Rejuvenating NiCad cells

Discussion in 'General Electronics Chat' started by jgessling, Sep 12, 2009.

  1. jgessling

    Thread Starter AAC Fanatic!

    Jul 31, 2009
    I've got a Tek 221 portable Oscope that uses 10 NiCad A cells. (flea market find). It has sat for who knows how long and I'd like to get it going. I've run across various ways of "zapping" the cells to get rid of internal shorts and make them usable again. One guy suggests using a welder to provide power, another with a capacitor, and another users just a couple other good cells. (urls below). Do these techniques work? Is the fix permanent or temporary? Using an electric welder seems pretty dangerous, but the other ways seems pretty straight forward. Any opinions?

    On a related note, I just got the Tek 221 service manual (scanned paper in PDF format) from . Those of you in need of manuals might want to check there.

    Using welder -
    Using capacitor -
    Using good cells -

    Thanks, Jim
  2. Bernard


    Aug 7, 2008
    In the early '70s I serviced a large number of pagers, persistant problem was shorted cells. I blew out the shorts with charged cap,maybe about 300 uF @ 200V. a paint dot on each revived cell showed many repeats, so then just discarded shorted cells. Recent reading indicated slow charging increased groth of dendrites.Battery University??
  3. flat5

    Active Member

    Nov 13, 2008
    Replace with the newer technology cells.
    I never heard of an "A" cell. Did you mean AA?
    Look around and you can buy 3 amp AA cells at a very good price.
    I did. You can use the old Ni-Cad slow charger too.
  4. Audioguru


    Dec 20, 2007
    Ni-Cads are old and hardly any company makes them anymore except the Chinese.
    Ni-MH cells are almost the same except have a capacity that is 5 times more.

    A cell with spikes of metal inside that causes a cell to short can have the spike burned out but the fix is temporary because there are many more spikes developed that soon will cause the same short.
    I don't think a Ni-MH cell develops metal spikes that cause a short.
  5. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    'Zap' the cell to start with, then trickle charge with a large cap directly in parallel so any recurrent shorts zap themselves.

    After a cell has been near 1.5V on trickle (< C/10 current) for a few hours, put a resistor (4.7 or 10 Ohm) across it and leave it until completely dead - to less than 0.1V and for at least 24 Hours.

    Repeat for three cycles and you should be able to tell then if they are useable.

    The shorts are caused by cold crystallisation, needle crystals form which pierce the seperators. These grow during charge so may need repeated zaps, but once fully charged a small trickle charge continuously reforms the plate surfaces and removes any needles.

    The 'Memory effect' is removed by the one-cell-at-a-time complete discharge.
    That is again caused by crystallisation of the metal in the plates, but at a deep level. This makes the plates less reactive, so causes a high internal resistance & making them appear flat when any significant current is drawn.

    They can still be discharged, but they will only give a low current. By discharging with a resistor & leaving for a day or two, a complete discharge is guaranteed.

    Note all the dire warnings about never discharging below 1V relate to cells in packs, not individual cells.
    If you try to deep-discharge a pack/battery, the weakest cell gets reverse polarised as the better ones continue to discharge and that will cause permanent damage - don't try to get the last few second use out of your battery drill, you are wrecking the battery...

    Whether Ni-Cad or Ni-MH, fully discharge and recharge (leaving on trickle for a few hours after the basic charge is complete) at least once a month to keep them in working order without serious loss of capacity, and alway store fully charged.
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
    zapping a cell is strictly a temporary fix. I built a zapper using a monster 30,000uF cap to bang the cells. It works, but they will always short again. Once they get internal shorts it means the insulators are bad and they need to be rejuvinated into the garbage can.
  7. jgessling

    Thread Starter AAC Fanatic!

    Jul 31, 2009
    Lots of good information and advice, thanks. Flat5, yes there is an A size battery. I thought I'd try rejuvenating the ones I've got before thinking about changing things. I guess I'm the weird battery guy, I've also got an Emerson model 558 portable tube radio (1947) that uses a 67.5 volt B battery for the tubes. I'll tackle that someday.