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Trickle charging NiCads

Discussion in 'Electronic Basics' started by pimpom, May 9, 2009.

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  1. pimpom

    pimpom Guest

    I know this is an old topic and I've done a search, but couldn't find a
    satisfactory answer to the question: What would be a reasonable charging
    current for a NiCad battery (in terms of its mAh rating) under the following
    conditions?

    1. Battery loading will be infrequent but unpredictable - could be once a
    day or a week or more.
    2. Unattended charging at or near constant current for 6-10 hours a day,
    everyday.
    3. Charging circuit needs to be simple - no dv or temperature detection.
    Load cut-off will be automatic.
    4. Battery service life need not be as long as that with optimum charging,
    but should not be drastically shortened.
    5. Does not matter if battery power is occasionally unavailable due to
    insufficient charging time between (infrequent) loads, but should otherwise
    be ready to provide power at any time.

    Thanks in advance.
     
  2. whit3rd

    whit3rd Guest

    So, charge at the ten-hour rate from a current source. 800 mAh
    battery
    (kind of a low-end AA) will want 80 mA for ten hours, roughly,
    so connect a wall-wart with enough volts into a resistor in series
    with
    the battery, or an unregulated supply with an LM317 + resistor
    regulated
    current source. The resistor should be sized to R= 1.25V / Icharge,
    and suitable to dissipate 1.25 x Icharge watts.
     
  3. pimpom

    pimpom Guest

    Problem is, it will be permanently connected to the charging circuit and
    will be charged 6-10 hrs a day regardless of whether it is discharged or not
    in between charges. Under those conditions, I don't think a C10 charge will
    be very healthy for the battery.
    I'm ok with designing the charging circuit to any degree of precision as far
    as current regulation is concerned. I just want to avoid a drastic reduction
    in useful battery life by constantly overcharging it, while keeping the
    circuit simple by omitting a complex full-charge detection.

    Perhaps this is a good time to explain the intended application. I want to
    make an emergency light using white LEDs that will turn on automatically in
    the event of a power failure. Nothing new in that. But this unit will be
    connected in parallel with a normal house light that's usually kept on every
    night from about 5 or 6 pm to 1-3 am.

    The emergency light will sense the state of the light switch even in the
    absence of mains power and turn on only if the switch is in the 'on'
    position - a fully automatic fit-and-forget operation. I've designed and
    tested the circuit but am not sure what level of trickle charging would be a
    good compromise.
     
  4. Phil Allison

    Phil Allison Guest

    "pimpom"

    ** Most NiCd makers suggest a C/50 rate for constant trickle charging.

    But why use NiCds at all??

    NiMH cells have lower self discharge rates and less issues like the
    formation of "dendrites" that plague NiCd cells left on trickle charge.


    ....... Phil
     
  5. Guest

    I was going to suggest a different battery type also. Why NiCds? If
    you don't mind the weight there are also lead acid gel cells. I
    haven't done much (read anything) with NiMH. But I thought I read
    that they didn't want to be trickle charged. They wanted pulses... I
    guess you can make a trickle pulser.

    George Herold
     
  6. Phil Allison

    Phil Allison Guest

    <
    "Phil Allison"
    "pimpom"
    I was going to suggest a different battery type also. Why NiCds? If
    you don't mind the weight there are also lead acid gel cells. I
    haven't done much (read anything) with NiMH. But I thought I read
    that they didn't want to be trickle charged. They wanted pulses... I
    guess you can make a trickle pulser.


    ** Trickle pulsing is probably a very neat idea - funny I have never seen
    it done other than in the standby-by mode of a pulse type fast Ni-Cd
    charger.

    The OP could set up a 555 timer, in astable mode, to produce a 1 second
    pulse each 30 seconds and have a CCS deliver say 500 mA to the battery
    during that pulse. Averages out as 16mA or C/50.

    OTOH - emergency systems nearly always use Gell Cells and with good
    results. Constant voltage ( current limited) charging is the go with them
    and is simple to implement



    ..... Phil
     
  7. pimpom

    pimpom Guest

    I understand that NiMH cells have fewer issues than NiCds. The main reason
    for using NiCds is that they're much cheaper and much more easily available
    than NiMHs where I live.
     
  8. pimpom

    pimpom Guest

    Thanks for your inpuits, everyone. NiMH, lead-acid gel, pulsed and timed
    charging are all technically sound ideas. But ATM, I'm committed to the idea
    of using one of those compact 3.6V NiCd packs that come with cheap Chinese
    import gadgets such as a US$2 rechargeable flashlight.

    My circuit uses 1 low-power transistor, one-half of LM393, 3x1N4007, 1 zener
    diode, 8 resistors and 2 caps on a 1"x2" pcb, and I want to keep it no more
    complex than that.
     
  9. pimpom

    pimpom Guest

    A charging timer would be superfluous here as the mains line to which it
    will be attached is regularly turned on every night during waking hours and
    turned off for the rest of the day - a pretty regular cycle. The uncertain
    factor is the discharge frequency which is neither regular nor predictable.
    It may go for days without being called upon to deliver power. You cite C/10
    and someone else cited C/50. That's quite a big difference.
     
  10. pimpom

    pimpom Guest

    Hmmm. It looks as if there's no definitive answer. Would you say that
    something like C/20 would be a reasonable compromise?
     
  11. Phil Allison

    Phil Allison Guest

    "RogerN"
    ** That is bullshit.


    ** Such a silly scheme will result in very long charging times AND the
    cells will never reach full charge.

    There is also the risk of charged cells heating and hence their voltage
    dropping and hence the charge rate increasing on and on to destruction.


    ....... Phil
     
  12. Phil Allison

    Phil Allison Guest

    "RogerNutcase"

    ** So says a brain dead FUCKING LIAR.


    ** Much less, in most cases.

    SO YOU LIE


    ** That is no answer at all to my complaints - so you have lost the debate
    already.

    Clearly, you are know nothing damn fool.


    ** None of which is even slightly relevant to the original question.


    ** Not very interested where you got all your wrong ideas from.

    But maker's mostly anonymously authored data sheets, pseudo technical
    wankers posing as "experts " for scumbag import operations & ridiculous
    software routines embedded in uCs by autistic code scribbling morons are an
    * inexhaustible supply * of rechargeable battery bollocks.

    Piss off.


    ...... Phil
     
  13. Guest

    Pimpom, Try googling "battery university". There is a bunch of stuff
    there, perhaps some answers to your questions.

    George Herold
     
  14. ehsjr

    ehsjr Guest


    Simple trickle charger with taper:

    Use an LM317 circuit to set the current to C/10. Add a TL431 circuit
    on the output, set to the full charge voltage, and designed to steal
    current such that the current available to the NiCds drops to ~C/50
    or less when the pack reaches full charge voltage.

    A partial schematic:

    Green
    + ---[LM317]---[D1]---+---[LED]---[R]---+
    | |
    +-------+------[R]--------+
    | | |
    [R] | |
    | [R] |
    Red [LED] | |
    | | |
    +-------+ [NiCd]
    | |
    [TL431] |
    | |
    Gnd ----------+-------------------------+

    When green is on and red off, current goes only to battery.
    When red comes on, current is "stolen" reducing the charge
    rate to ~ C/50 with properly chosen resistors. You don't
    need the LEDS, they're just nice indicators. I use 1.43
    volts per cell as the full charge voltage. Compute the
    parallel R's for ~1.8 volt drop when (C/10 - Iled) flows.

    Ed
     
  15. pimpom

    pimpom Guest

    Thanks for the follow-ups. I've visited the links you provided and those
    guys seem to know what they are talking about. Even if their information is
    not 100% accurate, they give a good overview of the issues involved.
     
  16. I would check into dendrites. Overcharging of NiCd cells, especially
    long term constant trickle charging, contributes to those. Once a
    dendrite punctures the separator, the cell has a foot in the grave.

    It comes down to what amount of trickle charging while fully charged
    does not cause enough dendrite growth to make cells die too much earlier
    than they would be declared dead for some other reason.

    Another thing - charging when already fully charged makes the cells
    warmer and that makes them age faster. My experience so far is that C/10
    warms cells a little noticeably, and that C/4 warms cells enough for
    reasonably reliable detection of full charge via either direct detection
    of warming (temperature sensor) or indirect detection of warming (voltage
    decreases after increasing).
    It appears to me that C/20 *usually* does not warm cells enough after
    they are fully charged to accelerate their aging much, though I would
    worry about the dendrite issue if the cells are cooking 24/7/365 at C/20.

    Should you find it practical to reliably detect full charge and then
    switch to a lower "trickle charge" current, such as with usage of one of
    those "smart charger" ICs that detects reversal of increasing voltage that
    results from C/4 or faster charge causing cells to heat up after achieving
    "completion of charging", it appears to me good to have such a "smart
    charger" reduce the charging current to something close to or slightly
    outpacing self-discharge of "topped-off" cells.
    I suspect that C/100 and C/200 will work well, C/200 may keep cells only
    80-90% "full", though I also suspect that C/50 may be, maybe "usually is",
    "not that big a deal" for NiCd cells being cooked at such rate 24/7/365.

    NiMH appears to me to have less tolerance than NiCd for trickle charging
    24/7/365 at rate exceeding self-discharge that occurs at "full charge".
    Not that I ventured major testing efforts, but some NiMH cell datasheets
    and some web-search-able items along those lines makes it appear to me
    that a NiMH cell has ususlly zero to sometimes very minimal,
    likely-well-tolerated overcharging when trickle-charged at C/200 while
    fully charged. (NiCd appears to me to "usually not die much earlier than
    life expectancy" if trickle-charged 24/7/365 at C/100, fair-to-good-chance
    at C/50, some-to-fair chance even at C/20.)

    - Don Klipstein ()
     
  17. Phil Allison

    Phil Allison Guest

    "pimpom"
    ** No way for you to conclude that unless YOU are a expert yourself.

    It is easy to find links that COMPLETELY contradict the ones given.

    Google " NiCd trickle charge dendrites " for some sobering info.


    ** No way for you to conclude that unless YOU are a expert yourself.

    Which of course you ain't.


    ....... Phil
     
  18. Guest

    Thanks for that Don, I was buying a pair of RC battery powered cars
    for my son and I a few years ago and after a bit of research concluded
    that NiCd's were more robust and less susceptable to abuse than the
    NiMH. Even though the NiMH were lighter and held more charge, I
    bought the NiCd's. I also read that the NiMH only store about 70% of
    the charge that is pumped through them. Whereas the NiCd's keep 90 to
    95% of the charge. I wonder if anyone has tested this?

    George Herold
     
  19. Phil Allison

    Phil Allison Guest

    <>


    Thanks for that Don, I was buying a pair of RC battery powered cars
    for my son and I a few years ago and after a bit of research concluded
    that NiCd's were more robust and less susceptable to abuse than the
    NiMH. Even though the NiMH were lighter and held more charge, I
    bought the NiCd's.

    ** Fool.


    I also read that the NiMH only store about 70% of
    the charge that is pumped through them. Whereas the NiCd's keep 90 to
    95% of the charge. I wonder if anyone has tested this?


    ** Even if true ( and it is not) how would it matter ??


    ...... Phil
     
  20. Guest

    Why? Cost per A-hr was less for NiCds. Also thought my 9 year old
    son could leave them plugged in for a day with out destroying them.

    Oh, have you measured this? It certainly makes no difference for an
    RC car powered from my wall socket, but I heard that Hybrid cars also
    use NiMH, If so that's certainly a factor in all the efficiency
    talk.
     
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