Conference vmszoo::rc

        I don't recommend trickle charging Nicads. I have several portable
        appliances of the type you mention. All the nicad powered units I
        only plug in when almost dead, and unplug a couple days later. It
        is best to trickle charge lead acid based cells (car & motorcycle
        batteries, gell cells, etc.), but nicads are best stored in a
        discharged state, and charged at the standard C/10 rate the day
        before you intend to use them. 

        If you must trickle charge them, realize that this may cause
        gradual loss of capacity, and will diminish the number of charge
        discharge cycles you will get from the battery. Its one advantage
        is that you don't forget about that pack, and it is ready for spur
        of the moment use. Trickle charge at C/100 rate, which for your
        pack would be 12ma. My previous note on battery charger design
        applies to this as well. 

        The 120 ma rate you mention is the C/10 standard charge rate.
        Charge at this rate for 16 hours, and not over 48. Anything longer
        than that can overheat the cells, causing them to leak and
        possibly explode. 

    I agree that it is best to discharge  nicads  most of the way
    before charging them, but disagree that they should be stored in
    a discharged state.  The GE design manual for Nicads says that if
    you leave them in a discharged state there is a higher probability of
    internal shorts. Apparently, when they have a good charge, any internal
    bridging gets blasted immediately by the battery current.  If
    discharged, the bridge is allowed to strengthen and then you can't
    get the battery charged.  I would recommend cycling nicads every
    few months and leave them fully charged when not using them.
    
    							Bill
    
    

Gotta agree with Bill on this one; you should store the nicads
charged for exactly the reasons he gives.  Also, I think that
most modern nicads can be left a C/10 indefinitely.  Bob is
correct about the undesirability of trickle charging.  You might
as well leave them at C/10.  I have some AAA batteries that I use
in my walkman; I put them on a C/10 charger and leave them for
several days while the batteries in the walkman wear down.  Just
rotate them as needed.  The batteries will stay on anywhere from
1 to 4 days, with no detectable heating or other effects (believe
me, I check!).

But, to answer your question.  You have a 6-cell pack with 7.2
volts and 1200 mA capacity.  You want to charge it at C/10.  Find
a "trickle" (just a common plug-in charger) that gives about 12 V
at anything above 150 mA.  Most will do this. The charger is
marked on its case.  Take the battery voltage and subtract it
from the charger voltage.  12 - 7.2 = 4.8 V.  You must drop this
voltage with a resistor.  For these purposes, the battery is
considered a short circuit.  So, 4.8 V at 120 mA, by Ohms law, is
40 ohms.  This is the resistor you'll need to regulate the
charging current.

Just find a charger, look at its voltage, be sure that its
current rating is sufficient, and calculate the resistor.  Just
for the record, the resistor is in series with the battery pack
and charger.

        Re: .2 

        I may not have been totally clear in .1; there is discharged (knee
        of curve) and DISCHARGED (0.00v); I meant the former. My
        understanding is that storing them fully charged contributes to
        the memory problem. Further, this charge tends to leak off, to the
        point where they are dead in about 6 months. 
        
        Re: .3
        
        A couple days at C/10 probably won't hurt, but I wouldn't leave
        them on charge at that rate for much longer than that. maybe the
        solution for such an application is a smart charger, that charges
        at C/10 when the battery voltage is low, and drops to C/100 as the
        battery reaches full charge. I suppose (hope) that is what the
        rechargable appliances do. This of course is more expensive than
        the simple charger I described in another note. 
        
        To restate .1, never allow a nicad to be DISCHARGED, but stop
        using it when it is discharged and leave it in that state until
        just before you are ready to use it again, then charge it up at
        the C/10 rate. 

    That's good info, thanks very much for your responses.
    Well after digging and digging, I came up with one:
    		SONY Power Adapter - AC-5A
    		Output - DC 4.5V 300 mA
                Input  - 100V, 110-120V, 220-240V (switch selectable)
    			 50/60 c/s 50 c/s 7VA 60 c/s 4VA
                         (I presume 50/60 hertz)
		Its in excellent condition.    
    
    What do you think ? Is it worth converting? If so, how?
    			
    

No cigar on that one; you charger puts out 4.5 V, but your pack
is 7.2 V (if I remember right).  You got the right idea,
though....

	Re: .6 re: .5

        I ran into that once; and somewhere I dug up a voltage doubler
        circuit. The end result was that I could trickle charge a gell
        cell from a 7v transformer. It worked for several years till that
        gell cell finally died. I just don't remember how a voltage
        doubler works, but I seem to recall that each phase of the sine
        wave charged opposite capacitors, giving a 2X voltage across the
        pair. maybe someone in the ANALOG or ELECTRO-HOBBY conference
        could fill in what I long forgot. 

Yeah, the problem with doublers is that they can't produce much
current.  I suppose that for trickle charging a gel cell at less
than say, 10 mA you could use one.  Really doesn't put out enough
poop for the nicads,  assuming that I still stick to my original
argument that its worthless to charge them at less than C/10.