NiCD discharging

Discussion in 'The Projects Forum' started by PlayaSlaya, Apr 24, 2010.

  1. PlayaSlaya

    Thread Starter Member

    Jun 22, 2009
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    Is there any way of quickly discharging a NiCd battery, other than just applying a load and waiting for it to discharge?

    Is there any certain output voltage that a NiCd has for when it is considered discharged? I'v done some reading and came up with an output voltage of 1V for a 1.2 V NiCd cell. Is this value accurate?
     
  2. Bychon

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    Mar 12, 2010
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    The charts I have show the Ni-cad as deep into its death dance at 1.1 volt.

    As for discharging it...A ni-cad battery is full of energy stored in chemical molecules. There is a certain amount of energy in there and it can't be destroyed. You can let it discharge through a load at some rate. I suppose you could dip the battery in a tub of mercury, and that would be fast, but the battery would probably warp or burst into flames. Other than letting the energy out slowly, I would have to guess, "magic wand" or get one of those alien space craft that drain car batteries when they pass overhead.
     
  3. retched

    AAC Fanatic!

    Dec 5, 2009
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    I would also recommend the space craft. ;)

    Batteries do not discharge like capacitors. If you discharge them too quickly, they get hot, then, the chemical reaction inside continues and "makes" more power.
     
  4. Bychon

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    Mar 12, 2010
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    What? No Harry Potter fans here?
     
  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    No.

    NiCd batteries are particularly prone to thermal run away compared to lead-acid batteries. They can catch on fire if discharged or charged improperly. They stopped using them in Lear Jets for that reason. I had one almost catch on fire in my pocket once when it shorted on some loose change. (That was years ago.) You must monitor both the current and temperature; although, you do not necessarily have to use an electronic means to monitor temperature.

    This link and its part 2 have a lot of information on batteries of all types: http://www.batteryuniversity.com/partone.htm

    John
     
  6. rjenkins

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    Nov 6, 2005
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    The 'around 1V per cell' limit is important if you are discharging a battery of several cells.

    When discharging a battery pack, if you go too deep, at some point the weakest cell will become reverse polarised due to the continuing current from the better capacity cells, and this can cause permanent damage.

    If you are working with individual cells, you can discharge them competely if required to restore capacity. I bend the leads on a ten ohm resistor so the wires are a spring fit across the cell and leave them for a few days.

    It's purely a 'first aid' measure to clear crystallisation from the plate material (and remove any NiCad memory effect), but for this it can be very effective.

    As far as possible, you should ensure all cells or batteries are used, with a full charge and a full (1.1V) discharge at least once a month.

    As long as you use either an 'intelligent' charger with auto cut off (not just a timer), or a trickle charger, you do not need to discharge the battery before every charge.

    Also note that the cells that need a proper discharge are the ones that appear to be low capacity - the internal resistance goes high due to crystallisation in the plate metal and the cell can then simply not give any appreciable current - they appear to be flat on any normal load. Once they are in this state, the 'several days' discharge is the only way to truly discharge them
     
    Last edited: Apr 24, 2010
  7. retched

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  8. jpanhalt

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    Hi Robert,

    A few days at 10 ohms is certainly well beyond the capacity of any NiCd cell I have used. I did a short search and could not find a single reference that recommends that deep of a discharge. Do you have some reference for that?

    BTW, I have used zapping for cells in non-critical applications.

    John
     
  9. rjenkins

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    Nov 6, 2005
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    The so called memory effect of NiCad cells is caused by cold crystallisation of the plates.

    If they are regularly charged after only a partial discharge, or left permanently on trickle charge, the deeper parts of the plate crystallise.

    The amorphous metal is reactive and the cell works normally to it's regular discharge depth. Once they are discharged to the crystalline part of the plate, the cell resistance goes through the roof and it appears to be flat in any normal use.
    (This is the 'voltage knee' effect referred to in other documents).

    The only way to recover cells from this is to leave them to discharge completely at whatever rate they can manage.

    Once recharged after that, the amorphous plate material is reformed and cell works as it should.

    The other problem with NiCads is that needle crystals can form, usually when they are left flat, which can pierce the cell separators. These tend to grow during charging so continuously shorting the cell.

    That's when a high current zap helps, it fuses the crystals causing the short.

    Once they are fully charged, a regular low rate trickle charge reforms the plate surface and prevents the surface needles forming.

    NiMH cells don't seem to suffer from the short circuit problem, and although they are not supposed to have the 'memory effect', I have found that the deep discharge technique significantly improves the capacity of cells that are often left on charge (like in cordless phones that are left on the charger base most of the time).

    If you have any cells that work but don't seem to have their proper capacity, try it!

    Edit: I again stress this is for SINGLE CELLS - discharging a NiCad or NiMH battery below the 1.1V/cell point will almost certainly damage or wreck it, unless you can access both ends of each individual cell and discharge each seperately...
     
    Last edited: Apr 24, 2010
  10. retched

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    Isnt that called Reverse charging? (I know.... that would be draining ;) ) When there is more power in the anode?

    When you have multiple cells and one dies, the others charge the cell backwards, that is what can cause the crystallization. So Ive read.

    When you charge a battery, you put power INTO the anode, but when you are discharging a pack of batteries in a normal load process, the dead cell gets power INTO the cathode.

    Any of this sound familiar?
     
  11. thatoneguy

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    Feb 19, 2009
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    If dealing with individual Cells, it is safe to deep discharge down to 0.9V at 1C. So if you have a AA NiCd, typical capacity of 1500mAH, discharge at a max of 1.5A until cell voltage is 0.9V, or temperature peaks.

    During discharge, the temp will peak faster when the cell isn't at full health.

    When working with a pack of cells, use the 1V/cell rule of thumb. I've repaired many packs of 6 cells where they were "Dead", but only 1 of the 6 cells were bad, but VERY bad. 1 cell would have reversed polarity, with the rest of the cells putting out nearly normal capacity. The one reversed cell makes the entire pack be "Bad"

    As far as rejuvenating NiCd batteries, once it gets to the point the cell need to be "shocked" to dissolve an internal crystal short, this isn't always a fix. If capacity doesn't ramp up quickly in 3 cycles on a battery conditioner, they probably aren't ever going to recover.
     
  12. jpanhalt

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    That is what I believed too. However, rjenkins has proposed using a 10 ohm resistor short for a few days. I am not as concerned about the rate of discharge as by the depth of discharge that would give. Going through a typical calculation, 10 ohms should give approximately 100 mA. A few days (take 3 as a minimum) is 7200 mA hours. That is several times the capacity of common, NiCd single cells.

    I tried a search on the question of depth of discharge for single cells and came up with nothing experimental. Even some ".edu" sites just repeated the same dogma about not over discharging, as apparently you and I still believe.

    So, is there any experimental evidence to support extremely deep discharge to rejuvenate individual cells?

    John
     
  13. thatoneguy

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    Here is the datasheet for Energizer NiMH cells:
    http://data.energizer.com/PDFs/nh15-2500.pdf

    Some kind of load-disconnect would be needed, whether it is at 1V or 0.9V, pulling a single cell below that can damage the chemistry.

    2 diodes in series with the resistors would put about 1.2V on them, unless germanium can be found. One each of Si and Ge diodes, both in series with the resistors, would effectively be a 1V cutoff.
     
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