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Thread: NiMH Battery Discharge

  1. #1
    Flashaholic
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    Default NiMH Battery Discharge

    Can a NiMH battery be completely discharged without damaging the cell? If not, how does it damage the cell? Is there a chance of reversing the polarity of the cell?
    Thanks.

  2. #2

    Default Re: NiMH Battery Discharge

    NiMHs should not be fully discharged. If you do, you may not be able to recharge them. Sometimes it's possible to revive overdischarged NiMHs by using a slow "dumb charger" to trickle-charge them, but it's not a safe bet. I haven't heard of overdischarge reversing cell polarity.

  3. #3

    Default Re: NiMH Battery Discharge

    Quote Originally Posted by Optic Nerve
    Can a NiMH battery be completely discharged without damaging the cell? If not, how does it damage the cell? Is there a chance of reversing the polarity of the cell?
    Thanks.
    There's no danger of reversing the polarity of the cell unless it's in a battery with other cells which can drive a reverse current through it after it reaches zero volts. Some manufacturers claim their cells can even tolerate a small amount of reverse charging without damage, although extensive reverse charging can damage a cell by causing it to vent and lose electrolyte. I was recently reading a paper at the Sanyo web site which described intentionally discharging cells to 0.4 volt to restore them after long storage.

    I have found that with NiCd cells, storing for a long period at zero volts seems to encourage dendrite growth, or at least doesn't furnish the energy to blow them as they form. The result is a shorted cell. But I've never seen this with a NiMH cell.

    I don't think I've ever seen any evidence that discharging a NiMH cell to zero volts will damage it, but it seems to persist in this forum as some kind of established lore. I'd appreciate it if someone who promotes this idea could furnish a link to a manufacturer's web site where this alleged problem is described.

    c_c

  4. #4

    Default Re: NiMH Battery Discharge

    1) No.
    2) Lots of info here.
    http://www.duracell.com/oem/recharge...metal_tech.asp
    http://www.duracell.com/oem/Pdf/others/TECHBULL.pdf
    3) Not in a single cell app but yes with multiple cells.

    Mike

  5. #5

    Default Re: NiMH Battery Discharge

    I have discharged a single cell several times to 0V without any negative effects.

    There is, however, one thing to be remembered. If the cell is left too long connected to the load at 0 V it may leak and indeed be damaged. The reason is that at 0 V the pressure inside the cell that keeps the safety vent closed is close to the atmospheric presuure. Thus the valve is not properly sealed and electrolyte may leak.

    So, discharging to 0 V must always be supervised! As soon as the cell reaches 0V it should be disconnected from the load. The voltage should bounce back to around 1 V quite quickly.
    Last edited by NiOOH; 12-28-2006 at 04:57 AM.

  6. #6
    Banned tron3's Avatar
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    Default Re: NiMH Battery Discharge

    FYI: I had a Radio Crap Alkaline reverse polarity in my Electronic Bible (4 cells) thus breaking it. Can't find that model any more.

    I had some NiCads for an OLD VHS-C camcorder which drained to zero. I thought the batteries had gone bad from age because they stopped charging, so I stopped using the camcorder years ago. Turns out the charger is dead. However, upon rigging some wires to the battery terminals, I was able to put a charge back into them.

  7. #7
    Flashaholic*
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    Default Re: NiMH Battery Discharge

    I've had brand new still wrapped leak, read zero volts, etc. Cycling didn't help, all had reduced capacity and some were tossed!
    Bill

    I'm a retired mechanic not a electronic/electrical engineer!

  8. #8

    Default Re: NiMH Battery Discharge

    Quote Originally Posted by wptski
    I've had brand new still wrapped leak, read zero volts, etc. Cycling didn't help, all had reduced capacity and some were tossed!
    Yeah, happened to me as well on a set of Uniross 2 Ah I bought from UK together with their Globetrotter charger. I didn't use the cells for about 6 -7 months. Finally, when I decided that their time had come, I discovered that 2 out of four had leaked a small amount of electrolyte. Both measured 0.0 V. The other two aparently had a bit slower self-discharge and were reading around 0.5 V, they were fine.

  9. #9

    Default Re: NiMH Battery Discharge

    Quote Originally Posted by Curious_character
    ...
    I don't think I've ever seen any evidence that discharging a NiMH cell to zero volts will damage it, but it seems to persist in this forum as some kind of established lore. I'd appreciate it if someone who promotes this idea could furnish a link to a manufacturer's web site where this alleged problem is described.

    c_c
    Thank you for posting this!!! I've read the Sanyo, Duracell OEM and Energizer documents along with a ton of other stuff and still thought over discharging an NiMH cell below .9 V caused damage.

    I went through the Sanyo and Duracell documents and you are correct about them not documenting damage from overdischarging NiMH except through reverse voltage in a multi cell app. They warn about over discharge and point to a later section but that section only describes damage from reverse voltage.

    Both documents do mention that one of the factors that affects cycle life is depth of discharge but I can't find anything that specifically covers discharge below .9 V. So some effect probably exists but it's obviously far more minor than I thought.

    Thankfully my long standing misconception has had little personal practical significance until the last year or so as all of my NiMH was multi cell. It's definitely relevant to me now though as I have several single cell lights running on NiMH.

    Again, THANK YOU for educating me! Frankly I feel a little stupid for not picking up on this myself but I'd MUCH rather feel stupid than remain ignorant.

    I was impressed enough to start reading some of your other posts. I'm STILL waiting for a P1D CE shipped on 12/19 (4 7s says give it another week ) but also have a PP 4AA Lux and a L0P SE like you. I also just got a used Extech light meter and found your Lux ("throw") readings very useful in checking the Extech's calibration. I hate to take this off topic but was wondering if you had a Lux reading on your L0P SE handy. Mine always seemed very dim on a ceiling bounce test compared to an L1P etc and the meter seems to confirm that. Your values on other lights are very close to those posted on flashlightreviews.com and others I've seen and I'd appreciate another data point. My apologies if you posted it previously and I missed it. My readings on the other lights we both have are all within a few per cent but my L0P is only about 60% of the Lux value on Doug's site.

    Was this a typo, when you posted "My standard P1 is nine times as bright as my L0P SE on high" and you actually meant vs your P1D CE? That seems to be the case based on flashlightreview's values.
    http://www.candlepowerforums.com/vb/...18&postcount=7

    Mike

  10. #10
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    Default Re: NiMH Battery Discharge

    Thank you guys for all the information.

  11. #11

    Default Re: NiMH Battery Discharge

    Quote Originally Posted by Mike abcd
    . . .Was this a typo, when you posted "My standard P1 is nine times as bright as my L0P SE on high" and you actually meant vs your P1D CE? That seems to be the case based on flashlightreview's values.
    http://www.candlepowerforums.com/vb/...18&postcount=7

    Mike
    No, I meant my standard P1. (I have both a standard P1 and a P1D CE.) My L0P SE disappointed me because it was much less bright than reviews indicated. A query I posted brought mainly the response that light meter measurements aren't to be trusted. Feeling that I could improve the throw by altering the focus, I went to extreme efforts to get the thing apart, without success. However, somewhere in the process, apparently due to heating, the throw did improve, with the lux level increasing by over 30%. I concluded that the heating caused the LED to move slightly, improving the focus.

    As I originally got the L0P SE, the lux at 1 meter (high setting) 5 minutes after turnon was 94 with alkaline, 101 with e2 lithium, and 101 with NiMH. After my abortive efforts to get it apart which included heating several times with a hair dryer, the lux at 1 meter with NiMH was 137. I routinely make repeat measurements within a couple of percent, so the increase is real. It's still not as bright as I had expected, but a definite improvement over the original. (Don't try this at home, kids. I got lucky, but you probably won't.)

    I want to thank you very much for your posting. People have various motivations for posting, but mine are to learn and to share what I've learned with others. Sometimes it's hard to tell if the postings have any effect. It's responses like yours that make it worthwhile.

    c_c

  12. #12

    Default Re: NiMH Battery Discharge

    c_c,

    Wow, your L0P SE seems to have been about 40% or less of what flashlightreviews measured. In your position, I would have sent it back. Mine is about 60% based on the medium setting. I'll have to recheck on high but it seemed more difficult to check there because the level was less stable at the higher current being pulled.

    I share your motivation on posting although I probably show less desirable personality traits more than you. I've found all your posts to be informative, well thought out and accurate.

    If you found my appreciation encouraging, your thank you deserves a polite You're Welcome. I guess we can put it down as one of my more useful contributions here.

    I hope you see fit to continue your valuable contributions here. I, for one, find them very helpful!

    Mike

    PS measured the same L0P SE PWM rate as you using a tachometer used to check prop/rotor speed I use with my r/c helis.

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