Conference vmszoo::rc

    I will begin this with a question about something my son relayed to
    me and I am unable to verify myself.
    
    My son indicated an article in an RC magazine somewhere indicated
    that certain types or quanities of matched NiCad packs must be
    periodically re-matched.  Is there any truth to this?  If so,
    how often would this be a recommended procedure?
    
    I can see that if the above is true, some pre-matched packs may not
    be such a hot deal, since the owner is not likely to have an array
    of batteries from which to re-match the older cells. That would lead
    me to think the way to run matched cells, assuming they change
    with use would be to buy a case of Sanyo cells, match up the cells
    into the maximum number of "packs" possible, then after some interval,
    tear them down and re-match them.
    
    
    A second question relates to observed advantages of matching.
    
    We have 2 sets of main batteries at present.  A generic set of 6 
    1200mah batteries (no labels, packed in 2 3-cell white shrink wrap
    tubes), and a set of 6 matched Sanyo 1200mah SCR batteries.
    
    I think the matched set is faster on his RC10 (on-road) while they 
    are up on their power curve. I understand the Sanyo SCR tends to be 
    this way anyway.  Would an unmatched set of SCR's be more difficult 
    to peak charge, or run down sooner (and more gradually), or be slower 
    (at the beginning)?  
    
    
    Walt

    
    	Walt , 
    
    		I believe before discussing matched cells that we should
    cover exactly what goes on with them . The ideal situation would
    be to have couple of "Factory hot" packs but these are far and few
    between . These packs are made up of the pick of the lot , about
    one in every 100 cells would qualify , but never make it to the public
    sales counter . Rather , they are "given" to the pro r/c racer for
    advertising .
    
    	What you get when you buy "matched" packs are packs that make
    the "second" pick . They hold a good charge and are acceptable for
    local event racing . But even these packs vary widely , some peaking
    higher than others . Then there are "stock" or "generic" cells .
    these are the average or less than average cells , sold much cheaper
    and found in massive quantities .
    
    	What Gives the cell its characteristics is peak voltage charge
    that it can hold for a given current rating . This means that a
    cell ( normally rated at 1.2 vdc ) could hold 1.4 vdc (volts dc)
    or higher . Buying a matched pack allows you to aquire a group of
    cells that may peak .5 to 1 1/2 volts or more , higher than any
    generic pack . This means that a 7.2 v pack running at 7.8 volts
    or more will give you more rpm on your motor , thus producing a
    higher top speed during a race , but still holding an equal amount
    of current ( eg : both a generic and matched pack could discharge
    at about the same time . ) This extra speed allows you more freedom
    to set up your car for various track conditions , thus optimizing
    performance and giving you that "edge" everyone is looking for .
    
    	More on this later , have to go but hope the info helped .
    
    	c u later
    
    
    	Carl
    

    I have to disagree some with what Carl has to say.

    Matched cells are not rated by their ability to hold voltage.  The
    voltage ANY battery can hold and produce is fixed. It is directly
    related to the electochemical reaction that occurs in the cell.
    Specifically, it is the difference in the electronegativity of the two
    reactive chemicals that produce voltage that determines the voltage of a
    bettery.  That's why Lithium batteries have such a high power density.
    Next to Fluorine, Lithium is one of the most electronegative elements.
    Fluorine is THE most electronegative.  The AMOUNT of each of the
    reactive elements, however, WILL directly affect the cell's capacity. 

    NiCads, like everything else in the world made by man or nature, are
    unique from unit to unit.  These differences are primarily related to
    the cell's ability to hold a charge, but also related to the cell's
    ability to deliver high current (since the two aspects go hand-in-glove).
    1200 MaH cells, for instance, will display quite a bit of variation,
    even when they come from the same production lot.  Unless there's
    something wrong in the manufacturing process, the variations between
    cells in a given lot plot out as a typical bell curve, with the curve
    centered around the manufacturers specified rated capacity of the cell. 

    Manufacturers usually discard cells that fall below the rated capacity
    (sometimes, though, they're sold to companies that package them as
    "consumer" NiCads, like what you can buy at Radio Shack for your
    portable radio.  Often the D cell sized NiCads you get from a place like
    RS are actually below rating sub-C cells in a D cell sized jacket).  The
    cells that "make the cut" get sold to the general manufacturing
    community. 

    Matched packs contain cells that have had their ratings determined
    empirically.  These cells all have capacities that fall within a certain
    percentage of each other (determined by the guys who matched them.  There
    is no way to tell, without matching them yourself, what the criteria
    used might be, and how much variation was allowed.  Charlie Watt's
    computerized NiCad pack cycler would perform this function admirably.).

    Often, if you're willing to sift through a BUNCH of cells, you can build
    packs with that will supply capacities as much as 50% over the rated
    capacity of the cells they're made from.  The 1400MaH packs that you may
    have seen around are built this way.  The highest I've seen are 1800MaH
    packs.  These are made from "A number one, first class" 1200 MaH cells
    that fall on the high side of the bell curve (although there are some
    1800MaH packs made from an "oversized" Sub-C cell or from the newer 1700
    MaH SCE cells, these don't count. For the purposes of this discussion I
    mean ONLY Sub-C sized 1200 MaH SCR's).

    The ability of a pack to deliver current is pretty much limited by the
    ability of the weakest cell in the pack.  When a cell in the pack
    reaches discharge, it will limit the current that can be delivered by
    the other cells.  If you have a pack consisting of 5 cells that can
    actually deliver 1400MaH, and one cell that can only deliver 1200MaH,
    you only get, essentially, 1200MaH of useable, high current, discharge
    from the pack. 

    You can see how it's a real advantage to have a matched pack, since this
    enables ALL the cells to deliver as much current as they can, for the
    maximum duration.  That's also why you need to equalize a NiCad pack.
    The variation in cell capacity means that, at the end of a run (the
    point at which the pack doesn't deliver enough current to satisfy your
    need for speed) some of the cells in the pack still may have
    substantially more charge left than others in the pack.  When you
    recharge a pack in this condition (particularly if you peak charge), the
    cells with the higher starting charge may end the charge cycle before
    the cells with the lower starting charge are actually fully charged.
    Then, when you use the pack, you start to get a loss of power when the
    lowest charged cell is discharged, even though most of the other cells
    in the pack still have some charge left.  This process results in a
    vicious circle, where, after many iterations of charge-discharge, the
    disparity between cells increases to the point that the packs rating is
    severly impacted.  The other problem, of course, is that the first cell
    to discharge ends up having reverse volatge applied to it by the other
    cells in the pack.  Vented cells can tolerate reverse voltage quite
    well, but it causes a lot of heat build up in the cell, and may
    eventually result in the cell's venting.  It only takes a few "vents" of
    a cell to make it useless.

    As for rematching, I've never heard this before, but in a way, it makes
    sense.  Even though the pack starts out being matched, the individual
    cells will degrade over time and use at different rates.  I would expect
    that you could get a couple of hundred cycles off a matched pack before
    you had to worry about this, though.  NiCads, when properly maintained,
    have amazingly long life cycles, and the factors that affect cell rating
    probably also have a direct bearing on cell life, meaning that the cells
    in a matched pack should tend to degrade at a similar rate. 

    An article in a recent RC car mag indicated that the SCE packs should
    be re-matched after only 3 races.  
    
    The article was aimed at 3 levels of car owners.  Casual users,
    stock class competitors and open class local competitors.  The next
    class of user would be the factory sponsored racer, I suppose.
    
    The above comment was made in the course of describing selection and
    use of batteries for the open class of local races, and in particular
    for modified motor cars running 7 cell packs and needing about 4
    minutes of run time.
    
    The idea of matching seemed fairly straight forward and reasonable.
    The need to re-match after a short time threw me though. If it is
    really needed, as opposed to individually cycling the cells of a set,
    to regain the "match" then I would think a lot of racers are
    wasting a good deal of money buying them matched to begin with.
    
    I like the idea of a periodic controlled cell by cell cycle. The
    Trinity folks recommend a 30 ohm resistor be plugged into their 
    matched packs after use (they dont actually say in the literature
    how long it should remain, maybe the resistor kit instructions do).
    Perhaps, their strategy is to ship very well matched packs (based
    on the price I would hope so) and with the use of the discharge
    resistor after each use, the cells will be completely and evenly
    discharged (addressing the problem of cells taking on different
    characteristics, leading to reversed cell problems over time).
    
    As a matter of fact (rather observation) the above may be exactly
    how Trinity proposes to keep a set matched over time.  Any thoughts
    on that?
    
    Walt

    What you describe is commonly referred to as equalizing.  Equalizing can
    be done in basically two ways.  One is to deep discharge the pack (a
    popular means is to use an automotive taillight bulb, and leave it
    connected until the filament stops glowing).  Another is to trickle
    charge the pack to full capacity.  In both cases the purpose is to allow
    all the cells in the pack to reach a common level of charge.  Once this
    is accomplished, the fully charged pack should provide the maximum it
    can, based on the capacity of each cell in the pack.

    When using the discharge method, it's important to limit the current
    being pulled from the pack.  Voltage reversal on the cells that
    discharge first is considerably less damaging when the current is low.
    I usually equalize my packs by trickle charging overnight very couple of
    fast-charge cycles.  It seems to work quite well for me.  I haven't
    noticed any degradation of my 7 cell SCE packs.  Remember, too, that I
    fly my packs in an airplane.  ANY loss of capacity is readily noticed,
    since the flight performance (which is fair to marginal to begin with)
    degrades an awful lot with a bad pack.

    I suspect that the article that talked about rematching was concerned
    with getting the absolute maximum from the packs in question.  A lot of
    the factory sponsored racers are pushing the power systems to the
    absolute maximum in order to win.  A few Millamp Hours might make all
    the difference to these guys.  I'de be more willing to accept that the
    pack need rematching because high current demands they place on their
    packs are causing rapid degradation, and as a result the slight
    variations between cells become more pronounced in a shorter time.  4
    minute runs mean they're drawing better than 25 amps from a SCE cell.
    These cells have slighty higher internal resistance than the SCR cells,
    which would only have to supply 18 amps to last 4 minutes.  Higher
    internal resistance, combined with higher current, means a P*SSLOAD more
    internal heat (remember I-squared-R).  More heat means higher likelyhood
    of venting, means faster degradation of cell life.

    Unless you're ultra competative, I wouldn't worry about it.

    My plane motors draw close to 20 amps, and, as I said, I haven't noticed
    any degradation.  I've flown the 7 cell SCE's at least 10 times each in
    the last few weeks.

RE: < Note 1036.3 by LEDS::COHEN >

    Randy,

    I understand most (or all) of what you said.  However, I've got some
    questions from this paragraph of yours:

>    Often, if you're willing to sift through a BUNCH of cells, you can build
>    packs with that will supply capacities as much as 50% over the rated
>    capacity of the cells they're made from.  The 1400MaH packs that you may
>    have seen around are built this way.  The highest I've seen are 1800MaH
>    packs.  These are made from "A number one, first class" 1200 MaH cells
>    that fall on the high side of the bell curve (although there are some
>    1800MaH packs made from an "oversized" Sub-C cell or from the newer 1700
>    MaH SCE cells, these don't count. For the purposes of this discussion I
>    mean ONLY Sub-C sized 1200 MaH SCR's).

    Is this really true?!?  It is possible to buy a pack made of "1200
    mAh Sub-C sized SCR Sanyo" cells that will deliver 1800 mAh???? 
    Where can I buy packs like this and how much will they cost?  What
    is the availability of these packs?  Are these packs only available
    to the manufacturer reps.?  

    I do not want a pack of 1700 mAh SCE cells - only the SCR cells.

    On a different note, I too believe that equalizing your pack(s) is
    very important and I do the same thing that Randy does - I trickle
    charge them overnight before using them.

                       _____
                      |     \
                      |      \                          Silent POWER!
      _        ___________    _________   |            Happy Landings!
     | \      |           |  |         |  |
     |--------|-  SANYO  + ]-|  ASTRO  |--|              - Dan Miner
     |_/      |___________|  |_________|  |
                      |       /           |     " The Earth needs more OZONE,
                      |      /                       not Caster Oil!! "    
                      |_____/

    OK, I can buy what has been said here so far...
    
    So, how important are all of these factors to those of us that use
    Nicads for control, not power??  Should we be concerned with matching
    and equalizing??  Or is cycling and slow charging (like c/10) and
    checking with a ESV with a load more than sufficient??
    
    Thanks,
    jeff
    

    
    	Gawrsh , I'm all red in the face !!! Thanks Randy for the
    correction of .2 . One should not try to reply to a note after a
    22 1/2 hour shift or words tend to get mixed up . If one were to
    reverse most of the words "voltage" and "current" it would be more
    accurate . Thanks again , and for a great re-explaination of cells
    and they're characteristics .
    
    
    	Carl
    

    Carl,

    No problem there, I kinda thought maybe you'de confused Voltage and
    Current, since, aside from that one mistake, everything else you said
    made sense.

    Jeff,

    I'de say that you really only need to be concerned about this kind of
    thing if you're wanting to get the most power with the least weight. For
    those of us flying electric planes, power to weight is REAL important,
    so it matters.  If it's just power to your servos and radio that
    concerns you, don't sweat it.  You don't fly your airborn power system
    until the last ounce of juice is squeezed from it, anyway, so it
    should'nt really matter to you if you only have 20% reserve left when
    you go home, as opposed to 25%.  It probably only becomes important if
    you're flying glider duration, and you've been up all day.

    Rick,

    I can't say where to get packs like I described.  I know that there are
    a few companies selling 1400 MaH packs that are actually 1200SCR cells.
    I've been told that, but have never actually seen, 1800MaH packs made up
    from 1200SCR's are available.  I know from reading about NiCads that in
    the manufacturing process, some cells in a given run will (law of
    probability, and all that) actually turn out to rate 50% or more than
    the specified capacity of the cells.  As to who might provide a service
    that sifts through a bunch to find the few "gems", I havn't a clue. You
    might try SR batteries, if anyone'de know about it, they should. You've
    got to remember that the 50% mark is pretty far out there on the bell
    curve, so a company producing packs with those kinds of cells is going
    to have to buy a BUNCH of cells to find the few really good ones, and
    having located them, they're gonna charge you a P*SSLOAD for the honor
    of using them.

    If, however, you do find such a manufacturer, by all means let me know,
    I'de like to buy some too.

     after reading all the replies a I wonder if you buy a 1400ma pack
     that is uing 1200ma cells that fell high on the bell curve
      in due time will it degrade to a 1200ma pack???
      if so, even if the pack was cycled properly and trickled charged
    all the time how long of a time period are you talking about
    for this to happen.
      
    thanks Jon.