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Rate of charge of NiMH AAA cells at home

Discussion in 'Electronic Equipment' started by Alex Coleman, Oct 27, 2005.

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  1. Alex Coleman

    Alex Coleman Guest

    I have a reasonably modern charger (a few months old) and some old
    rechargeable cells (about 5 years old). Is it wise to use the two
    together?

    -----

    The charger is specified to give a charge rate of 350 mA.
    I think the charge voltage is 2.8 V per pair of cells.
    The charger has a "negative-delta-V" sensor.

    The AAA cells are NiMH and are each rated as holding 0.185 mAh.

    (Q.1) Will this 350 mA *RATE* of charge be too much for such
    cells to take resulting in noticeable adverse effects?

    ------

    Assuming the charge in the cells is fully depleted and assuming
    that the charge is taken up in a linear way, then the figures
    suggest a maximum charge time of 30 minutes.

    (Q.2) Is it highly likely that the "negative-delta-V" will be
    sensed only after the cells have gone into overcharge (venting gas
    and messing up who-knows-what internally)?
     
  2. mike

    mike Guest

    The 2.8V is a nominal number that is loosely related to actual.
    A more useful number would be the overvoltage termination number.
    negative-delta-V will overcharge NiMH.
    You sure about that? VERY old NiCd cells may have been that low.
    Assuming you really meant 185mAH. I don't recall seeing a AAA NiMH
    much below 500mAH.
    Probably, but more important is the charge termination method.
    You can buy 8.5 minute charge cells these days.

    This is a risky assumption. Cells are rarely fully depleted
    in actual use. Sooner or later, you're gonna stuff mostly
    charged cells in the charger. The 2.8V number implies that two
    cells are charged in series. Things get really bursty
    when you stuff in one fully charged and one depleted cell.
    No matter how determined you are, you'll do it eventually.
    Ask me how I know ;-)

    and assuming
    Need to be certain of the cell capacity numbers before drawing any
    conclusions. It certainly is possible to charge cells so fast
    that they vent before they even get warm.

    mike

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  3. ehsjr

    ehsjr Guest

    That - .185 mAh - is impossibly low. Aside from that, don't
    use your charger on those cells.

    The cells are 5 years old, and I assume you don't have a charger
    for them. So it seems that it's not worth investing any time/energy
    in buying or building a charger. However, if you want to do it as
    an experiment, build a trickle charger as follows:

    -----
    Vcc ----|LM317|---+
    ----- |
    Adj [68R]
    | |
    +------+--- To nicd +

    Gnd ------------------ To nicd -

    Vcc can be anything from +6 to +30. Worst case, the
    LM317 will need to dissipate about 1/2 watt. With 12
    volts Vcc, it will need to dissipate 200 mW.
    The circuit limits the charge current to ~18 mA
    You can leave the cells in the charger for > 24
    hours with no ill effect. Without knowing the specifics
    of your cells, assume that they require a full 24 hour
    charge with this circuit. You can charge them in series
    or one at atime with this circuit - it makes no difference.

    Ed
     
  4. You forgot the reverse protection diode. When the cell(s) is/are
    connected without power, they will force current back into the circuit,
    with who knows what results.
     
  5. ehsjr

    ehsjr Guest

    It's not needed. The datasheet says no protection diode is needed
    for output voltage of 25 volts or less, and shows the circuit
    without that protection.

    Ed
     
  6. Ikke

    Ikke Guest

    Your cells wil live MUCH longer when you charge them at a low rate.

    P.
     
  7. I thought that most analog chips cannot tolerate a reverse voltage of
    more than negative .3V - at least that's what the datasheets say. And
    that would be especially true for regulators because they have such a
    low internal resistance and high current capability. I just looked at
    the datasheet for Nat'l LM317 and that's what it says: -0.3V
    _absolute_maximum_ rating. That would be far exceeded by the -1.5V or
    more of the cell(s).
     
  8. Doug McLaren

    Doug McLaren Guest

    | Alex Coleman wrote:

    | > The charger is specified to give a charge rate of 350 mA.
    | > I think the charge voltage is 2.8 V per pair of cells.
    |
    | The 2.8V is a nominal number that is loosely related to actual.
    | A more useful number would be the overvoltage termination number.

    What is this `overvoltage termination number' you speak of?

    You can't determine that a NiCd or NiMH cell is fully charged or not
    only based on a single instantaneous voltage reading -- you just can't
    say `oh, it reads 1.42 volts -- it's fully charged now!'

    (That does work on LiPo and Pb cells, however.)

    | > The charger has a "negative-delta-V" sensor.
    |
    | negative-delta-V will overcharge NiMH.

    If it does, it's only by a small amount, if done properly. NiMH cells
    do have a negative voltage peak just like NiCd cells do, but it's
    smaller, so older chargers that look for a larger peak can often miss
    it, and then THAT will overcharge your NiMH cell. But
    `negative-delta-V' _does_ work properly on NiMH if it's sensitive
    enough.

    (And as a rule of thumb, the older a charger is, the less likely it is
    to be sensitive enough, since they started making them more sensitive
    to handle NiMH cells when NiMH cells started becoming popular. Of
    course, being _too_ sensitive is a bad thing too, as it can cause
    false peaks.)

    | > The AAA cells are NiMH and are each rated as holding 0.185 mAh.
    |
    | You sure about that? VERY old NiCd cells may have been that low.
    | Assuming you really meant 185mAH. I don't recall seeing a AAA NiMH
    | much below 500mAH.

    First of all, his units are obviously wrong. It might be 185 mAh or
    0.185 Ah, but not 0.185 mAh (which would be off by a factor of 1000.)

    But you're right -- I've never seen AAA NiMH cells that small, and the
    smallest AAA NiCd cells I've seen in a while have been about 250 mAh.

    Note that in most cases you should not charge NiMH cells at over 1 C.
    So if your cells really are 185 mAh, they should not be charged at
    over 185 mA.

    (Exceptions include sub-C cells, which can be charged at 2C safely,
    and then there's the new NiMH cells which say they can be charged in
    15 minutes or even faster, which would be 4C or more, but I've never
    used those new cells myself.)

    Is it possible that you've got AA (not AAA) cells, 1850 mAh capacity,
    and the charge rate suggested on the package is 0.185 mA for 12-14
    hours or so? That would make a lot more sense.

    | > (Q.1) Will this 350 mA *RATE* of charge be too much for such
    | > cells to take resulting in noticeable adverse effects?
    |
    | Probably, but more important is the charge termination method.

    Both are pretty important, actually. 350 mA would cook 180 mAh NiMH
    cells pretty quickly, for example. They might tolerate it for a few
    charges, but they wouldn't last long. (And overcharging at 350 mA
    .... ouch.)
     
  9. mike

    mike Guest

    The overvoltage termination is somewhere around 1.6V/cell.
    It's intended as a fail-safe to reduce the number of exploded cells
    when something goes horribly wrong in the charging process.

    Same reason there's also a timer to shut off charging after 1.5C or so.


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  10. mike

    mike Guest

    There was a well-known manufacturer who mounted BIG electrolyics on
    a metal plate with a hole in the plate where the cap vent was located.
    Somebody put a bunch of 'em in backwards, covering the vent hole.
    Rumor was that they had to replace a LOT of ceiling in the cycle room.
    mike

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    Wanted, Serial cable for Dell Axim X5 PDA.
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  11. One other thought that might be a concern. The battery's reverse
    voltage is put across the filter caps in the PS. It would be more of a
    concern if there were two cells in series with negative 3V or so across
    the charger. Putting reverse voltage across an electrolytic can cause
    it to "unform", and if the voltage is high enough, it might explode.

    A guy at work was telling me last week about his shenanigans when he
    worked for Xerox. He said for kicks he would put a 'lytic across a PS
    and cause it to explode like a firecracker. Sometimes the rubber bung
    would shoot up into the ceiling and imbed itself into the acoustic
    ceiling! :))

    I think that if I did that, I'd go to someone else's workbench, so
    there'd be on tell-tale evidence above _my_ workbench! :-O
     
  12. ehsjr

    ehsjr Guest

    The battery voltage is the same polarity as the capacitor. The
    "reverse current" scenario you described was power loss on
    the input side, not a battery installed backwards. Are you now
    talking about a scenario where the battery is installed backwards?
    That would require a diode to protect things.

    Regarding diode protection and the -.3 volt differential you
    mentioned in your other reply: That would seem to mandate that
    *any* time you had a source connected to the output of *any*
    value, a protection diode would be mandatory. Yet the datasheet
    states it is not needed below 25 volts (see page 8 & 9 for the
    discussion of protection diodes) and it shows two battery charger
    circuits (including the one I drew) without protection diodes on
    page 19. That suggests that the internal backwards polarity path
    resistance is low enough to keep the differential within .3v unless
    the voltage on the output pin exceeds 25 volts. The discussion on
    diode protection indicates that the diode protects Vout against
    *short circuit* on the input, and the diode to protect the adj pin
    protects against either an input or an output *short*. The condition
    of loss of power on the input side is simply not a factor when
    Vout < 25 volts. With no short on the input, if current flows from
    Vout to Vin, it will be small, and will simply keep the power supply
    cap charged through the low internal resistance of the chip.

    Ed
     
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