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suck battery to zero

Discussion in 'Electronic Design' started by rt, Dec 13, 2004.

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  1. rt

    rt Guest

    For years now you've been able to buy AA powered electronics that
    seem to last forever and use the batteries down to zero.
    What's going on in there? Are they using switching ps?
  2. Anders F

    Anders F Guest

    Definitely! =)

  3. Yes- they step it up to more like 3V in most cases I think. I just got
    an MP3 player for my kid that runs off of a single 1.2V NiMH AAA cell
    for hours and hours.

    Best regards,
    Spehro Pefhany
  4. Mark Jones

    Mark Jones Guest

    Hey Spehro.

    Isn't that killing the rechargeable batteries to run them down that
    far? IIRC, the discharge rate of any battery is non-linear. They
    typically peter out way before 0v.

    Maybe NiMH can be functionally discharged to zero, dunno. I've been
    working with a lithium-ion design for it's good temperature range.
    Typical Li-ion cell voltage ranges from 4.100v +/-1% charged to to
    2.5v "discharged", with voltages outside these ranges being "Very
    Bad"(R). So, stacks of these batteries must use some kind of charge
    equalization circuit or individual charger, otherwise risk
    overcharging and destroying a cell. Constant-current charging
    methodology doesn't help in this regard. Then cells under 2.5v must be
    trickle-charged to gently regain capacity... batteries sure have grown
    complex over the years... :)

  5. Guest

    Word of advice....if you don't want to spend the money on rechargables
    get Alaklines! Don't waste your money on Heavy duty batteries. Yes they
    are cheaper but last like 2 mins hahaha
  6. To be pedantic (and it matters here), a battery is two or more cells,
    normally connected in series. It is definitely a Bad Thing to run a
    battery down to zero, as one cell of that battery will probably reach
    zero before the other(s), and as the other cell(s) continue to
    discharge, the weakest will be recharged with reverse polarity.

    If you are just dealing with a single cell, this reverse charging
    can't happen (but it probably isn't good to discharge a cell to zero

    Peter Bennett, VE7CEI
    peterbb4 (at)
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  7. Ian Stirling

    Ian Stirling Guest

    To nitpick the original point, with a constant power load, you can't
    generally run the battery down below half its original voltage.
    Because at that point, if you try to keep maintaining the same power
    out, the voltage just collapses to zero immediately.
  8. Deep discharge of NiMH batteries is NOT recommended-- it can damage
    the battery. You get hardly any additional operating time below the
    recommended 1.0V per cell end voltage anyway, so it's best to actively
    shut down the circuit at about that voltage. Nominal voltage is 1.2V
    per cell.

    For long life per charge- use a high capacity cell (you can draw 1.5A
    for HOURS from modern AA cells) and design your circuit to use as
    little current as practical. For long battery life- follow all the
    battery manufacturer's recommendations.

    Best regards,
    Spehro Pefhany
  9. Ian Stirling

    Ian Stirling Guest

    But, going down to 0.7V (compared to say 1V) can give significantly
    longer run-times, at low power levels with alkaline cells.
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