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2 Lithium Manganese Dioxide cells in parallel. ?

Discussion in 'Electronic Repair' started by N_Cook, May 11, 2013.

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  1. N_Cook

    N_Cook Guest

    Well nearly , the 2 negatives joined and 2 epitaxial diodes (avoiding mutual
    discharge possibility) from the positives commoned and then on to the
    memory, is there some circuitwise reason? Both cells are from the same batch
    , so not as though "all your eggs in one basket" circumvention.
    Is the chemistry of this type of cell odd? 3V cells originally , could be 20
    years, but removed from the pcb and out of circuit still 3V. Put 150K across
    either cell and it drops in voltage to 2.9V or so compared to just DVM
    loading. Remove and immediately recovers to 3V.
    Tried 5K6 over for 12 hours , dropped to 2.6V , measured a few minutes ago,
    removed the R and immediately recovered to 2.9V, I expect when I get back to
    it , it will be back to 3V with just DVM load
  2. I'm not sure what your question is. Or if you have a question.

    3V lithium coin cells are commonly used to back up memory, including 5V
    systems. They can last for years; I've never seen two used to add capacity.

    I have no idea why such a light load (150K) would cause the voltage to
    immediately drop by 3%.
  3. N_Cook

    N_Cook Guest

    I've only ever seen a single cell or if multiple ones, then in series. On
    first confronting , much confusion. Realised they were tied together but not
    realised in parallel.
    Each one measured 3V , in turn, but put the DVM over the other 2 ends then
    near enough 0V, scatching head.
    Put DVM on resistance scale and measured a varying low resistance of 20 to
    60 ohms , varying slowly like across a large C capacitor , but no large
    capacitors present. The approximately 0 ohms between 2 nearly equal 3V cells
    was being interpreted as low ohms.

    When I get replacement cell/cells? (why spend out on 2 ?) I'll try the 5K6
    over night and see what effect on a known good cell , or at least new from
    the supplier. These cells were 1/2AA size Sanyo CR14250SE if that is
    relevant, ie not the usual PC large button cells
  4. N_Cook

    N_Cook Guest

    For the same reason that Boeing should have stopped them being incorporated
    on their recent product fiasco?

    Both batteries are tanged and would require desoldering/soldering to replace
    either. If designed to allow one failure then I'd expect at least for them
    to be from different batches if not different suppliers, both same make and
    batch number here
  5. I'm taking about lithium coin cells used in battery-backed
    NASA was "nuts" about such things. It was worried about outgassing from the
    rubber feet on HP calculators. These were removed.
  6. burst lithium cells do smell truly awful.

    those things are up there in space though, so maybe nasa has exceptions,
    or just doesn't care about all of them.
  7. well, rubber does outgas, crappier types more than others, but how do you
    know which is which? It seems like an odd but sensible thing to do.
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