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another odd battery problem

Discussion in 'Electronic Repair' started by William Sommerwerck, Jan 9, 2014.

  1. Any nicad experts out there? This is an interesting one. (I'm not looking for
    a "solution", just an explanation, out of curiosity.)

    Many years ago I bought the Sony TCD-D3 DAT Walkman, which fits a fat pocket.
    I made a few live recordings, as it was a lot less bulky than a processor/VCR
    system.

    The other day I pulled it out to confirm it was still working. I'd bought two
    extra batteries with the unit, and put all three on charge. They all charged
    up, but the third battery wouldn't power the unit. Its end-of-charge voltage
    was nearly 7V (though these batteries are nominally 6V), and the other two
    charged to about 6.4V.

    When I put the third battery on charge again, it rose to about 7.5V, even
    before charge was terminated. I let it sit overnight, and it self-discharged
    (?) to about 5.5V. (The others //did not// show significant self-discharge.)
    It still wouldn't power the unit.

    "Obviously" this battery pack is defective. I'm just wondering if anyone knows
    what's going on.

    It's no big deal, as I have two good packs (which are 20+ years old!), and the
    AC adapter. Also, the packs are "external", so I could (if I wanted) saw open
    the pack and replace the cells. Or simply throw together a homebrew pack to
    fit the DC jack.

    Just askin'.


    "We already know the answers -- we just haven't asked the right questions."
    -- Edwin Land
     
  2. "Dave M" wrote in message
    I assume the discharge is + to +.


    I'll give this serious thought. The worst that can happen is that the battery
    is ruined. (!!!) Thanks.

    I'm still curious about the battery's unnaturally high voltage.
     
  3. Phil Allison

    Phil Allison Guest

    "William Sommerwanker "

    ** You are describing the standard "old age" failure mode of most NiCds.

    1. The internal resistance of some or all the cells in a pack rises - often
    dramatically.

    2. The energy capacity drops by a large factor.

    3. The pack will not hold charge due to internal leakage.

    4. The terminal voltage is depressed, from 1.25V to about 1.2V per cell.

    A single overheating event can result in the same outcome - whether by
    overcharging or otherwise.

    Reason ?

    NiCd cells have a small amount of water in them - lose that and the cell is
    stuffed.



    .... Phil
     
  4. Phil Allison

    Phil Allison Guest

    "Phil Allison"


    .... Phil
     
  5. mike

    mike Guest

    I'd be careful. Energy is proportional to the square of the voltage.
    I use 12,000 uF and 10-20V, mostly because that's what was handy.
    You want to hit the sweet spot where you instantly vaporize the dendrite,
    but generate no more heat than necessary. Too little energy or too
    much energy is worse than just right. That's why suggestions to
    use a car battery are bad. Little chance of getting it just right.
    Way too much chance of getting enough energy to explode something.

    500uF AND 200V is WAY too much.

    The process leaves the hole in the insulator/separator.
    The field density is high and the dendrite wants to grow back.
    And discharge thru the hole causes the high self-discharge rate.

    I've had a lot of success restoring 40 year old F-NiCd's for
    uses that don't require more than a few days storage life.
    But, these days, you can get equivalent capacity in a pair of
    AA NiMH cells.

    Don't discount the safety issues with high voltage.
    The "experts" who look at only part of the problem will call me an idiot,
    because 200V probably won't kill you.
    Two problems with that..."probably" is great for statistics, but
    provides little solace to your family when you're the ONE who died.
    And B, the shock isn't the only problem.
    I've got scars where the muscle contractions that thru me across the
    room caused me to rip my arm open on some sharp object in the path.
    Luckily, I haven't fallen and cracked my head open on the furniture...
    yet...

    Eye Protection is a MUST. Sparks fly.
     
  6. Fine. Nothing new. But why the high voltage?
     
  7. Phil Allison

    Phil Allison Guest

    "William Sommerwanker the Over Snipper form HELL "

    ** All new to YOU fuckhead.

    ** When on charge - point 1 applies.

    Cease charge the pack and the voltage drops almost immediately.

    V = I x R ..........

    Wot an idiot.



    ..... Phil
     
  8. "Phil Allison" wrote in message
    ** All new to YOU, fuckhead.

    I've killed men for less than that.

    I wasn't expecting you to repeat common knowledge about rechargeable
    batteries.

    ** When on charge - point 1 applies.
    ** Cease charge the pack and the voltage drops almost immediately.
    ** V = I x R ..........
    ** Wot an idiot.

    You don't see it, do you? Of course not.

    The same thought crossed my mind. The question is... why should a cell's
    voltage -- which is supposedly determined by the battery's chemistry --
    essentially double (assuming only one cell is bad)? Why should the cell become
    a sort of electrochemical chameleon, trying to match the voltage across it?
     
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