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RECHARGEABLE BATTERIES AND LAPTOP COMPUTERS
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No other topic seems to inspire more opinion and comment than
the proper care and handling of rechargeable laptop and notebook
computer batteries. Should you slow or fast charge them? What is
the true life of a rechargeable battery after which it must be
disposed? Do rechargeable batteries have a "memory" effect? Can
nickel-cadmium batteries explode when charging or discharging?
Although this tutorial may seem technical in places, try to read
ALL of it since battery power may be your only source of laptop
power on many occasions.
For many portable computers a variety of rechargeable battery
options exist today. But frequently it comes down to the old
standby: nickel-cadmium batteries. Ubiquitous in consumer
electronic items such as shavers, flashlights, toothbrushes and
radios, nickel-cadmiums or "nicads" are a reasonable balance of
power, cost and weight and are used by many computer
manufacturers as the portable power source of choice. Let's
scratch the surface on the topic since there is QUITE A BIT the
manufacturer doesn't tell you about nicads....
Glance at the following chemical equation which is at the heart
of the nickel-cadmium cell reaction. Don't get overly anxious
because high school chemistry was not your favorite subject.
We'll take things slowly....
<-----
Cd + 2 NiOOH + 2 KOH -----> Cd(OH) + 2NiO + 2 KOH
2
In this highly simplified reaction sequence, electricity is
generated when the reaction proceeds in the direction of the
right pointing arrow, the discharge cycle. If the reaction
proceeds in the left direction the cell is charging.
In simplest terms, a nicad cell (a battery is constructed of
several cells hooked together) has a positively charged plate of
nickelic hydroxide and a negative plate of metallic cadmium. The
liquid between the positive and negatives plates which
facilitates this chemical reaction is usually a dilute solution
of potassium hydroxide - similar to lye or the Draino (tm)
solution your pour down the sink to clean your plumbing. When
discharging and thus producing electricity, the nickelic
hydroxide is reduced to nickelous hydroxide as hydroxyl ions
from the potassium hydroxide electrolyte combine with the
cadmium metal of the negative plate of the cell to form cadmium
hydroxide. Cadmium is oxidized when this happens and electrons
are provided into the external circuit, such as your laptop
computer.
When charging, the process reverses and hydroxyl ions combine
with the nickel which accepts electrons from the external
charging circuit. Notice that the electrolyte, potassium
hydroxide is unchanged with two atoms or units produced on both
sides of the chemical equation whether charging or discharging.
This is why you do not need to add more water to a nicad battery
which operates as a sealed reaction container. It regenerates
its electrolyte in both the charge and discharge cycles.
All of this is an ideal nicad cell. The real world of computers
and rechargeable batteries is not quite that simple. The first
SERIOUS item to consider is that all nicad cells and batteries
generate gas during both the charging, and to a lesser extent,
discharging cycle.
During recharging, oxygen gas is generated at the positive
electrode while hydrogen gas is produced at the negative
electrode. In other types of rechargeable cells, a standard lead
acid car battery for example, these gasses are usually released
into the atmosphere. The nicad cell does not have this luxury
since it must operate cleanly and with minimum release of gasses
or liquids. To minimize hydrogen gas release, nicad cells
usually have an oversized negative electrode which tends to
reabsorb hydrogen gas. In addition oxygen is recycled by
combining with metallic cadmium to produce cadmium oxide. So
called "fast-charging" nicad cells prevent gas buildup and
dissipate some of the heat generated during the quick charge
cycle by further enlarging the electrodes. Heat and gas buildup
is thus controlled and kept to tolerable limits in quick charge
nicads.
The first of several lessons which can be derived from this
technical discussion is that the buildup of hydrogen and oxygen
gas during the charging cycle is normally dissipated unless HIGH
recharging rates are attempted or unusually high temperatures
are produced. If the nicad cell is charged at abnormally high
rates the oxygen gas cannot dissipate and will EXPLOSIVELY
rupture the cell.
A safety system of sorts exists within the design structure of
most nicad cells via a pressure venting system - a plastic
diaphragm membrane at the top of the cell and small external
hole or "exhaust vent." In theory the system safely vents excess
pressure and then reseals. In practice the resealing is never
complete and the cell may continue to ooze caustic electrolyte
or worse the vent may not open soon enough and the cell may
simply explode. The vent is really designed for SEVERE charging
or discharging rates. In normal use it should NEVER activate; if
it does, the battery should be discarded. In cases of massive
overcharge or discharge the safety vent is usually too little
too late and a dangerous battery explosion takes place anyway.
During rapid discharge - short circuiting the nicad cell or
battery with a piece of wire, for example - gas buildup and heat
can be generated and a violent explosion can occur. Another
reason why nicads can explosively burst when short circuited and
forced to discharge quickly is that they have relatively low
"internal resistance" which allows them to dump their electrical
capacity quickly and with explosive force.
Common zinc carbon batteries have a much higher internal
resistance and when shorted may produce serious burns to your
fingers from melting wire but usually will not explode due to
sudden gas buildup. On the point of sudden nicad discharge by
short circuit you might be tempted to say that it would be highly
unlikely with a portable computer battery. Not so. Tales are
told of laptop computer batteries which have exploded when a
careless owner shoved several fully charged nicad batteries in a
travel case with a set of spare keys. If the keys accidentally
contact both the positive and negative poles of the nicad
simultaneously, a violent explosion reaction can occur!
Clearly nicads have some unusual features to be respected and
understood. Be careful with charged nicads and treat them as the
small "hand grenades" which they can become. Heat, sudden short
circuits and high rates of charging are the problem in this
area.
The correct operating temperature for discharging and recharging
nicads is from 65F to 85F, according to most manufacturers. High
and low ranges of from +32F to 115F are possible as upper and
lower limits if nicads MUST be used in extreme environments
although discharge and recharge efficiency may be adversely
affected - it may require more power to fully charge the battery,
charge may not be held for as long on the shelf after charging
and finally discharge may not produce a full three or four hour
computing session at these severe temperature ranges.
Electrically, individual nicad cells - the units which are
hooked together to produce the final battery - have a charged
voltage of 1.25 volts. Nominally this drops to 1.2 volts under
actual discharge use or "load" in the electrical device.
Individual cells are strung together in "series" with the
positive terminal of one cell touching the negative terminal of
the next cell in sequence to raise the voltage to that suitable
for the electrical device. Thus two cells hooked in "series" as
a battery produce 2 X 1.2 volts = 2.4 volts. Likewise, three
cells connected as a battery produce 3.6 volts. By the way,
ordinary flashlight batteries of the carbon zinc type have a
nominal voltage of 1.5 volts compared to the 1.2 volts of the
nicad cell.
Nicad batteries have an unusual and highly characteristic
discharge behavior which is best described as "a stable
discharge plateau then sudden voltage drop." Essentially a fully
charged nicad battery provides constant voltage and current
until near its exhaustion at which point the voltage SUDDENLY
DROPS and the cell is, for practical purposes, completely
discharged.
Compare this to standard carbon zinc and alkaline batteries
which gradually drop in voltage and amperage through the
discharge cycle of the battery. In use nicads tend to be stable,
then die suddenly at the end while conventional non-rechargeable
batteries slowly decay in voltage as their power is consumed.
One conclusion you might draw from this is that when your
portable computer beeps that the nicad battery voltage is
nearing exhaustion you literally have only moments of use left!
The good news is that nicads produce dependable power through
their discharge cycle which is highly desirable with digital
data and computer memory devices.
The "memory effect" of nicads is perhaps the most discussed and
misunderstood phenomenon associated with nicad cells and
batteries. An undesirable and somewhat unique characteristic of
nicad batteries that they can develop a "memory" which can
decrease either the capacity or voltage of the battery.
The first type of memory problem in nicads - voltage memory - is
caused by sustained charging over many days or months. This
memory effect can be accelerated by high ambient temperature
extreme duration of charge and high rate of charge. In effect
the battery is charged for such a long period of time or at such
a high rate or high temperature that the efficiency of the
chemical reaction is impaired and proper terminal voltage
readings are not achieved.
In the second, more common "memory capacity" problem, the nicad
loses the capability to deliver its full power capacity. One
cause of this peculiar memory problem is the FREQUENT PARTIAL
DISCHARGE of the battery - use for perhaps 30 minutes - and then
full recharge again. In effect the nicad battery "learns" that
only part of its capacity is used and over several cycles of
"partial depletion and then full recharge" that less then full
capacity is needed. It will then be unable to deliver a full
two or three hour standard discharge in normal use. Fortunately
memory effects are usually temporary and can be reversed.
The chemical basis for these two memory effects is not fully
understood, but may have to do with obscure oxidation reactions
which temporarily coat the internal electrodes of the battery
with thin layers of complex non-reactive chemical compounds
which can be removed by more fully "exercising" a nicad through a
complete charge/discharge cycle.
It is claimed by many manufacturers that this odd memory effect
of nicads has been largely eliminated due to modern
manufacturing methods. However to some degree this may in fact
be a result of newer charging systems and the relatively
complete discharge of nicad power by modern laptops. In effect
the batteries are charged and discharged in a more appropriate
manner by most laptop users so memory effects "appear" to be no
longer a problem.
Both memory problems - voltage memory and capacity memory - are
usually temporary and can be corrected by discharging the
battery to or very near its exhaustion point (optimum drawdown
voltage is about 1.0 to .9 volts for a standard 1.2 volt nicad)
and then recharging it to full capacity. Repeat this discharge-
recharge cycle from 2 to five times and frequently the nicad
will lose its memory for the "partial capacity" and again
provide a full 3 or 4 hours of use in most laptops. Actually,
frequent FULL discharge and recharge prolongs the life of a
nicad. The more you use them the longer they last!
Most folks who want to completely discharge laptop nicads simply
leave the computer on until it runs down. A much faster method is
to use the following batch file which continuously reads the
directory of a disk and writes the contents to a disk file.
The continuous disk access drains nicad power much faster. If
you are not familiar with batch files, read the batch file
tutorial elsewhere in this program. Here's the three line batch
file. To stop the batch file at any time press the control and
break keys simultaneously. When finished you may wish to erase
both the batch file and the small file named "test" which it
creates.
:start
dir>test
goto start
As an aside, the newer nickel-hydride batteries used in some
laptop and notebook computers do not seem to suffer from memory
effects. But these batteries are more expensive and not in
common use by most laptop manufacturers.
Nicads do eventually fail. And for various reasons. Temporary or
partial failure due to memory effects was discussed in the
previous paragraphs.
Permanent failure - usually between 3 to 5 years into the life
of a typical nicad can happen due to the growth of
characteristic "whiskers" of conducting chemical compounds which
effectively bridge the internal gap between the positive and
negative electrodes inside the battery. Effectively these small
contamination deposits gradually short circuit the battery
internally which leads to inability to charge or discharge. Some
clever electronic hobbyists build high current "surge" power
supplies which can burn open these internal deposits and reopen
the gap between positive and negative electrodes. A risky
practice at best - given the explosive reputation of nicads - but
"zapping" nicads in this manner has been documented as one way
to add life to an otherwise dying battery. A risky an usually
ill-advised attempt to salvage an otherwise dying battery.
A different permanent failure can result from premature loss of the
liquid electrolyte from the battery. High temperature and/or
high charging rates are usually the cause here. Quick-charge
batteries frequently fail due to this problem if their charging
circuits are not properly designed. If the top edge of the cell
which contains the fail safe pressure release valve has a
buildup of white corrosion powder this is probably the residue
ot the expelled electrolyte and the cell may be on its way to
failure and should be replaced. Note that you can only see this
corrosion buildup on the top of the SINGLE nicad cells which are
usually encased within a surrounding plastic battery housing.
The plastic housing may show little problem externally.
Generally, however, the average computer user should not attempt
to open the protective plastic case of the battery to examine
each cell. If the manufacturer seals several individual nicad
cells in a plastic battery container it is for GOOD reason and
your own personal safety. As a rule quick charge nicads do not
last as long a regular nicads due to heat build up during the
charging cycle.
So how long will a nicad battery last before complete failure
occurs? Manufacturers estimate LOW figures between 500 and 1,000
full charge and discharge cycles or about 3 to 5 five years, as
noted above. Some nicads have been known to approach 5,000 to
10,000 charge and discharge cycles before permanent failure.
Excessive quick charging, heat buildup, infrequent use and lack
of full charge all contribute to shortened nicad lifespan.
Charging and discharging mathematics...
Charging nicads is generally done automatically by a charging
circuit. Two practical pieces of advice: 1) if the battery
becomes VERY hot something could be wrong 2) if the manufacturer
tells you that the battery will be fully charged after a certain
length of time although it can be left charging longer you will
probably do the nicad a favor by removing it after full charge
is reached. Some clever nicad users simply attach an inexpensive
electrical timer - similar to those used to turn lights on and
off in the evening - directly to the nicad charger to prevent
overcharging.
Generally nicads have a proper charging rate which depends on
each manufacturers recommendation. For standard nicads which are
NOT quick charge types the proper slow or "trickle" charge rate
is determined by dividing the ampere hour capacity of the
battery by 10. For example if a nicad has a total capacity of 1
ampere hour, dividing this by ten (1/10) produces a correct
trickle charging rate of .1 amps or 100 milliamps. Quick-charge
nicads can accept a charge rapidly and the suggested charging
rate is determined by dividing the ampere hour capacity of the
battery by 3 rather than by 10. These figures represent the
trickle charge rate which theoretically means the nicad "could"
be safely left charging indefinitely without harm.
Higher efficiency chargers are designed not to simply trickle
charge nicads but start a discharged battery at a HIGH rate of
charge and then taper the charging current back quickly to the
safer "trickle" charge rate once full charge is reached. Usually
for regular nicads this "initial surge charge" can be as high as
the ampere hour capacity divided by 3. For quick charge nicads
this "initial surge charge" can be as high as the ampere hour
capacity divided by 1. Obviously these are very high charge
rates and are provided to discharged batteries and then
quickly discontinued once full charge is approached. Clearly a
charging circuit of this sophistication is expensive and may
even contain its own microprocessor to sense the discharge level
of the nicad and calculate the optimum charge rate, time and
trickle charge transition. Since we have previously discussed
the adverse affect of heat on nicads it is essential to note
that NICADS SHOULD BE CHARGED IN A COOL OR ROOM TEMPERATURE
location since they normally generate heat when charged. If you
minimize heat buildup - especially during the charging cycle -
you will prolong the useful life of your nicad battery.
Discharging a nicad - especially if you are trying to remove a
"memory" problem such as that discussed earlier does NOT mean
discharging a cell to zero volts. Usually the correct discharge
voltage is about 1.0 volts. This may seem odd when you consider
that the fully charged cell has a 1.2 volt reading, but in fact
at 1.0 volts a typical nicad cell has released about 90% to 95%
of its energy - another eccentric, but predictable behavior of
nicads given the rapid "voltage drop off" as they near the end
of their three or four hour life in a laptop computer.
Shelf life. While carbon zinc and alkaline batteries can hold
their charge for years, nicads lose their charge relatively
quickly. Although it varies, one quick rule of thumb is that a
typical fully charged nicad will lose roughly 25% to 35% of full
charge in one month. Then another 25% to 35% of THE CHARGE
REMAINING in the next month. And so on and so on. Thus if you
have several nicad batteries you want to charge for a trip you
will be taking in a month, it is probably better to charge ALL
OF THEM the final week just before the trip rather than the
month before. For want of a better phrase, this might be called
"shelf discharge" and is normal with all nicads and has to do
with slight electrical leakage and chemical compound decay
internally within a charged nicad which sits on a shelf. Cooling
or refrigerating the nicad (but NOT freezing) will slow this
"shelf discharge" since you are cooling and slowing the
breakdown reaction. In fact ALL batteries will last longer when
refrigerated until they are used. Simply store them in
individual sealed plastic bags (to minimize moisture
condensation) and place them in the refrigerator.
And so we conclude with a little summary....
1) Do exactly what the manufacturer suggests for both
discharging and recharging a nicad.
2) Keep temperatures - especially during charging - cool or at
normal room temperature.
3) Never short circuit a nicad intentionally or accidentally.
4) Try cycling a nicad through several COMPLETE discharge and
recharge cycles if it "appears" to be faulty an incapable of
operating your equipment for a normal three or four hour
operating period.
5) Remove nicads from charging circuits or discontinue charging
when full charge has been reached.
6) Watch for white flaky corrosion deposits on the upper edge of
the cell near the pressure vent this can mean impending cell
failure and electrolyte loss.
7) Dispose of permanently defective nicads properly - contact
the manufacturer for instructions since cadmium is a dangerous
toxic metal and has been banned from many dump sites. Try
calling your local city hall and ask who can answer a question
about cadmium metal waste disposal.
8) When the nicad battery power begins to drop near the end of a
discharge cycle it will drop VERY QUICKLY due to the rapid
characteristic dropoff of nicads. Prepare for laptop shutdown
quickly.
9) Cycle your nicads through a FULL DEEP discharge and FULL
COMPLETE recharge frequently - they will last LONGER before you
must dispose of them and deliver MORE power when used.
10) Infrequently used nicads should be charged and discharged at
least once or twice every two or three months to prolong their
usable lifetime before permanent failure.
11) If your nicads are stated by the manufacturer to be quick
charge type, you can probably prolong their life by slow or
trickle charging them (if your charger provides that option)
since you will minimize heat and gas buildup within the cell.
Just because they can be quick charged does not mean they MUST
be quick charged. Nicads last longer and deliver more power when
not driven to extremes of temperature or overcharging.
Tutorial finished. Be sure to order your FOUR BONUS DISKS which
expand this software package with vital tools, updates and
additional tutorial material for laptop users! Send $20.00 to
Seattle Scientific Photography, Department LAP, PO Box 1506,
Mercer Island, WA 98040. Bonus disks shipped promptly! Some
portions of this software package use sections from the larger
PC-Learn tutorial system which you will also receive with your
order. Modifications, custom program versions, site and LAN
licenses of this package for business or corporate use are
possible, contact the author. This software is shareware - an
honor system which means TRY BEFORE YOU BUY. Press escape key to
return to menu.