If you read last week”s column on black holes, you will recall
that a fellow named Chandrasekhar had trouble convincing his
peers that black holes were real. However, over half a century
later, he got the Nobel Prize, in part for his black hole work. A
few days ago the $1.5 billion X-ray telescope Chandra, named in
his honor, was launched into orbit. One of the main objectives of
this telescope is the study of X-rays emitted by material about to
be sucked into a black hole. Chandra is expected to supplement
and expand the views of the universe so beautifully portrayed by
the Hubble telescope. One thing that interested me was that
there was a small problem with a premature shutdown of the
shuttle”s engine and that the Chandra”s own rocket engine would
be needed for correcting the telescope”s orbit. The article I read
said the Chandra batteries had more energy than anticipated and
implied there wouldn”t be a problem.
This was not the case for another exciting event last week, a
summer camp production in which our 11-year-old
granddaughter played a prosecutor in the court trial of the month
of February. February, portrayed by a short young lad (one of
only two boys enrolled in this “Off-Broadway” summer
program), was charged with not carrying its fair share of our
calendar”s load, with only 28 days. For this slacking off, the
prosecution was trying to get February expelled from the
calendar. I was delegated to record this event on videotape.
Uncharacteristically, I was farsighted enough to charge the
nickel-cadmium battery and make sure there was enough tape to
last the predicted half-hour run time of the performance. All was
going smoothly until Abraham Lincoln appeared in defense of
February and the low battery indicator in my camcorder started
flashing. Sure enough, by the time George Washington and
Susan B. Anthony appeared, also in defense of February, the
battery was “dead”. I was reduced to turning off the camcorder
to allow the battery to rest; then record a snippet before having
again to rest the battery and record another snippet. Disgusted,
my wife and granddaughter both suggested I get a new
camcorder. I couldn”t convince them that the problem was really
a classic case of the “memory effect”, so often encountered with
rechargeable nickel-cadmium batteries.
What is this memory effect? And does it exist? Have you had
the same problem of a rechargeable battery expiring before its
time? The problem may lie in the way you use your camcorder,
cordless phone or other device powered by a nickel-cadmium
battery. Specifically, let”s say that your battery is rated to run
your camcorder for 45 minutes before you need to recharge the
battery. Now assume that you are like me and, even though
every minute of that special birthday party is truly worthy of
being recorded for posterity, you only manage 15 minutes before
the camcorder becomes dead weight and you”re exhausted.
(Remember that I”m 71 years old.) So you put the camcorder
away for a few months until Christmas or Chanukah comes,
when you again charge the battery so as not to miss anything.
You do this because you”re a savvy type who knows that nickel-
cadmium batteries lose capacity on standing, what is known as
“self-discharge”. Self-discharge also is the reason that if you
leave your car standing for several months while you winter in
Hawaii your lead-acid battery is completely dead! (This
happened to a friend, not me, when she returned and found her
Jaguar wouldn”t start.) Back to the holidays, again you tire after
only 15 minutes of filming, put the camcorder away until the
next birthday and repeat the process over a period of several
years. Never have you come close to utilizing the full capability
of that battery to run the camcorder for 45 minutes.
Well, that nickel-cadmium battery is smarter than you think and
“remembers” that you”ve only asked it to work 15 minutes at a
time. Now comes this serious dramatic event and you”re rested
and primed to record for at least 35 minutes. But that silly
battery says, “Hey, I”ve gotten used to the shorter working hours
and I”m just not going to work any longer than 15 minutes unless
I get special treatment!” At this point, of course, it”s too late and
you don”t even know what special treatment this crazy battery
wants. There are even knowledgeable skeptics who maintain
that this “memory” effect does not exist. One of these is my
colleague Al, who has literally written books on nickel-cadmium
batteries. Al also gives lectures on alkaline batteries (which
include nickel-cadmium batteries) in the short course on modern
battery technology that we gave last month in Amsterdam. As
course director, for years I have been insisting that Al include a
section on the memory effect in our notes. He finally agreed to
put in a section on the “so-called” memory effect and is still
adamant that there should be no problem. Yet, invariably, one of
the participants in the course brings up the subject.
Why these conflicting views? Let”s see what actually causes the
trouble. There are a number of factors but the simplest
explanation involves the cadmium electrode and “Ostwald
ripening”, named of course after Ostwald. We tend to talk
about Nobel Prize winners in this column and I believe this
Ostwald is the Wilhelm Ostwald who won the Nobel in 1909.
This “ripening” has nothing to do with fruits or vegetables but
rather with what happens when you have, for example, a mixture
of particles or crystals of different sizes of a substance, say
cadmium. What happens as time passes is that, under the right
circumstances, the bigger particles or crystals get bigger, the
smaller particles get smaller and eventually there is a big
majority of big crystals and very few little crystals. It”s sort of
like little particles of dust gathering together to form those
embarrassing dustballs that reveal the deficiencies in one”s
housekeeping to those you”d like most to impress.
In a battery, the same thing can happen to cadmium on
discharging and charging the battery. If you don”t fully discharge
the battery, i.e., you don”t film the full 45 minutes of party
activity, some cadmium crystals remain behind. Then, when you
charge the battery, the cadmium that is deposited will tend to sit
down on the crystals that are already formed and those crystals
get bigger. On the next discharge, the smaller crystals will tend
to react first and as the battery cycles back and forth Ostwald
ripening occurs and large cadmium crystals form at the expense
of the small ones. Now, in a device like a camcorder where the
current required to power the equipment is reasonably high, one
wants to have lots of little crystals of cadmium so as to provide a
large surface area. You need this large surface area so that lots
of cadmium atoms can react at the same time with the electrolyte
to give off enough electrons to scoot around in your camcorder
and make it run. To do this you want lots of electrolyte in
contact with the cadmium. In other words, the electrode should
be porous, like a sponge. With large crystals, the electrolyte has
to wait in line to feed in to the smaller area and there aren”t
enough electrons generated to power the camcorder.
On the other hand, if you use your camcorder for the full 45
minutes each time you film, you”ve used up most of the cadmium
before you have to charge the battery. Now, on charging, the
cadmium doesn”t have any big crystals to sit down on so it forms
a lot of small crystals. One calls this seeding or spontaneous
nucleation of crystals. The next time you want to film 30
minutes of play there aren”t those big crystals to hold you back
and the battery, having been given special treatment, happily
obliges and pours out the electrons you expect from it. So, the
lesson is that either you film near the full rated time every time
or you give the battery some other special treatment.
Here”s where my friend Al makes his point. He says that there
aren”t bad batteries but only bad chargers and that good chargers
will solve the “so-called” memory problem. Specifically, a
“good” charger will discharge a nickel-cadmium battery fully
before it charges the battery, thus eliminating those big cadmium
crystals that louse up the act. Al also says that you can charge
the battery in your refrigerator and that will cause smaller
crystals to form. However, I find it inconvenient enough just to
charge my camcorder battery normally, let alone worrying about
how to get the wires into my refrigerator. One point to note.
“Good” chargers cost more money
I hope that if you”ve experienced the memory effect, now you”ll
have some appreciation why the camcorder or cordless phone
manufacturer may tell you to discharge your battery fully every
so often. With the camcorder you can just set it down on a table
or leave it on a tripod, which I don”t have, and leave it turned on.
If you”re like me, chances are the results will be better than the
pictures I get with all the zooming in and out and the useless
views of ceilings and floors while searching through the
viewfinder for my subjects. And, if you fail to anticipate the
memory effect as I did, at least you now have a scientifically
valid excuse to give your spouse when those golden moments
don”t get recorded. Just say, “Honey, it was Ostwald ripening!”
By the way, the testimony of Lincoln, Washington, Tom Edison,
Susan B. Anthony, Valentine”s Day and Groundhog convinced
the jury that February was not guilty. I did get on tape the judge
rapping his (her) gavel adjourning the court so at least there was
closure!
.
Allen F. Bortrum
