275mA, and counting so far within the hour. Cell is still cool to the touch. I would want to charge the cell slower but 200mA is the lowest I can go lol.

    Keep you posted. And I had no choice for this particular cell lol.
    Last edited by MarioJP; 07-03-2010 at 04:08 PM.

  14. #44
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    Default Re: Overdischarge of NiZn Cells

    Quote Originally Posted by MarioJP View Post
    Quite simple actually, when its charger won't do it the La-crosse can lol.
    I find this quite interesting. I personally never use the PowerGenix charger, and always charge the cells on my Maccor battery tester using the CC/CV protocol in the PowerGenix spec sheets. I have not yet seen a problem with these cells.

    Can somebody comment on how the LaCrosse charger would be different from the PowerGenix charger?

    Cheers,
    Battery Guy
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  15. #45
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    Default Re: Overdischarge of NiZn Cells

    Quote Originally Posted by Battery Guy View Post
    Can somebody comment on how the LaCrosse charger would be different from the PowerGenix charger?
    The LaCrosse charger is specifically meant for NiMH cells. It is not designed to charge NiZn cells! However, if it never takes the cells above 1.9 V it presumably can't harm them.
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  16. #46

    Default Re: Overdischarge of NiZn Cells

    Did not say charger was meant for these cells but it does charges.

    In any case cell#4 was getting warm to hot and while the voltage was still at 1.8v. I can see why the cells gets hot in the powergenix charger. Voltage does not hit 1.9v mark!


    Update: Well was not able to hit the 1.9 at 200mA rate, as the cell got quite hot. After that this cell became difficult to charge and voltage drop to 1.79. Reason why it was getting difficult because I was getting way too many terminations at random. Does not matter if it was 1 minute or 10 minutes.

    So decided to amp it up once the cell was cooled of course lol. Going from 200mA to blasting the cell at 1500mA charge rate!. Results were no termination, but the cell got too hot to handle and still was not able to achieve the 1.9v goal, instead it was stuck at 1.80v and eventually trip the thermal sensor, causing to display the infamous triple 0's!!.

    Next step put the cell #4 in the freezer!!.

    keep you posted
    Last edited by MarioJP; 07-04-2010 at 12:46 AM.

  17. #47
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    Default Re: Overdischarge of NiZn Cells

    Quote Originally Posted by Mr Happy View Post
    The LaCrosse charger is specifically meant for NiMH cells. It is not designed to charge NiZn cells! However, if it never takes the cells above 1.9 V it presumably can't harm them.
    Does the LaCrosse charger even go up to 1.9V?
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  18. #48

    Default Re: Overdischarge of NiZn Cells

    Quote Originally Posted by Battery Guy View Post
    Does the LaCrosse charger even go up to 1.9V?
    That my friend is what I am about to post lol.

    Results are in!. After putting cell #4 in the freezer for couple of hours' and put back in the charger (while still cold) set the charging current 1A. The cell voltage finally hit 1.9v mark! (Should of taken pictures). And it was not a struggle like last time.

    Voltage started to climb quickly less than a minute and stopped at 1.9v!

    Anyways I can see why, while in the powergenix charger these cells gets really hot!.

    Once the cells are damaged, they become very stubborn. Still not sure however if freezing the cell caused it to hit 1.9v. With that said somebody needs to do an extensive tests on these cells from the day they were new to how they degrade overtime.

    In any case. Unless the powergenix charger has some sort of thermal protection, I would keep an eye out when charging "stubborn" cells".
    Last edited by MarioJP; 07-04-2010 at 09:33 AM.

  19. #49
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    Default Re: Overdischarge of NiZn Cells

    Quote Originally Posted by MarioJP View Post
    That my friend is what I am about to post lol.

    Results are in!. After putting cell #4 in the freezer for couple of hours' and put back in the charger (while still cold) set the charging current 1A. The cell voltage finally hit 1.9v mark! (Should of taken pictures). And it was not a struggle like last time.

    Voltage started to climb quickly less than a minute and stopped at 1.9v!
    Why would the LaCrosse charger even go up to 1.9V? That is very surprising.

    Quote Originally Posted by MarioJP View Post
    With that said somebody needs to do an extensive tests on these cells from the day they were new to how they degrade overtime.
    At present I am doing cycling tests to look at normal cycling and overcharge cycling behavior. I plan to add some overdischarge cycling tests to the test regimen. Hope to have some good results in a few weeks.

    Cheers,
    Battery Guy
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  20. #50

    Default Re: Overdischarge of NiZn Cells

    Quote Originally Posted by Battery Guy View Post
    Why would the LaCrosse charger even go up to 1.9V? That is very surprising.
    More surprising question, why did it detect a cell with the voltage of 0.6??. In any case, let's just say that the firmware on this charger has exploits getting around the "full display" that I can think of 2 exploits already

    But then again the manual makes it sounds its not really a exploit at all lol. It clearly states in bold font letters. this charger is strictly for charging NiMh batteries. Vs will detect primary cells from accidentally being charged. What is it trying to say? that you can but you really shouldn't lol.

    At present I am doing cycling tests to look at normal cycling and overcharge cycling behavior. I plan to add some overdischarge cycling tests to the test regimen. Hope to have some good results in a few weeks

    Cheers,
    Battery Guy.
    This will be interesting to see. I still think these cells are too fragile.
    Last edited by MarioJP; 07-04-2010 at 10:48 AM.

  21. #51
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    Default Re: Overdischarge of NiZn Cells

    Quote Originally Posted by MarioJP View Post
    More surprising question, why did it detect a cell with the voltage of 0.6??. In any case, let's just say that the firmware on this charger has exploits getting around the "full display" that I can think of 2 exploits already

    But then again the manual makes it sounds its not really a exploit at all lol. It clearly states in bold font letters. this charger is strictly for charging NiMh batteries. Vs will detect primary cells from accidentally being charged. What is it trying to say? that you can but you really shouldn't lol.

    This will be interesting to see. I still think these cells are too fragile.
    My guess would be that it determines if a cell is a primary cell by an internal resistance measurement. An alkaline cell will typically be >150 ohms, and a lithium primary will typically be >90 ohms. Compare that to typical NiMH cells that are <50 ohms.

    Based on my measurements, the NiZn cells have an internal resistance comparable to NiMH, so the charger probably cannot tell the difference.

    I still cannot understand why a charger designed for NiMH would go up to a voltage of 1.9V.

    Cheers,
    Battery Guy
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  22. #52

    Default Re: Overdischarge of NiZn Cells

    Quote Originally Posted by Battery Guy View Post
    My guess would be that it determines if a cell is a primary cell by an internal resistance measurement. An alkaline cell will typically be >150 ohms, and a lithium primary will typically be >90 ohms. Compare that to typical NiMH cells that are <50 ohms.

    Based on my measurements, the NiZn cells have an internal resistance comparable to NiMH, so the charger probably cannot tell the difference.

    I still cannot understand why a charger designed for NiMH would go up to a voltage of 1.9V.

    Cheers,
    Battery Guy
    I really doubt it even checks for resistance for the simple fact that I have NiMh cells that has been rejected by other chargers but has no problem charging in the La crosse, so I would not be surprised if it starts to charge alkaline cells lol.

    and 2 it did in fact started to charge a alkaline cell (whoops lol) when it was discharging a alkaline cell out of curiosity to find out what the capacities are. Lucky it was only for couple of seconds and yanked the cell out lol.

    and for the why this charger hits the 1.9v?. what really is happening is the charger is in fact working like it suppose to as any voltage above 1.40v will display "FULL" automatically. that is iF you go by normal operating procedure according to the manual lol.

    What is happening is when you put a cell, fully charged you automatically see the "FULL" on the display like it suppose to right.

    This is where the exploit or in my opinion could be an exploit comes in.

    You have couple of seconds (first 4 seconds according to manual) to make a change. Within this "time window period" you can override the "FULL" and select the charge rate by pressing the current key, ignoring all conditions, states, of the cell.

    Once you have selected the desired current, controls locks and display starts to flash, and starts charging a fully charged cell lol. Now for NiMh cell the full will kick in shortly as there will be -dv. Could be 5 or 10 minutes later.

    for a ni-zn however it just keeps going for half an hour to an hour before terminating, sometimes does not terminate at all lol, and just keeps going all the way to 1.9v. And if you don't stop it the voltage will continue to climb to almost 2V!.

    And that my friend is exploit #1! you didn't hear it from me

    I also use this exploit to top off my eneloops at 200mA and stop the charge when voltage is between 1.52v-1.53v

    going back to the ni-zn results. This gave me a clue to why your powergenix charger just keeps dumping current on those poor already damaged cells to the point they are hot. Like I stated before. Voltage does not hit where it suppose to for the charge to slow down. I am curious about the other fault protection. What about the time elapse?? and how is the thermal sensor working out?

    If cells gets too hot where you can start to smell the plastic either the sensor is not working or there is none lol. Could be the time elapsed is too long.?? I tested the other 3 cells out of curiosity now, and they easily hit the 1.9v without a struggle or without a fight!!.

    because cell #4 was a ongoing battle! lol.
    Last edited by MarioJP; 07-05-2010 at 12:34 AM.

  23. #53

    Default Re: Overdischarge of NiZn Cells

    Quote Originally Posted by Battery Guy View Post
    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
    Even though it was many years ago, this post lives on and has provided exactly the information I was looking for SEVEN years later.
    Thanks!

    Long Live the old but great posts!

  24. #54
    Flashaholic
    Join Date
    Oct 2006
    Location
    Somewhere west of where you are
    Posts
    165

    Default Re: Overdischarge of NiZn Cells

    Yes, it is excellent and contains useful info about the "point of no return" being 0.4 volts per cell.

    And I also found that I can charge my BPI "2500 mWh" NiZn cells in a Lacrosse while watching them closely.
    It ain't easy being me, but someone's gotta do it.

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