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Thread: ni-cd theory

  1. #1 ni-cd theory 
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    nicad batts loose their effectiveness from cycling because the develop tiny
    crystal structures between the 2 poles, thus eventually shorting the battery,
    or causing a quick discharge cycle and extensive heat while charging.

    This phenomena can be accelerated by high current charge/discharge cycles
    (yes quick chargers kill ni-cads). after the battery becomes shorted, it CAN
    be recovered at times. Applying a high current charge (>5 amps for sub C cells) and destroy the crystaline structure for a short time.

    IS is doing it right, by using a peak charger that will turn off the charge current and switch to a trickle charge that will keep the battery "topped off"
    . BUT, even these high performace chargers can destroy a battery in short
    order, even with the current limiting, pulse width modulation that all modern chargers use, a used, semi dead battery can be damaged further if it gets hot
    while charging (over 120F) this cause crystal structures to form faster, and gases to form faster than the mechanics of the cell can release into the environment, causing them to burst, swell, deform and short.

    "memory" is common in a ni-cd cell, but can be easily removed, or "reprogrammed" by cycling the cell to under 1 volt and recharging, at the
    SAME current rates for 2 or 3 cycles will remove any memory. (I don't know the full theory behind memory, but I'm sure it's all over the web)

    The worse problem with ni-cads is reverse polarity. when the cell hits ZERO volts, it's poles can reverse, this usually causes the ZERO volt battery pack because one cell has reversed polarity. This can USUALLY
    be fixed by applying high current, high voltage pulses to the cell in the correct polarity. This usually take 5-12 volts and 15 amps of current for a 1200mah sub c cell. Pulses of 1 second or less do not cause heating and deterioration of the cell. I've had cells return to their correct polatiry in 3 to 20 pulses. The cell should then be IMMEDIATLY be SLOW charged and left for 24 hours to rest, then cycled twice before returning to service.


    Overall, we abuse all out ni-cd batteries as compared to the manufacturer
    specs, then wonder why they don't last for their "1000 cycles", but, it's the price we pay for wanting cordless drills that draw 20 amps, RC cars that hit 150-200 amps of consumtion, and want them recharged in 15 minutes instead of 15 hours.

    I'll go over the theory of NI-MH later, I'm getting long winded and putting
    everyone to sleep ... They do have SOME downfalls, but are MUCH
    more tollerent of abuse, overcharge, deep discharge, etc,etc,etc


    Steve


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  3. #2 opps 
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    Guess I was asleep...this shouldn't be a new topic... I apologize to all

    Steve


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  4. #3  
    Forum Isotope (In)Sanity's Avatar
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    I actually managed to recover my cordless drill batteries using a "Peak" charger. It's actually a charger made for RC planes and is pretty smart. It will do both Ni-Cd and Ni-Mh. I use it to fast charge the Ni-Mh batteries as I found the plug in wall chargers just leave you with either an overcharged battery or a half charged battery (in most cases).

    From what I understand with Ni-Cd batteries that developed the "memory" effect is to discharge them down to a pretty low voltage (but not zero) and then peak charge them. Repeat cycle a few times and it often will work. It did for me.
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