🚀 go-pugleaf

In article <57370@microsoft.UUCP>, gordonl@microsoft.UUCP (Gordon LETWIN) writes:
> In article <17660102@hpfcdj.HP.COM>, myers@hpfcdj.HP.COM (Bob Myers) writes:
> > Once again, with feeling:

> > There IS NO "memory effect" in nicads, at least none that has ever been
> > documented by the battery manufacturers since the very early days of these
> > cells.  

> You should take what Mr. Myers says with a grain of salt.  Motorolla,
> a very major presense in portable radios and therefore a world expert
> on Nicad batterys, specifically says that there *is* memory effect on
> nicads.  If you buy one of their MicroTacs they tell you to full discharge
> it's nicad battery periodically.  They say not to do that with the lead acid
> battery also provided.

> Obviously, this isn't proof that Mr. Myers is wrong, but I'll place my bets
> on Motorolla - I expect they know quite a bit more about the subject then
> Mr. Myers.


*IS NOT*, and your mother wears army boots!

Actually, I can't quote a definitive source, but from my circa-1980
conversations with several of General Electric's NiCd battery gurus,
I recall that what is usually called the "memory effect" is more properly
known as "Voltage depression".  There *is* a "memory effect", but it is
rarely, if ever seen.  Here is the story on each of these:

Memory Effect:  This was seen on an early satellite (the orbiting type),
and was caused by a charging system that would cycle the batteries so that
they were discharged to *exactly* the same point evry time, then recharged
back to *exactly* the same state of charge.  After many repeated charge/
discharge cycles, it was found that the batteries would not supply power
when discharge was attempted past the discharge setpoint.  The solution
was to vary the recharge threshold in some manner.

The conditions that cause the memory effect are just *not* present in
typical use, even highly cyclical use.  Probably the best way to avoid
the memory effect is to take the batteries into *overcharge* when recharging.
This is the typical recharging method, by the way.

Voltage depression:  This is often referred to as the "memory effect", but
due to prior claim the term "memory effect" is properly reserved for the
condition described in the above paragraphs.  Voltage depression is a state
where the NiCd cell voltage under load drops below the nominal while the cell
still retains a significant charge.  In equipment where the required operating
voltage is set for nominal cell voltage, this will result in drastically
reduced operating time between charges.

The typical cause of voltage depression is long-term trickle charging at
low charge rates (less than C/20).  My understanding is that under these
conditions, a surface barrier is formed on the cells that increases the
output resistance (I could be all wet here...)

Below, I will attempt to construct a graph showing typical discharge curves
for these conditions.

Cell Voltage

1.3Vx+
    @x+
1.2V  @x+@x+@x++@x+++x+++x++x++++++++++++++++++++++++
                 @              x                      +
1.1V               @@@@@@@@@@@@@@@@@x@@@@@@@@@@@@@@@@     +
                                       x              @     +
1.0V                                     x              @     +
                                           x              @     +
0.9V                                         x              @    +
                                              x                @  +
0.8V                                           x                 @ +
                                               x                  @ +
0.7V                                            x                  @+
                                                x                   @+
0.6V                                             x                  @+

     |     |     |     |     |     |     |     |     |     |     |     |
     0     1     2     3     4     5     6     7     8     9    10    11 Hours

Legend: '+': Nominal NiCd at C/10 discharge rate
        '@': Voltage depression case
        "x": Memory effect case


There!  That wasn't so hard, after all!  Please consider the above graph
an artist's conception; don't rely on the absolute numbers.

If anyone out there can provise a definitive source, or refutation of this,
please post it.

-- 
      Paul M. Elliott      Optilink Corporation     (707) 795-9444
               {uunet, pyramid, tekbspa}!optilink!elliott
"an archetypal entity..., superimposed on our culture by a cosmic template."